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Trace metals in the groundwater of coastal sandy aquifers significantly influence the coastal ecosystem, yet the spatiotemporal controls of these metals remain unclear. In this paper, we comprehensively revealed the distribution patterns, key controlling factors, potential ecological risks, and fluxes to the ocean of groundwater trace metals (As, Ba, Cr, Cd, Fe, Mn, Pb, and Zn) in a coastal deep sandy aquifer. The results showed a clear zonation of trace metals in the groundwater in relation to the mixing extent between seawater and terrestrial freshwater. The freshwater zone exhibited a relatively low concentration of trace metals, whereas the freshwater‐seawater transition zone showed a substantial quantity of dissolved Fe, Mn, As, and Ba. Seawater‐groundwater interactions significantly affected the Fe, Mn, As, and Ba concentrations through redox potential and pH gradients. The tide‐driven saline water zone was vulnerable to oceanic environments and anthropogenic activities, resulting in the enrichment of trace metals such as Zn and Cd. Driven by recirculated submarine groundwater discharge (SGD), the concentrations of trace metals in the density‐driven saline circulation zone were found to be higher than those in the surrounding areas. The ecological risk index suggested that the freshwater‐seawater transition zone posted the highest ecological risks. Trace metal fluxes (i.e., Fe, Mn, and As) via SGD significantly contributed to the total input into the sea, which may have potential impacts on the coastal environments. Our study highlighted the importance of seawater‐groundwater interactions on trace element cycling in coastal sandy aquifers. Key Points: The distribution of groundwater trace metals in a coastal deep sandy aquifer with different seasons was investigated, and four zones were identifiedKey controlling factors, ecological risks, and fluxes of trace metals in different zones were evaluatedSeawater‐groundwater interactions aiding trace metal addition/removal insfluenced trace element cycling on a sandy coast [ABSTRACT FROM AUTHOR]

To address groundwater issues, it is often necessary to develop geological and hydrogeological models. Combining geological, geophysical and hydrogeological data available on a site to build such models is often a challenge. This paper presents a methodology to integrate such data within a geologically consistent model with robust error estimation. The methodology combines the Ensemble Smoother with Multiple Data Assimilation (ESMDA) algorithm with a hierarchical geological modeling approach (ArchPy). Geophysical and hydrogeological field data are jointly assimilated in a stochastic ESMDA framework. To speed up the inversion process, forward responses are computed in lower‐dimensional spaces relevant to each physical problem. By doing so, the final models take into account multiple data sources and regional conceptual geological knowledge. This study illustrates the applicability of this novel approach using actual data from the upper Aare Valley, Switzerland. The results of cross‐validation show that the combination of different data types, each sensitive to different spatial dimensions, enhances the quality of the model within a reasonable computing time. The proposed methodology allows the automatic generation of groundwater models with robust uncertainty estimation and could be applied to a wide variety of hydrogeological issues. Plain Language Summary: When dealing with groundwater, it is necessary to develop underground models. However, taking into account all the different data types on a site is time‐consuming, and there is a lack of uncertainty quantification. In this study, we develop an approach that automatically combines different types of data, including boreholes, geophysical data, and hydrogeological measurements. All data are combined using a stochastic algorithm and produce an ensemble of plausible and data‐compatible models. These models can be used, for example, to forecast groundwater availability, pollutant distribution, or the effect of climate change on groundwater. Key Points: A methodology is proposed to assimilate geological, hydrogeological, and geophysical data in consistent stochastic modelsThe methodology combines a hierarchical geological modeling technique with the ensemble smoother with multiple data assimilationThe applicability of the methodology is demonstrated using actual field data from the upper Aare valley aquifer in Switzerland [ABSTRACT FROM AUTHOR]

This paper focuses on deriving new approximate analytical solutions in wedge‐shaped aquifers. The proposed methodology is applicable to any type of aquifer namely, leaky, confined and unconfined, under both steady state and transient flow conditions. By applying the method of images and separating the flow field into sections using physical arguments, approximate analytical expressions are obtained for the drawdown function, which in contrast to the conventional theory, are applicable to any arbitrary wedge angle. Moreover, the solutions fully observe the boundary conditions, while they preserve the continuity of the drawdown, which can be calculated directly at any point of the flow field. Nevertheless, comparison of the results of the new approximate analytical solutions to numerical ones, has been considered necessary to check their validity. MODFLOW, a well‐known numerical tool is used to calculate the numerical results. The discrepancies between the numerical results and those of the approximate analytical solution are negligible. The main advantages of the proposed methodology are the following: (a) The model needs only finite number of terms compared to conventional analytical and numerical solutions that involve infinite series, (b) The computational load is low, so it can be easily used in conjunction with meta‐heuristic algorithms to solve groundwater resources optimization problems, (c) Stream depletion rate can be calculated rather accurately and (d) The method is applicable to related flow problems. Key Points: New functions for drawdown calculation in wedge‐shaped aquifers are presentedThe proposed functions are suitable to be used in conjuction with meta‐heuristic algorithms to solve groundwater optimization problemsConvergence of the results obtained from the set of functions and numerical methods (MODFLOW) point out the validity of the proposed solutions [ABSTRACT FROM AUTHOR]

Low‐elevation coastal areas are increasingly vulnerable to seawater flooding as sea levels rise and the frequency and intensity of large storms increase with climate change. Seawater flooding can lead to the salinization of fresh coastal aquifers by vertical saltwater intrusion (SWI). Vertical SWI is often overlooked in coastal zone threat assessments despite the risk it poses to critical freshwater resources and salt‐intolerant ecosystems that sustain coastal populations. This review synthesizes field and modeling approaches for investigating vertical SWI and the practical and theoretical understanding of salinization and flushing processes obtained from prior studies. The synthesis explores complex vertical SWI dynamics that are influenced by density‐dependent flow and oceanic, hydrologic, geologic, climatic, and anthropogenic forcings acting on coastal aquifers across spatial and temporal scales. Key knowledge gaps, management challenges, and research opportunities are identified to help advance our understanding of the vulnerability of fresh coastal groundwater. Past modeling studies often focus on idealized aquifer systems, and thus future work could consider more diverse geologic, climatic, and topographic environments. Concurrent field and modeling programs should be sustained over time to capture interactions between physical processes, repeated salinization and flushing events, and delayed aquifer responses. Finally, this review highlights the need for improved coordination and knowledge translation across disciplines (e.g., coastal engineering, hydrogeology, oceanography, social science) to gain a more holistic understanding of vertical SWI. There also needs to be more education of communities, policy makers, and managers to motivate societal action to address coastal groundwater vulnerability in a changing climate. Plain Language Summary: Along the world's coastlines, seawater flooding on the land surface by storms and high tide events can drive the downward infiltration of seawater into coastal aquifers that are an important source of fresh groundwater. Rising sea levels and the intensification of coastal storms are expected to exacerbate seawater flooding events globally. The salinization of fresh groundwater along the coast following floods is often "out of sight, out of mind," but this process threatens freshwater resources that sustain dense coastal human populations and valuable, sensitive ecosystems. This paper reviews prior studies that have applied monitoring and/or computer modeling methods to better understand processes and controls on vertical saltwater intrusion. Our review details how surface flood dynamics, groundwater processes, environmental characteristics, and human activities combine to create complex patterns of salinization and flushing over space and time. From our synthesis we identify future research needs, including long‐term, globally dispersed monitoring and collaboration across scientific disciplines. We further emphasize the need for communication with local stakeholders about this topic to motivate informed management and research initiatives to understand and reduce the vulnerability of coastal fresh groundwater resources in a changing climate. Key Points: Vertical saltwater intrusion following coastal floods threatens fresh groundwater in an age of rising seas and intensifying stormsWe review salinization and flushing processes identified with field and modeling methods and identify future research opportunitiesKnowledge gaps can be addressed through multidisciplinary studies to advance science and inform water resources management and policy [ABSTRACT FROM AUTHOR]

The drawdown time is important for the design and assessment of infiltration basins. This paper investigates its dependence on soil heterogeneity, and simple analytical solutions for the mean and the variance are given. The solutions are tested through Monte Carlo simulations in various realistic scenarios, using a modified Green Ampt model for layered soils and a model based on the integration of the 1‐D Richards equation. The impact of the employed approximations is assessed, and the leading role of the spatial distribution of the saturated hydraulic conductivity is emphasized. The effects of other relevant design and natural factors, including the initial water level and water content distribution, are also investigated. Plain Language Summary: Infiltration basins are receiving an increasing interest in water resources management as an alternative to surface water storage in the managed aquifer recharge and as a green infrastructure at the urban scale. One of the main parameters in the design of the infiltration basins is the drawdown time, that is, the time needed to empty the basin after it has been filled with water, for instance, after a flood event or by a managed treatment plant outlet. This work provides simple analytical solutions for the evaluation along a stochastic approach of the drawdown time, accounting for the soil heterogeneity and leading to a probabilistic design of the infiltration structure. Key Points: The time needed to empty infiltration basins, or drawdown time τ, is an important quantity for their design and assessmentThis work proposes analytical solutions for the mean and variance of τ considering the soil heterogeneitySolutions have been tested through a set of Monte Carlo numerical simulations, showing that the probability density function is lognormal and investigating the effects of design and natural factors [ABSTRACT FROM AUTHOR]

Reactive transport models (RTMs) are essential tools that simulate the coupling of advective, diffusive, and reactive processes in the subsurface, but their complexity makes them difficult to understand, develop and improve without accompanying statistical analyses. Although global sensitivity analysis (SA) can address these issues, the computational cost associated with most global SA techniques limits their use with RTMs. In this study, we apply distance‐based generalized sensitivity analysis (DGSA), a novel and computationally efficient method of global SA, to a floodplain‐scale RTM and compare DGSA results to those from local SA. Our test case focuses on the impact of 17 uncertain environmental parameters on spatially and temporally variable redox conditions within a floodplain aquifer. The input parameters considered include flow and diffusion rates, geochemical reaction rates, and the spatial distribution of sediment facies. Sensitivity was evaluated for three distinct components of the model response, encompassing both multidimensional and categorical output. Parameter rankings differ between local SA and DGSA, due to nonlinear effects of individual parameters and interaction effects between parameters. DGSA results show that fluid residence time, which is controlled by aquifer permeability, generally exerts a stronger control on redox conditions than do geochemical reaction rates. Sensitivity indices also demonstrate that sulfate reduction is key for establishing and maintaining reducing conditions throughout the aquifer. These results provide insights into the key drivers of heterogeneous redox processes within floodplain aquifers, as well as the main sources of uncertainty when modeling complex subsurface systems. Plain Language Summary: Models that simulate the movement of groundwater and contaminants in aquifers, known as reactive transport models (RTMs), are complex. Multiple competing processes, including the physics of groundwater flow and the chemistry of microbial interactions, make the results of such models difficult to understand without additional statistical analyses. Sensitivity analysis, a technique for calculating the effect of each input variable on model output, is one potential tool for interpreting complex models, but it is rarely performed on RTMs due to the computational resources required. In this paper, we show that distance‐based generalized sensitivity analysis (DGSA) can be used to interpret reactive transport models and has several advantages over other techniques. As a case study, we use DGSA to measure the effect of 17 input variables on a model that simulates iron cycling within a shallow aquifer. Results show that groundwater flow rates control dissolved iron concentrations and are generally more influential than geochemical reaction rates. Though we focus on iron, the findings are relevant for other microbially driven reactions, which can control the mobility of many nutrients, contaminants, and metals in groundwater. Key Points: Global sensitivity analysis is a valuable tool for improving process understanding in subsurface biogeochemical modelsParameter rankings from local and global sensitivity analyses can differ when applied to reactive transport modelsIn certain settings, sediment permeability exerts a more dominant control on redox cycling than do geochemical reaction rates [ABSTRACT FROM AUTHOR]

Strains occur at shallow depths in response to pressure changes during well tests in an underlying aquifer, and recent developments in instrumentation have made it feasible to measure essentially the full strain tensor. Simulations using poroelastic analyses indicate that shallow normal strains are approximately proportional to the logarithm of time when a well is injecting into or pumping from a deep aquifer or reservoir. The drawdown is also a linear function of log time, as shown by the classic Cooper‐Jacob type‐curve analysis. The time when the semilog straight line intercepts the zero‐strain axis is similar to the time determined from the Cooper‐Jacob pressure analysis, and it can be used to estimate hydraulic diffusivity, suggesting that horizontal strain data can be used directly to estimate aquifer properties. This approach was validated using measurements from shallow (30‐m deep) borehole strainmeters during an injection test at a 530‐m‐deep sandstone aquifer/reservoir in Oklahoma. The results show intercept times for the shallow normal strain data are essentially the same as for deep pressure data from an equivalent radial distance. The slopes of the semilog plots of the pressure and the strain increase at the same time, suggesting that they both respond to a lateral aquifer boundary. Significantly, though, strain was measured at shallow depths while the pressure data were measured at 530‐m depth. This suggests that strain data from shallow depths could be an effective way to improve the characterization of an underlying aquifer. Plain Language Summary: Drilled wells are used to recover water, oil and gas, minerals, heat, and other valuable resources, and they are also used to dispose of carbon dioxide, sewage, and other unwanted wastes. Most tests to evaluate well performance involve measuring the pressure change in a monitoring boring that taps a permeable layer, such as an aquifer or reservoir. The pressure change is analyzed by fitting it to an appropriate mathematical analysis, called a "type curve." This is an effective approach, but monitoring borings can be expensive and they are almost always in short supply, so we are interested in evaluating alternatives. Dropping the pressure in an aquifer causes it to shrink by a tiny amount, and this in turn deforms the overlying rocks. We used high precision instruments called borehole strainmeters to measure the very small strains that occurred at shallow depths (30 m) when a well test was being conducted in a much deeper permeable sandstone at 530‐m depth. We also measured the pressure at three monitoring wells completed in the sandstone. Data from the tests show that the strain signal at shallow depths is similar to the pressure signal in the sandstone. Pressure measurements from the monitoring wells were analyzed using a type curve that is standard for the test that was conducted. We show in the paper that a similar type curve function can be used to analyze the strain signal and the results are similar to those from the pressure analysis. This finding is significant because it shows that strain measurements made at shallow depths can be analyzed using a straightforward approach to help characterize much deeper aquifers and reservoirs. This can reduce risk and cost, and improve the reliability of wells used recover resources or store wastes. Key Points: New method that uses strain measurements for interpreting well tests are suggestedStrain measured at shallow depths above the aquifer can be used instead of the pressure in the aquifer itself to estimate aquifer propertiesThe suggested method of strain measurements could lead to a new way for characterizing aquifers using pumping tests [ABSTRACT FROM AUTHOR]

Recession analysis is a powerful tool for determining the hydraulic characteristics of a riparian aquifer. The basis of the method relies on two analytical solutions of the nonlinear Boussinesq equation: one applies to a uniform water table profile in a semi‐infinite aquifer while the other pertains to a hypothetical initial profile in a finite aquifer. Both solutions assume that the water depth in the adjoining stream is negligible. In the present work, a nonlinear solution of the one‐dimensional Boussinesq equation is derived using the traveling wave approximation. The solution better represents the field conditions as it pertains to a uniform water table profile in a finite aquifer that is adjoining a stream with a non‐zero water level. It allows the derivation of simple algebraic expressions for the recession of the water table, the flow rate, and the associated drainage volume. Approximations relating the discharge rate to the outflow volume were also obtained for practical implementation in the estimation of the hydraulic parameters of the aquifer. A comparison of the proposed methodology of parameter estimation with the standard method of recession analysis showed that the present approach is superior as it avoids the pitfalls associated with the classical method, especially when daily discharge values with inherent observational errors are used. Application of the recession model to real‐world cases demonstrates its effectiveness in the estimation of the catchment‐scale hydraulic conductivity and drainable porosity. Plain Language Summary: The hydraulic characteristics of aquifers are required for many hydrologic applications and analysis of recession flow is one valuable tool for characterizing an aquifer system. The analysis relies on two solutions of the governing Boussinesq equation, each with restrictive conditions. The present study relaxes those assumptions, derives working relationships, and proposes a more effective method for the estimation of the hydraulic properties of aquifers, the evaluation of the baseflow contribution to streamflow, and the quantification of the aquifer depletion in drought periods. Key Points: Practical expressions for the discharge rate and drainage volume are derivedDischarge‐volume relations for the estimation of the hydraulic parameters are presentedA simple equation for the water table recession is obtained for plausible flow conditions [ABSTRACT FROM AUTHOR]

Do observation wells in fractured porous aquifers measure water head in the fracture network, water head in the matrix, or some combination of both? This question necessarily arises when calibrating dual‐continuum hydrogeological models against on‐field data. One can assume that observation wells measure fracture water head, because matrix permeability is negligible compared to fracture permeability. Nevertheless, this reasoning is invalid for wells poorly‐connected to the fractures. Yet, the possibility of such a poor connection at given depths has never been implemented in a physics‐based manner when comparing matrix and fracture water heads simulated by dual‐continuum models to on‐field data. To fill this knowledge gap, a physically based, easy to calibrate, open‐source postprocessing tool, POWeR‐FADS (Program for Observation Well Representation in Fractured Aquifer Dual‐continuum Simulations), available at https://github.com/BJeannot1/POWeR-FADS, has been developed. It introduces as parameters well geometry and the altitude of lowest interception of the fractures by the well. From these, POWeR‐FADS nonintrusively postprocesses time series of matrix and fracture water heads at the well, as simulated by any planar, bidimensional dual‐continuum hydrogeological model, to calculate water exchanges involving the observation well and thus the evolution of water level in the well. Synthetic test cases show that POWeR‐FADS makes it possible to simulate peculiar behaviors that are similar to patterns actually observed by the authors in on‐site observation wells of a fractured porous aquifer, like "floors" in observed water levels, delayed but sharp rises at the beginning of recharge events, or inflexion points accelerating the drawdown velocity during the recession phase. Key Points: A program is made to simulate the water level in observation wells from matrix and fracture water heads computed by dual‐continuum modelsFor wells poorly connected to the fractures, the code simulates patterns that could not be modeled by a dual‐continuum model aloneThis postprocessing tool is easy to use, as its most sensitive parameter can be deduced from drill cores or observation well data [ABSTRACT FROM AUTHOR]

Accurate modeling of water flow and solute transport in unsaturated soils is of significant importance for precision agriculture, environmental protection and aquifer management. Traditional modeling approaches are considerably challenging since they require well‐defined boundaries and initial conditions. Physics‐informed neural networks (PINNs) have recently been developed to learn and solve forward and inverse problems also constrained to a set of partial differential equations and are more flexible than traditional modeling approaches. However, hydrological applications of PINNs used so far spatial measurements of hydraulic head, water content and/or solute concentrations, which were well distributed in the subsurface for training the system. Such measurements are hard to obtain in real‐world applications. Here, we propose to train PINNs with non‐invasive geoelectrical tools for simulating two‐dimensional water flow and solute transport during infiltration and redistribution processes with unknown initial conditions. Two‐dimensional flow and transport numerical simulations were used as benchmarks to examine the suitability of the described approach. Results have shown that the trained PINNs system was able to reproduce the spatiotemporal distribution of both water content and pore‐water salinity during both processes with high accuracy, using five time‐lapse geoelectrical measurements and matric head measurements at a single location. The trained PINNs system reconstructed the initial conditions of both state parameters at both processes. It was also able to separate the measured electrical signal into its two components, that is, water content and pore‐water salinity. The subsurface geoelectrical tomograms were significantly improved compared to those obtained from a classical inversion of the raw geoelectrical data. Key Points: Physics‐informed neural networks were trained with geoelectrical data to simulate water flow and solute transport at the subsurfaceInitial conditions, as well as spatiotemporal patterns of both water content and solute concentrations, were reproducedBulk electrical conductivity tomograms were significantly improved compared with the ones obtained with classical inversion of the raw data [ABSTRACT FROM AUTHOR]

Unsaturated flow influences both the seawater extent under steady‐state conditions and the propagation of tides in coastal aquifers. However, its effects on salt distributions in tidally influenced coastal aquifers are little investigated. The present study used numerical simulations and data from laboratory experiments to analyze the effects of unsaturated flow on density‐dependent solute transport in coastal unconfined aquifers. The effects of the inland boundary condition (i.e., constant‐head or constant‐flux) were tested. Compared to a stable sea level, the results show that unsaturated flow has a more pronounced influence on salt distributions in coastal unconfined aquifers when tides are considered, regardless of the type of inland boundary condition. Neglect of unsaturated flow effects leads to expansion of the upper saline plume (USP), shrinkage of the saltwater wedge (seaward movement of saltwater wedge), and overestimation of water and salt exchange across the aquifer‐ocean interface. This is caused by a lower head in the nearshore area during high‐tide periods with the unsaturated zone effects removed. Thus, without the unsaturated zone, stronger head gradients within the nearshore aquifer occur at high tide, leading to stronger tidally driven seawater infiltration and hence a larger USP. Counterintuitively, ignoring unsaturated flow effects leads to greater average inland head over a tidal period, which shifts the saltwater wedge seaward. It is concluded that unsaturated zone effects should not be neglected for modeling tide‐affected seawater intrusion, especially if quantification of near‐shore conditions is important. Key Points: Tidal fluctuations intensify unsaturated flow effects on salt distributions in coastal unconfined aquifersNeglecting unsaturated flow effects leads to a more extensive upper saline plume and shrinking of the saltwater wedgeNeglecting unsaturated flow effects intensifies the water and salt exchanges across the aquifer‐ocean interface [ABSTRACT FROM AUTHOR]

Seasonally snow‐covered catchments in the western United States supply water to growing populations as both annual snowmelt‐driven streamflow and multi‐year groundwater recharge. Although interannual variability in streamflow is driven largely by precipitation, runoff efficiency (the ratio of streamflow to precipitation) in individual catchments varies by 50% or more. Recent work suggests that interannual variability in groundwater storage, inferred from winter baseflow, is a primary control on runoff efficiency, highlighting a need to quantify both the time scales on which groundwater storage varies and the hydro‐climatic drivers of storage. Using over a century of daily stream discharge data from 10 seasonally snow‐covered catchments in northern Utah, we find that temporal variability in winter baseflow, an index of groundwater storage, measured from mean daily January discharge, exhibits a 2–5‐ and 12–15‐year periodicity, driven by regional precipitation patterns and snowmelt dynamics. Specifically, multiple linear regression (MLR) modeling using antecedent hydro‐climatic variables demonstrates that winter baseflow (groundwater storage) was positively related to 3–4 years of antecedent annual precipitation, negatively related to the previous year's mean annual temperature, and positively related to 1–4 antecedent years of snowmelt rate and duration. Because antecedent baseflow (groundwater storage) is strongly related to runoff efficiency, these results suggest that more frequent and longer droughts in a future climate will reduce surface water supplies faster than otherwise expected. More broadly, these results highlight the importance of including the influence of antecedent climate on groundwater storage when modeling and managing water supplies from seasonally snow‐covered catchments. Key Points: Interannual changes in groundwater storage inferred from baseflow in 10 headwater catchments exhibit coherent 2–5 and 12–15 year periodicityInterannual variability in groundwater storage is related to 1–4 years of antecedent precipitation (+) snowmelt rate (+) and temperature (−)Groundwater storage in warmer/drier catchments is related to longer periods of antecedent climate than cooler/wetter catchments [ABSTRACT FROM AUTHOR]

Managed aquifer recharge (MAR) can provide long‐term storage of excess surface water for later use. While decades of research have focused on the physical processes of MAR and identifying suitable MAR locations, very little research has been done on how to consider competing factors and tradeoffs in siting MAR facilities. This study proposes the use of a simulation‐optimization (SO) framework to map out a cost‐effectiveness frontier for MAR by combining an evolutionary algorithm with two objective functions that seek to maximize groundwater storage gains while minimizing MAR cost. We present the theoretical framework along with a real‐world application to California's Central Valley. The result of the SO framework is a Pareto front that allows identifying suitable MAR locations for different levels of groundwater storage gain and associated MAR project costs, so stakeholders can evaluate different choices based on cost, benefits, and tradeoffs of MAR sites. Application of the SO framework to the Central Valley shows groundwater can be recharged from high‐magnitude (95th percentile) flows at a marginal cost of $57 to $110 million per km3. If the 10 percent largest flows are recharged the total groundwater storage gain would double and the marginal costs would drop to between $30 and $50 million per km3. If recharge water is sourced from outside local basins (e.g., the Sacramento‐San Joaquin Delta), groundwater storage gain is approximately 25%–80% greater than can be achieved by recharging local flows, but the total cost is about 10%–15% higher because of additional lift cost. Key Points: Combining a simulation‐optimization framework with a groundwater model can identify suitable managed aquifer recharge locationsRecharging water from local rivers within the same basin is the most cost‐efficient managed aquifer recharge approach per unit of groundwater storage gainDiverting more water may not yield the most efficient groundwater storage gain due to higher transportation cost [ABSTRACT FROM AUTHOR]

We study the sensitivity of aquifer‐scale estimates of transmissivity (T) and storativity (S) to the variance and correlation length scale of aquifer heterogeneity, when such estimates are obtained by the traditional approach of analyzing pumping test data. We consider both constant‐rate and variable‐rate pumping tests, and a variety of Theis‐based solution methodologies (single‐ and multiple‐observation well methods, and interpreted single value or transient values for T and S parameters) applied in pumping test data analysis. Our results indicate that achieving reliable inference of effective T and S requires that pumping be continued until the radius of the test‐induced cone of depression exceeds a "representative length" that corresponds to ∼15 times than the correlation length scale of the aquifer heterogeneity. Independent of solution type, pumping history, correlation length scale, magnitude of the sill, and location of observation well, the estimates for T will converge toward the geometric mean. For S, the estimation uncertainty is relatively large for observation wells that are close to the pumping well, but diminishes for observation wells located beyond the representative distance, resulting in convergence to the arithmetic mean. Given that these results have practical implications for how pumping tests should be carried out, we present a simple "rule‐of‐thumb" for estimating the correlation length scale of the heterogeneity of an aquifer. Key Points: The T estimates for constant and heterogeneous S aquifers differ for every observation well; instead, the difference for S strongly depends on observation well distancePumping rate and history does not affect the inferred aquifer parameters; however, Theis‐based solution type results in different interpretationsAs a rule‐of‐thumb, temporal variability of T and S diminishes when the depression cone radius exceeds 15 times the correlation length scale plus the observation well distance [ABSTRACT FROM AUTHOR]

In water‐stressed hyperarid basins, questions mount over the impacts of anthropogenic groundwater extraction and climate‐driven perturbations on groundwater‐surface water interactions and the resilience of ecosystem‐critical surface water. Coupling groundwater with surface water observations from Sentinel‐2 data provides an unprecedented opportunity to evaluate surface water connectivity with local aquifers following intense precipitation events in arid basins. Surface water area and groundwater level data were analyzed for trends following precipitation, including peak lag time, post‐peak recession rates, and changes in hydraulic gradients. Results indicate variable connectivity following large precipitation events between surface water change and groundwater level fluctuations in the upgradient freshwater aquifer, whereas the downgradient brine‐to‐brackish area of the aquifer indicated virtually no connectivity with the aquifer. Comparison between precipitation and surface water response indicate distinct responses based on the physical relationship of the surface water body with the brine‐to‐brackish area of the aquifer. Lumped parameter modeling of surface water inundation also constrains the possible hydrologic dynamics of the post‐precipitation response. While modeled influx to surface water seems primarily controlled by watershed hydraulics rather than direct hydraulic connectivity of the aquifers, the relationship between surface water and adjacent groundwater levels coupled with surface water area indicates that local aquifers are primarily connected to the surface water bodies through discharge via subsurface infiltration. Modeling results imply that the existence of brine‐adjacent surface water in arid basins relies on upgradient discharge from freshwater aquifers. Our results further support that marginal surface water systems can serve as a critical recharge mechanism to local aquifers. Plain Language Summary: Surface water is important for desert ecosystems, but the future of surface water remains uncertain because of climate change and human consumption. Understanding how surface water responds to precipitation and interacts with groundwater is useful for better predicting future water availability. Advancements in the quality of satellite imagery provide an opportunity to combine satellite data with rain gauge and water level data to investigate the connection of surface water to rainfall patterns and groundwater. Observations from flooding behavior show two different types of surface water. Both types can be categorized by proximity to the brine that exists in the aquifer underlying the basin floor: transitional pools occur at the edge of the transition to brine while terminal lagoon systems occur in the brackish groundwater zone. Calculations indicate that the two groups have different inflow and outflow mechanisms that occur over varying timescales. These mechanisms indicate that intense rainfall events will lead to increased inundation over longer periods of time for transitional surface water bodies. However, despite rapid rates of recession, terminal pools will also experience prolonged inundation above background levels for up to a year after large rainfall events because of increased spring discharge from rainfall‐driven recharge to groundwater. Key Points: Post‐precipitation inflow and outflow mechanisms of salar‐adjacent surface water are distinct from recharge and discharge in local aquifersSurface water response to precipitation can be predicted and categorized based on its position to the brine groundwater and the salt flatSurface water resilience to drought relies on groundwater discharge from upgradient springs in hyperarid basins [ABSTRACT FROM AUTHOR]

In the Parowan Valley of Utah, groundwater levels have declined by as much as 30 m over the past 50 years with accompanying subsidence rates of up to 5 cm/year. Traditional methods to estimate groundwater storage change use a combination of groundwater level and storativity estimates, but there is often considerable uncertainty in these. In this study, we demonstrate a new method that relies on a combination of geodetic data from InSAR, as well as groundwater level and pumping data, to estimate both the total groundwater storage loss and the percentages of storage loss in fine‐ and coarse‐grained layers within an aquifer system. We find that when aggregated over all of Parowan Valley, fine‐ and coarse‐grained layers account for roughly equal portions of the total groundwater storage loss. However, in confined aquifers, fine‐grained layers account for most of the storage loss. This has important implications on the source of groundwater in depleting aquifer systems, as many models do not account for fine‐grained layers as a source of water. We find that in the Parowan Valley, the aquifer depletion is roughly 12.5% of the volume of pumped groundwater, meaning that the remainder of pumped groundwater is sourced from net inflow. This study presents the first method that combines geodetic and in situ groundwater data to provide estimates of groundwater storage change that account for both coarse‐ and fine‐grained intervals, which are typically present in significant amounts in the major unconsolidated aquifer systems of the world. Plain Language Summary: As surface water becomes more scarce, groundwater represents an important source of freshwater. However, it is being depleted in many basins of the western US and world. The Parowan Valley, in southwest Utah, has seen groundwater levels decline by as much as 30 m over the past 50 years. This has resulted in subsidence rates of up to 5 cm/year. Monitoring groundwater storage loss is critical for implementing sustainable groundwater management plans. Subsidence can be measured with a satellite data set called InSAR (Interferometric Synthetic Aperture Radar) with high accuracy. This can then be coupled with ground‐based measurements to improve our understanding of groundwater storage loss. We implement this method in the Parowan Valley, and find that roughly half of the estimated groundwater storage loss comes from consolidation of fine‐grained materials within the aquifer, which are typically ignored in groundwater budgets. We also find that groundwater depletion is roughly 12.5% of the volume of pumped groundwater, meaning that if pumping were reduced by that amount, depletion of groundwater would likely cease. However, changing recharge and inflow of groundwater could result in renewed depletion in the future (5–10 years), and re‐assessing the water budget regularly is crucial for effective management. Key Points: New method developed to estimate groundwater storage loss using InSAR, groundwater‐level and pumping dataFine‐grained layers account for most aquifer storage loss in confined aquifersWater lost from fine‐grained layers is a significant but unsustainable source of water for confined aquifers [ABSTRACT FROM AUTHOR]

Among the impacts of earthquakes on aquifers, permeability change is one of the most challenging to quantify, since techniques to measure permeability evolution are scarce. The study of tidal response of boreholes is one of the most promising, yet complex to use in practice. We used 14 years of piezometric level measurements and two concurrent source signals, earth tidal strain and barometric pressure, for which we separated the respective contribution in a state‐of‐the‐art tidal analysis. We developed a new general analytical hydrogeological model, based on geological observations of a confined bi‐layer aquifer. It is able to match combined observations of earth and barometric tide phase lags which could not be explained by existing models. We demonstrate that its relative complexity can be overcome thanks to the results of tidal analysis, yielding a simpler model adapted to the Fond Lahaye site of the Martinique Island. The resulting evolution of diffusivity and loading efficiency, was validated independently with several pumping tests occurring all along the studied period. The transient diffusivity increases and decreases indicate which earthquakes impacted the aquifer, enabling to establish an empirical magnitude‐distance relationship criterion. This criterion confirms the suspected dependence on dynamic stresses, which decrease as the square of the hypocentral distance. Additionally, we investigate two other factors of diffusivity changes: heavy rainfall events and aquifer withdrawals, which demonstrates the sensitivity of volcanic aquifers properties to environmental and anthropogenic influence. Key Points: A model for the tidal response of bi‐layer aquifers was developed, focusing on pressure diffusion and exchange between two confined layersUsing 14 years of hourly piezometric data, the temporal evolution of aquifer diffusivity was inferred and confirmed independently by pumping testsThe change of permeability is shown to be due to earthquakes, extreme rainfall events and aquifer withdrawals in upstream borehole [ABSTRACT FROM AUTHOR]

Although neutral in situ leaching through CO2 + O2 is employed to extract uranium (U) in sandstone by in situ leaching (ISL), mechanisms of U mobilization and O2 consumption remained unclear. To address this gap, 18 groundwater samples were taken from the Qianjiadian sandstone U ore field, including seven samples from production wells in mining area M1 (mining for 5 years), six samples from production wells in mining area M2 (mining for 4 years), and five samples from monitoring wells (GC), to quantify U‐mobilizing processes in the mining aquifer by employing hydrogeochemical compositions and multi‐isotopes. The introduction of O2 and CO2 efficiently stimulated U mobilization in the mining aquifer. The injected CO2 critically promoted the dissolution of carbonate minerals, which enhanced the formation of uranyl carbonate (predominantly CaUO2(CO3)22− and Ca2UO2(CO3)3(aq)) and thus facilitated U mobility. Generally, δ34SSO4 and δ18OSO4 in M2 and M1 were significantly lower than those in GC (p < 0.01). A Bayesian isotope mixing model of δ34SSO4 and δ18OSO4 showed that the contribution of pyrite oxidation to SO42− concentration increased from 1.7% in GC to 13.6% in M2 and to 15.0% in M1. During ISL, pyrite, ammonium, and dissolved organic carbon were major compounds competing with U(IV) for introduced O2 in the ore‐bearing aquifer. Most of the consumed O2 was used for pyrite oxidation (56.2%) and U(IV) oxidation (39.3%), following the thermodynamic sequence of those redox reactions. The current results highlighted the significance of increasing O2 utilization efficiency in improving the performance of ISL operations. [ABSTRACT FROM AUTHOR]

Heat transport in fractured aquifers is determined by the combined effects of flow velocity heterogeneity in the fracture system, and diffusive exchange between the fluid in the fractures and the rock matrix, which can be assumed as impervious. We analyze the impacts of this diffusive exchange on the response to heat transport, as opposite to the pure advective displacement, which governs solute transport. We focus on the post‐peak behavior where we observe pre‐asymptotic regimes with slopes that differ from the signature of matrix diffusion, which exhibits a decay rate of −3/2. This deviation is driven by the variability of both velocity field and fracture aperture field. We derive theoretical models that predict these pre‐asymptotic tails under three extreme cases that can be related with specific network structures, that is, networks dominated by large or small fractures, networks with highly or poorly channelized flow. These theoretical predictions are compared with results from numerical simulations in different sets of three‐dimensional discrete fracture networks. We determine that the combined observation of solute and heat transport responses allows classifying the network in terms of connectivity structure, and partially characterizing the fracture aperture variability in terms of upscaled parameters. Key Points: Heat transport in heterogeneous fractured aquifers exhibits post‐peak tails that express the characteristics of the fracture networkDFN simulations show that heat BTC transition into diffusive regime depends whether the system is dominated by large or small fracturesThe combined observation of solute and heat decay rates allows characterizing connectivity structure and fracture aperture variability [ABSTRACT FROM AUTHOR]

Tidal and barometric water‐level responses in wells have been widely used to calculate the hydraulic properties of aquifer systems. The effect of anisotropy induced by shale content on such responses has not received significant attention. In this study, we examine how the presence of shale (anisotropy, extremely low porosity/permeability; approximately 10−4 to 10−3 mD), which occurs as interlayers in aquifers, affects the tidal responses of the leaky model. Our findings show that the number of wells with shale in the screened section of the study aquifer is limited. Here, we focus on the study of the limited number of wells in the North China Platform to ensure a similar geological background for each well. Calculations indicate that wells, even with a small amount of shale (≥∼5%) in the observation aquifer, may exhibit strong anisotropy and heterogeneity, deviating from the theoretical analytical solutions obtained using isotropic and homogeneous assumption models. Generally, the higher the shale content in aquifer lithology, the greater the phase shifts deviated. Thus, theoretical ideal models with isotropic and homogeneous assumptions may only obtain a rough estimation for wells with shale in aquifer lithology, suggesting that we avoid setting observation aquifers onto shale layers. Plain Language Summary: As special lithology, shale is fragile, compact, and anisotropic, and because of its extremely low porosity/permeability (10−4 to 10−3 mD), it will also induce the heterogeneity of the whole layer containing both shale and other lithology rocks. Therefore, aquifers containing shale might exhibit unique characteristics. However, there is mostly no previous research on this topic in the hydro‐geophysics region, perhaps because the number of wells with shale in aquifer lithology is very limited. Key Points: Well aquifers, even with little amount of shale (∼5%), might have strong anisotropy and heterogeneity and deviate from the ideal modelsThe more shale content in the aquifer lithology the larger the deviation occursFurther call for us to avoid setting the observation aquifers onto the shale (low permeability, friability, and anisotropy) layers [ABSTRACT FROM AUTHOR]

Characterizing aquifer heterogeneity is paramount for accurate flow and transport modeling. In this work, we present a new approach for statistical analysis of hydraulic properties in continuous pumping tomography tests of a phreatic aquifer. The method entails determining equivalent hydraulic conductivity (Keq), specific storage (Ss,eq), and specific yield (Sy,eq) at many locations in the field and then calculating statistical moments of the equivalent properties, assuming they are random space variables. Equivalent properties are defined as the ones pertinent to a homogeneous aquifer for which the head time dependent signal fits the one observed in the pumping test. Calculation is carried out in a novel approach, by matching measured head data to two separate semianalytical solutions considering two time periods, early and late. We apply this approach to the Boise Hydrogeophysical Research Site and find that the equivalent property spatial averages decrease with horizontal distance from the pumping well and appear to stabilize at sufficiently large distances, in line with existing theory for Keq. The squared coefficient of variation shows similar behavior, with values indicating a weakly heterogeneous aquifer. Furthermore, estimated values for Keq, Ss,eq, and Sy,eq are in agreement with literature values for the site. Key Points: New approach for statistical analysis of equivalent hydraulic properties (conductivity, specific storage, and specific yield) is presentedMethod is applied to tomographic pumping test data from the Boise aquifer to find mean and variance of equivalent propertiesThe mean is shown to decrease with distance from pumping well and appears to converge to the effective value at a distance [ABSTRACT FROM AUTHOR]

The common‐pool nature of groundwater resources creates incentives to over pump that contribute to their rapid global depletion. In transboundary aquifers, users are separated by a territorial border and might face substantially different economic and hydrogeologic conditions that can alternatively dampen or amplify incentives to over pump. We develop a theoretical model that couples principles of game theory and groundwater flow to capture the combined effect of well locations and user asymmetries on pumping incentives. We find that heterogeneities across users (here referred to as asymmetries) in terms of either energy cost, groundwater profitability or aquifer response tend to dampen incentives to over pump. However, combinations of two or more types of asymmetry can substantially amplify common‐pool overdraft, particularly when the same user simultaneously faces comparatively higher costs (or aquifer response) and profitability. We use this theoretical insight to interpret the emergence of the Disi agreement between Saudi Arabia and Jordan in association with the Disi‐Amman water pipeline. By using bounded non‐dimensional parameters to encode user asymmetries and groundwater connectivity, the theory provides a tractable generalized framework to understand the premature depletion of shared aquifers, whether transboundary or not. Key Points: Economic and hydrogeologic differences between users affect common‐pool externalities in shared aquifersWhen combined, asymmetries in energy cost, groundwater profitability and aquifer response can exacerbate incentives to overpumpChanging asymmetry conditions might have facilitated the world's first distance‐based groundwater treaty [ABSTRACT FROM AUTHOR]

Backward particle tracking (BPT) remains a subject of research and applications in hydrology for decades, and most BPT models and software suits assume Fickian diffusion even though pollutant transport in natural aquifers is usually non‐Fickian. To fill this knowledge gap, BPT models were presented in this study for pollutants undergoing sub/super‐diffusion in multi‐dimensional geological media. The resultant BPT governing equations were adjoint models derived for four equally possible, vector fractional diffusion equations. BPT solvers followed a two‐step backward Lagrangian scheme, which differed from its forward Lagrangian scheme in super‐dispersive jumps and boundary conditions. BPT parameter estimability depended on the type of parameters and aquifers according to Monte Carlo analyses: (a) most parameters could be estimated using medium properties or tracer snapshots, while the mean velocity and the space‐dependent dispersion coefficient had poor estimability, and (b) fractured aquifers had a lower estimability for BPT parameters than alluvial aquifers. Mathematical evaluation and field applications showed that the adjoint of the vector fractional‐divergence advection‐dispersion equation (FD‐ADE) identified the source location of pollutants undergoing multiscaling super‐diffusion better than the other BPT models. The adjoint of the vector FD‐ADE and its Lagrangian solver, therefore, were recommended to backtrack pollutants undergoing anomalous transport in aquifers with various degrees of heterogeneity and any dimension and boundary conditions. Key Points: Backward particle tracking (BPT) models and solvers backtrack sub/super‐diffusion in aquifers with any sizes, dimension, and boundary conditionsBackward particle tracking model parameter estimability depends on the type of parameters and aquifers (fractured vs. porous media)Backward fractional‐divergence advection‐dispersion equation is the best fractional‐derivative model for BPT of anomalous transport [ABSTRACT FROM AUTHOR]

Groundwater monitoring networks are direct sources of information for revealing subsurface system dynamic processes. However, designing such networks is difficult due to uncertainties in the spatial heterogeneity of aquifer parameters such as permeability (k). This study combines deep learning and information theory with an optimization framework to address network design problems in heterogeneous aquifer systems. The framework first employs a generative adversarial network to parameterize heterogeneous k distribution using a low‐dimensional latent representation. Then, surrogate models are developed based on the deep neural networks to perform uncertainty quantification of pressure heads and solute concentrations at locations of pre‐designed candidate monitoring stations. The monitoring stations are then ranked using the greedy search algorithm based on the maximum information minimum redundancy (MIMR) criterion. In order to depict the importance of each candidate monitoring location, the hotspot maps of the selection probability (Ps) are derived from MIMR repetition results. Comprehensive monitoring networks derived from the hotspot maps are then conducted as the final monitoring stations to improve monitoring information compared to MIMR results. Additionally, nine entropy quantization strategies are compared to evaluate their effects on monitoring network optimization results. Results indicate that caution should be taken when selecting entropy quantization strategies to achieve the accuracy required for model calibration and to improve the efficiency of monitoring optimization. Considering high‐dimensional uncertainties associated with aquifer parameters, the developed framework can provide important insights for monitoring network designs in various earth observational projects. Plain Language Summary: Groundwater is widely distributed throughout the crust and lithosphere playing a critical role in geological evolution processes and environmental problems. This study presents an integrated framework for designing groundwater monitoring networks in heterogeneous aquifer systems. The proposed model takes advantage of the strength of deep‐learning algorithms and deals with high‐dimensional and highly non‐linear problems of entropy‐based methods for designing the monitoring networks. The framework can efficiently provide monitor locations for complex aquifers. The monitoring data collected by the designed network can significantly reduce uncertainties associated with high‐dimensional parameters and contain the least amount of redundant information. Further, the proposed method is significant in practice as credible groundwater forecasting requires model calibrations that use monitoring data. Key Points: Effective designed monitoring networks are critical for reducing model uncertainties and properly reflecting subsurface dynamic processesDeep learning and maximum information minimum redundancy criterion were combined to design optimal monitoring networks for heterogeneous aquifersThe designed monitoring networks were verified to be influenced by different entropy quantification strategies [ABSTRACT FROM AUTHOR]

Coastal deltaic aquifers are vulnerable to degradation from seawater intrusion, geogenic and anthropogenic contamination, and groundwater abstraction. The distribution and transport of contaminants are highly dependent on the subsurface sedimentary architecture, such as the presence of channelized features that preferentially conduct flow. Surface deposition changes in response to sea‐level rise (SLR) and sediment supply, but it remains unclear how these surface changes affect the distribution and transport of groundwater solutes in aquifers. Here, we explore the influence of SLR and sediment supply on aquifer heterogeneity and resulting effects on contaminant transport. We use realizations of subsurface heterogeneity generated by a process‐based numerical model, DeltaRCM, which simulates the evolution of a deltaic aquifer with different input sand fractions and rates of SLR. We simulate groundwater flow and solute transport through these deposits in three contamination scenarios: (a) vertical transport from widespread contamination at the land surface, (b) vertical transport from river water infiltration, and (c) lateral seawater intrusion. The simulations show that the vulnerability of deltaic aquifers to seawater intrusion correlates to sand fraction, while vertical transport of contaminants, such as widespread shallow contamination and river water infiltration, is influenced by channel stacking patterns. This analysis provides new insights into the connection between the depositional system properties and vulnerability to different modes of groundwater contamination. It also illustrates how vulnerability may vary locally within a delta due to depositional differences. Results suggest that groundwater management strategies may be improved by considering surface features, location within the delta, and the external forcings during aquifer deposition. Plain Language Summary: The findings of this study provide insight into the vulnerability of deltaic aquifers to three contamination processes: (a) widespread contaminant transport from the land surface, (b) river water infiltration, and (c) seawater intrusion. We consider how contamination is affected by the location of contaminants and the processes associated with the accumulation of sediments in deltas. Our work shows that vulnerability to contamination depends on how the aquifer is deposited. The results also demonstrate that the distribution of sandy channels preserved in the subsurface, as well as rivers on the surface, controls vertical contaminant transport. We find that these effects vary from upstream to downstream in the delta because of spatial differences in depositional processes. These findings will help to improve predictions of groundwater contamination and manage groundwater development in deltas around the world. Key Points: In deltaic aquifers, contaminant distribution is controlled by the sand fraction and channel stacking patternsRivers on the surface have a strong influence on vertical contaminant transport in aquifersDue to differences in channel distribution, vertical contamination varies nonlinearly from upstream to downstream regions [ABSTRACT FROM AUTHOR]

Previous studies found that Finite Particle Method (FPM), an improved Smoothed Particle Hydrodynamics (SPH) method, yields more accurate solutions of the advection‐dispersion equation (ADE) than SPH method does when simulating solute transport in heterogeneous porous media. FPM however is computationally more expensive than SPH because FPM needs to solve a correction matrix equation for each particle. While decoupled FPM (DFPM) can reduce computational cost of FPM by using diagonal terms of the matrix equation, DFPM is less accurate than FPM due to discarding off‐diagonal terms of the matrix equation. This study develops the Decoupled Finite Particle Method with Normalized Kernel (DFPM‐NK) to improve computational accuracy of DFPM by using normalized kernels in DFPM. We evaluate computational performance of SPH, FPM, DFPM, and DFPM‐NK using two numerical experiments of ADE with non‐reactive and reactive solute transport in a heterogeneous aquifer. Results of the two experiments indicate that, among the four methods, SPH has the least computational time, but has the worst computational accuracy. DFPM‐NK is substantially more efficient than FPM, and has similar accuracy of FPM. On the other hand, DFPM‐NK and DFPM have similar computational cost, but DFMP‐NK is significantly more accurate than DFPM, especially for heterogeneous hydraulic conductivity fields. We thus recommend to use DFPM‐NK for computationally expensive ADE problems without sacrificing computational accuracy. Key Points: A decoupled Finite Particle Method with normalized kernel (DFPM‐NK) is developed to simulate groundwater solute transportDFPM is computationally more efficient than Finite Particle Method (FPM) due to using diagonal elements of correction matrixDFPM‐NK is computationally more accurate than DFPM due to using normalized kernel [ABSTRACT FROM AUTHOR]

Steady flow generated by an injecting and a pumping well (doublet) takes place in a porous formation where the spatially variable hydraulic conductivity K is modeled as a stationary, lognormal, random field with anisotropic two‐point autocorrelation. The latter is characterized by a vertical integral scale, that is, Iv, smaller than the horizontal one, that is, I. A solute, either passive or reactive, is injected in the medium, and we aim at computing the breakthrough curve (BTC) and its moments not only at the recovery (pumping) well, but also at any location between the two wells. The strong coupling between K and the nonuniformity of the flow renders the problem very difficult. Nevertheless, a simple (analytical) solution is obtained by adopting a few assumptions: (a) wells are replaced by lines of singularity, (b) a perturbation solution which regards the variance σY2 ${\sigma }_{Y}^{2}$ of the log‐conductivity Y = ln K as a perturbation parameter is employed, (c) the study is limited to strongly anisotropic heterogeneous formations (for which the anisotropy ratio λ = Iv/I is much smaller than one), and (d) the impact of pore‐scale dispersion is neglected. Central for the computation of the BTC is the statistics of the travel time of a fluid particle released at the injecting well and reaching a control plane located at any position x1 along the distance connecting the two wells. It is shown that the spatial variability of Y acts de facto like a dispersion mechanism: it enhances spreading, especially in the early arrivals. Useful closed form expressions for moments of the travel time along the central trajectory are also obtained. Finally, the theoretical framework presented in this study is applied to two transport experiments in order to compute the second‐order (temporal) moment as function of x1, and therefore to quantify dispersion occurring in the zone delimited by the two wells. Plain Language Summary: Transport takes place between an injecting well and a pumping one through to a porous formation. The controlling parameter is the conductivity which, unlike the classical approach, here is regarded, in line with field findings, as spatially variable. This renders the problem at stake extremely difficult to solve. However, a simple solution is achieved by adopting a few simplifying assumptions, which nevertheless resemble most of the existing aquifers, and therefore it is applicable to numerous real‐world situations. It is shown that the proposed solution finds application in the identification of the aquifer's parameters as well as the quantification of efficiency of decontamination procedures. Finally, the theoretical framework is applied to a couple of transport experiments, in order to illustrate (and to quantify) how dispersion process develops in the zone delimited by the two wells. Key Points: Transport in a doublet flow is investigated by means of the breakthrough curve (BTC) and its momentsA simple solution is achieved by dealing with a strongly anisotropic heterogeneous formationResults are applied to a couple of field‐scale transport experiments [ABSTRACT FROM AUTHOR]

Flow‐direction‐dependent (FDD) dispersivity in coastal aquifers (CAs) may strongly affect the inland extend of seawater intrusion (SWI) and the accompanying vertical salinity distribution. FDD dispersivity may predict greater inland intrusion of the saltwater wedge, but less vertical spreading of salinity than does the classical flow‐direction‐independent (FDI) dispersivity, the standard currently employed in most numerical CA models. Dispersion processes play a key role in the SWI process and directly affect CA pumped water quality. Constant FDI dispersivities may be inappropriate in representing mixing processes due to large differences between depth and horizontal salinity transport scales, and due to typical structured heterogeneities in aquifer fabrics. Comparison of FDI and FDD model forecasts for the classical Henry problem (HP) steady‐state SWI, based on a new HP semianalytical solution with FDD and on a numerical FDI model modified to additionally represent FDD, highlights the theoretical types of differences implied by these alternative dispersivity assumptions and exactly how each parameter affects the solution. Large differences between FDI and FDD dispersivity forecasts of time‐dependent SWI in large scale heterogeneous aquifers occur in a typical CA (Akkar CA, Lebanon). The FDD model forecasts that future salinities in pumping wells will exceed the potable water limit, whereas the FDI model greatly underestimates the historic inland intrusion of the saltwater wedge and forecasts no impact on future Akkar CA potable water supply. These results indicate the importance of employing the appropriate dispersion process representation when creating model‐based SWI forecasts, especially for developing effective CA management strategies. Key Points: Most existing modeling‐based studies of seawater intrusion (SWI) assume constant dispersivitiesWe investigate effect of flow‐direction‐dependent dispersivities on SWI based on the Henry problem and a typical coastal aquifer in LebanonThese results indicate the importance of employing an appropriate dispersion process representation when creating model‐based SWI forecasts [ABSTRACT FROM AUTHOR]

Groundwater dynamics in continental rift zone settings remain poorly understood because of the spatial heterogeneity in flow, storage, and recharge dynamics. The Central Kenya Rift is an excellent example where, though groundwater is important for domestic, irrigation, and geothermal energy exploitation, its hydrogeological properties remain largely unknown. Existing conceptual groundwater models assume flow from the high‐elevation, humid rift flanks to the low‐elevation, semiarid rift floor, but the role of the faults that fracture the aquifers is commonly unaccounted for. We applied geochemical, isotopic (δ18O, δD, 87Sr/86Sr, 3H‐3He), multivariate statistical methods, and knowledge of geological structures to determine recharge sources, flow, and residence times to revise the conceptual flow model. Results show that groundwater is primarily recharged by meteoric waters, river input, and Lake recharge. The faults impart a control on the groundwater flow within four sub‐compartments. Major differences in flow patterns exist between the eastern and western rift flanks: surface and groundwater transfer from the eastern flanks to the rift floor occurs via relay ramp structures, and flow on the west side takes place laterally from the high escarpment. Although 3H‐3He dating shows that the age of groundwater ranges from a few to >>50 years, most of the groundwater in the rift‐floor area is free of 3H and was recharged before the 1960s. Hence, we propose that these areas receive episodic recharge and represent the most sensitive groundwater resources in the rift. To inform sustainable groundwater development, a robust monitoring network is required to capture the heterogeneous groundwater dynamics. Key Points: Fault ramps are important groundwater transfer zones within the rift blocksFault displacements exert control on recharge and groundwater availability within the rift floorMajority of the water samples from the rift floor lack tritium, meaning that active recharge is minimal [ABSTRACT FROM AUTHOR]

Many irrigated agricultural areas seek to prolong the lifetime of their groundwater resources by reducing pumping. However, it is unclear how lagged responses, such as reduced groundwater recharge caused by more efficient irrigation, may impact the long‐term effectiveness of conservation initiatives. Here, we use a variably saturated, simplified surrogate groundwater model to: (a) analyze aquifer responses to pumping reductions, (b) quantify time lags between reductions and groundwater level responses, and (c) identify the physical controls on lagged responses. We explore a range of plausible model parameters for an area of the High Plains aquifer (USA) where stakeholder‐driven conservation has slowed groundwater depletion. We identify two types of lagged responses that reduce the long‐term effectiveness of groundwater conservation, recharge‐dominated and lateral‐flow‐dominated, with vertical hydraulic conductivity (KZ) the major controlling variable. When high KZ allows percolation to reach the aquifer, more efficient irrigation reduces groundwater recharge. By contrast, when low KZ impedes vertical flow, short term changes in recharge are negligible, but pumping reductions alter the lateral flow between the groundwater conservation area and the surrounding regions (lateral‐flow‐dominated response). For the modeled area, we found that a pumping reduction of 30% resulted in median usable lifetime extensions of 20 or 25 years, depending on the dominant lagged response mechanism (recharge‐ vs. lateral‐flow‐dominated). These estimates are far shorter than estimates that do not account for lagged responses. Results indicate that conservation‐based pumping reductions can extend aquifer lifetimes, but lagged responses can create a sizable difference between the initially perceived and actual long‐term effectiveness. Key Points: The long‐term effectiveness of pumping reduction‐based groundwater conservation is dependent on lagged processesVertical hydraulic conductivity (KZ) controls if lagged responses are lateral‐flow dominated or recharge‐dominatedIgnoring lagged processes overestimates aquifer lifetime by 32 and 133 years in lateral‐flow and recharge‐dominated settings, respectively [ABSTRACT FROM AUTHOR]

Large‐scale groundwater models are required to estimate groundwater availability and to inform water management strategies on the national scale. However, parameterization of large‐scale groundwater models covering areas of major river basins and more is challenging due to the lack of observational data and the mismatch between the scales of modeling and measurements. In this work, we propose to bridge the scale gap and derive regional hydraulic parameters by spectral analysis of groundwater level fluctuations. We hypothesize that specific locations in aquifers can reveal regional parameters of the hydraulic system. We first generate ensembles of synthetic but realistic aquifers which systematically differ in complexity. Applying Liang and Zhang's (2013), https://doi.org/10.1016/j.jhydrol.2012.11.044, semi‐analytical solution for the spectrum of hydraulic head time series, we identify for each ensemble member and at different locations representative aquifer parameters. Next, we extend our study to investigate the use of spectral analysis in more complex numerical models and in real settings. Our analyses indicate that the variance of inferred effective transmissivity and storativity values for stochastic aquifer ensembles is small for observation points which are far away from the Dirichlet boundary. Moreover, the head time series has to cover a period which is roughly 10 times as long as the characteristic time of the aquifer. In deterministic aquifer models we infer equivalent, regionally valid parameters. A sensitivity analysis further reveals that as long as the aquifer length and the position of the groundwater measurement location is roughly known, the parameters can be robustly estimated. Plain Language Summary: We build large‐scale (regional) computer models of the subsurface flow conditions in order to quantify the long‐term shift in groundwater storage and response on the national level under changing climatic conditions and increasing human water demands. These models must be fed with hydrogeological parameters obtained from subsurface observation wells, drilling logs, and hydraulic tests in conjunction with (hydro)geological and geostatistical methods. In some regions these wells are sparsely distributed and derived parameters are representative only for small areas. We hypothesize that groundwater level records can reveal regional aquifer information when analyzed in the spectral domain. In order to bridge that scale gap and because groundwater level time series are generally available, we propose to infer regional parameters by analyzing the frequency content (spectrum) of long groundwater level time series. The required parameters were determined using mathematical formulations of the theoretical spectrum for simplified settings. We tested the methodology in computer models with limited complexity and found that the groundwater level time series indeed contain regional information if the time of observation is sufficiently long. Lastly, we apply the spectral analysis to real groundwater data to test the capability of the method to infer regional aquifer parameters in real aquifers. Key Points: We successfully tested the spectral analysis of groundwater level fluctuations in numerical models and obtained regional aquifer parametersIn a sensitivity analysis of the spectral analysis using field data, the storativity and the response times could be robustly estimatedThe application of the suggested methodology to the field data from a catchment in central Germany produced plausible results [ABSTRACT FROM AUTHOR]

Imaging subsurface small‐scale features and monitoring transport of tracer plumes at a fine resolution is of interest to characterize transport processes in aquifers. Full‐waveform inversion (FWI) of crosshole ground penetrating radar (GPR) measurements enables aquifer characterization at decimeter‐scale resolution. The method produces images of both relative dielectric permittivity (εr) and bulk electrical conductivity (σb) that can be related to hydraulic aquifer properties and tracer distributions. To test the potential of time‐lapse GPR FWI for imaging tracer plumes, we conducted a numerical tracer experiment by injecting saline water, desalinated water, and ethanol in a heterogeneous aquifer. The saline and desalinated tracers only changed σb, whereas ethanol changed both εr and σb. Tracer concentrations were retrieved from the inverted εr and σb models using information about petrophysical parameters. GPR FWI εr and σb tracer images could recover preferential paths of ∼0.2 m width, while the derived σb structures are smoother. FWI of 50 time‐lapse data sets demonstrated the potential of the FWI to derive spatially resolved breakthrough curves of the saline and ethanol tracer in the image plane between the boreholes. Thus, high‐resolution imaging with GPR FWI of tracers that produce a high permittivity contrast against the background has a great potential for characterization of heterogeneous transport in aquifers. Key Points: Time‐lapse crosshole ground penetrating radar (GPR) full‐waveform inversion (FWI) shows a potential to monitor high‐resolution of ∼0.2 m transport of tracer in aquiferStarting model strategies for FWI were tested in order to optimize plume reconstruction, to be used later for field data setsGood recovery of conductivity and especially of permittivity changes suggests to monitor adequate tracers by GPR FWI for transport imaging [ABSTRACT FROM AUTHOR]

Abnormal fluid pressures (above or below hydrostatic pressure) can develop and persist in sedimentary basins. The common occurrence of abnormal pressures may cause challenges for project permitting of geological carbon sequestration (GCS), particularly in reservoirs with pre‐injection overpressure. The leaky wells that may exist in some sedimentary basins can provide flow paths between deep brine aquifers and shallower freshwater aquifers. Pre‐injection relative overpressures can cause brine leakage through leaky wells even before any injection occurs. The tendency for flow through leaky wells is coupled with the process of pressure dissipation that occurs through acquitards. Specifically, with non‐zero permeability, acquitards can dissipate pressure over large areal extents and thereby reduce leakage rates through leaky wells. This study presents development of a semi‐analytical solution for hydraulic head and brine leakage in multilayered aquifer–acquitard systems with geologic pressure forcing. The geologic forcing that causes abnormal pressures in the multilayered system can coexist with any number of injection, extraction, and leaky wells that also affect fluid pressure. The semi‐analytical model is applied to explore how leakage through leaky wells varies as functions of pressurization rate, along with acquitard and leaky well properties in an overpressured multilayered system. The results show that although injection‐induced pressures can dissipate rapidly through suitably permeable acquitards, coexisting geologic forcing may create sustained rates of brine leakage into freshwater aquifers through leaky wells. In GCS, a very low‐permeability acquitard with high capillary entry pressure to free‐phase CO2 is desired to serve as the caprock to prevent leakage of CO2 from the storage reservoir. Nevertheless, the results from this study show that the brine leakage impact to shallow freshwater aquifers through leaky wells might substantially decrease with increasing acquitard permeability values, as long as small acquitard permeability and high capillary entry pressure serve to prevent CO2 leakage. Key Points: This study extends semi‐analytical solutions for leakage in abnormally pressured multilayered aquifer systems undergoing fluid injectionNew solution presented predicts brine leakage from leaky wells caused by coexisting geologic forcing and injectionAlthough the injection‐induced pressures may dissipate through permeable aquitards, coexisting geologic forcing may add to the manmade forcing to create sustained brine leakage rates into aquifers through leaky wells [ABSTRACT FROM AUTHOR]

Steady well‐type flow was monitored in an aquifer that was artificially packed in order to reproduce a given, highly heterogeneous, statistical distribution of the log‐conductivity Y. In particular, we focus on pumping tests carried out at 10 volumetric flow rates. The experimental arrangement was composed by a pumping well and several surrounding observation piezometers. The unique feature of this experimental study is that the high heterogeneity structure of Y is known fairly well. Thus, the study lends itself as a valuable tool to corroborate theoretical findings about flows driven by sources through porous formations, where the variance σY2 ${\sigma }_{Y}^{2}$ (in the present study equal to 3.79) of Y is large. Besides discussing experimental findings, we tackle the crucial issue of upscaling the hydraulic conductivity in a well‐flow configuration. In particular, we deal with the equivalent conductivity (EC) as that pertaining to a homogeneous (fictitious) medium which conveys the same volumetric flow rate of the real one, under the same boundary conditions. Hence, the EC can be identified straightforwardly by means of head measurements. Even if we show that the EC is a proper parameter to reproduce measurements, it is experimentally shown (in line with the theoretical results) to be position‐dependent, and therefore, it cannot be regarded (unlike groundwater‐type flow) as a formation's property. This implies that the EC applies only to the configuration at stake. Then, we show that the EC, combined with a recent model of effective conductivity in well‐flows through highly heterogeneous porous formation, leads to a reasonably reliable estimate of σY2 ${\sigma }_{Y}^{2}$, some limitations and approximations, notwithstanding. It is hoped that the present experimental study will be useful for other researchers who are engaged with similar research‐topics. Key Points: Steady well‐flow through a strongly heterogeneous formation is investigated by means of an experimental studyThe formation was artificially packed to reproduce a given, statistical distribution of the hydraulic conductivityThe equivalent conductivity is proved to be a proper parameter to simulate the average flow [ABSTRACT FROM AUTHOR]

Microbial pollution of aquifers is a persistent water quality problem globally which poses significant risks to public health. Karst aquifer systems are exceptionally vulnerable to pollution from fecal contamination sources as a result of rapid recharge of water from the surface via discrete pathways linked to highly conductive, solutionally enlarged conduits alongside strong aquifer heterogeneity. Consequently, rapid changes in microbial water quality, which are difficult to monitor with expensive and time‐consuming conventional microbiological methods, are a major concern in karst environments. This study examined flow cytometric (FCM) fingerprinting of bacterial cells in groundwater together with fecal indicator bacteria (FIB) at nine separate karst springs of varying catchment size over a 14 month period in order to assess whether such a technique can provide faster and more descriptive information about microbial pollution through such karst aquifer systems. Moreover, the data have also been evaluated with respect to the potential of using turbidity as an easy‐to‐measure proxy indicator of microbial pollution in a novel way. We argue that FCM provides additional data from which enhanced insights into fecal pollution sources and its fate and transport in such karst catchments can be gained. We also present valuable new information on the potential and limitations of turbidity as an indicator of fecal groundwater contamination in karst. FCM has the potential to become a more widely used tool in the field of contaminant hydrogeology. Key Points: Flow cytometry can provide enhanced insights into the fate and transport of fecal pollution in karst aquifersStable bacterial communities in karst aquifers can limit the potential of flow cytometry to predict changes in fecal indicator bacteriaEnterococci should be analyzed with Escherichiacoli if turbidity is to be evaluated as an inexpensive groundwater quality indicator in karst springs [ABSTRACT FROM AUTHOR]

Aquifer remediation with in‐situ soil washing techniques and enhanced oil removal typically involves the injection of liquid solutions into the geological formation to displace and mobilize non‐aqueous phase liquids (NAPLs). The efficiency of these systems is oftentimes low because the displacing fluid bypasses large quantities of NAPL due to the inherent complexity of a heterogeneous natural system. Here, engineered injection and extraction (EIE) generated by rotating periodic injection is proposed as a method to enhance NAPL removal and mixing. To evaluate the method, we perform two‐phase flow simulations in multiple realizations of random permeability fields with different correlation structures and connectivity between injection and extraction wells embedded in a five‐spot pattern. Results show that EIE can significantly improve removal efficiency and mixing depending on several controlling factors. The effects of EIE are more significant under unfavorable conditions, that is, when injection and extraction wells are well‐connected through preferential channels, permeabilities are highly heterogeneous, and/or the mobility ratio between the wetting and the non‐wetting fluids is larger than one. Removal efficiency reaches its maximum value when the Kubo number is close to one, that is, when the saturation front travels one range of the permeability field in an injection pulse. These effects can develop in just a few cycles. However, removal efficiency should undergo first an early stage with detrimental effects in order to maximize removal in the long term. EIE not only enhances NAPL removal and mixing but also reduces the uncertainty, making the system more reliable and less dependent on heterogeneity. Key Points: Engineered injection and extraction enhances non‐aqueous phase liquids removal and mixing in heterogeneous porous media while reducing uncertaintyRemoval efficiency increases to a maximum value when the injection pulse satisfies that the Kubo number is close to oneThe relative benefit of chaotic advection is substantially larger in conditions of unfavorable aquifer heterogeneity and connectivity [ABSTRACT FROM AUTHOR]

Assessing water travel times in hyporheic and adjacent alluvial sediments is important to quantify exchange rates, biogeochemical turnover, and pollution dynamics across groundwater‐surface water interfaces and in floodplain aquifers. Heat and 222Rn are useful natural tracers for this purpose. While heat transport is commonly simulated via the convection‐conduction equation, the transport of 222Rn is often simulated assuming purely advective transport and a homogenous distribution of 222Rn production rates across the model domain. In the present study, we explicitly model the production, transport, and radioactive decay of 222Rn after surface water infiltration into an alluvial aquifer with the numerical models MODFLOW‐NWT and MT3D‐USGS. Using field observations, numerical modeling, and laboratory experiments, we show that 222Rn production rates vary substantially in the alluvial aquifer of a lowland river. Distributions of mean river‐to‐groundwater travel time in the alluvial aquifer, estimated via a Bayesian approach, shifted considerably depending on whether 222Rn and time series of groundwater heads and temperature were jointly inverted and whether 222Rn production rates were allowed to vary spatially. With distance to the river, differences between the median river‐to‐groundwater travel time and apparent 222Rn ages increased by factors ranging from 1.4 at 4 m to 11.9 at 59 m, a finding that highlights the need to simulate 222Rn transport explicitly. The joint inversion of 222Rn, groundwater heads, and temperature reduced uncertainty associated with mean travel times, suggesting that the uncertainty introduced by spatially varying 222Rn production rates can at least partly be compensated by using a combination of natural tracers. Plain Language Summary: Rivers and groundwater bodies exchange water back and forth. Knowing the time that water spends traveling from rivers to certain points in groundwater is important for water management, particularly in situations where drinking water is produced via bank filtration: It allows to predict what happens to natural and toxic compounds that come from the river and to assess the quantity of water moving from rivers to aquifers. Here, we present a modeling approach to estimate travel times of groundwater originating from a nearby river. The approach uses the open‐source software packages MODFLOW‐NWT and MT3D‐USGS and a combination of natural tracers, such as the noble gas 222Rn, which is produced in the subsurface with varying rates but decays with a constant rate constant. We demonstrate that by using both 222Rn and other natural tracers, such as heat, we estimate travel times from rivers to points in groundwater with less uncertainty. Key Points: A flexible approach to explicitly model the transport of 222Rn in saturated porous media using MODFLOW‐NWT and MT3D‐USGS is presentedSpatially varying 222Rn production rates, found to vary by a factor of two at relatively small spatial scales (<100 m), are consideredA joint inversion of hydraulic heads, temperature, and 222Rn reduced the high degree of uncertainty in the estimated groundwater travel times [ABSTRACT FROM AUTHOR]

This study proposes a new analytical slug test model describing confined flow considering the effects of the formation damage and non‐Darcy flow near the wellbore. The former, primarily caused by drilling fluid invasion, introduces an extra wellbore head loss near the well screen. The latter, mainly resulting from the relatively high‐velocity flow across the gravel‐pack screen, adopts the Forchheimer equation to consider the joint effects of the laminar flow and non‐Darcy flow. The proposed model is nonlinear due to the quadratic term for non‐Darcy flow in the Forchheimer equation. The new solutions for head responses in the well and formation are derived using the Laplace transform method and integration by parts formula. Furthermore, the present solution is applied to estimate the aquifer parameters via analyzing the water level data from field slug tests conducted in the Minnelusa aquifer near Spearfish, South Dakota. We found that the present solution provides very good fit to the field data from tests in fractured aquifers, notably the data exhibits a rapid decline at late times on the normalized head vs. logarithmic time plots. Key Points: We present a slug test model by considering near‐well formation damage and nonlinear flowThe solution of the model is developed by Laplace transform method and integration by parts formulaPresent solution is used to analyze field water level data from three test wells in fractured aquifer for determining aquifer parameters [ABSTRACT FROM AUTHOR]

In 1986, a seminal data set from the Canadian Forces Base (C.F.B.) Borden aquifer, Ontario, Canada, was published, illustrating, in unprecedented detail, the spatial distribution of hydraulic conductivity (K). Among many contributions attached to that data set was a geostatistical examination of field‐based data for comparison with theoretical predictions of macro‐dispersivity. However, that work treated K as a static parameter and the sole source of flow variability. Here, point velocity probes (PVPs) are used to extend the earlier work by collecting a novel high‐resolution data set of groundwater velocity (v) measurements in the C.F.B. Borden aquifer. Velocity is a dynamic parameter of fundamental importance, closely tied to solute dispersion. Over 400 velocity measurements were collected along a transect perpendicular to flow, analyzed geostatistically, and compared with the analysis of the B‐B′ cross‐section of K reported by Sudicky (1986), https://doi.org/10.1029/wr022i013p02069. The PVP measurements exhibited geostatistics similar to those previously estimated by Sudicky (1986), https://doi.org/10.1029/wr022i013p02069. This finding suggests, as implicitly assumed in the previous work, the distribution of v is primarily controlled by K. PVPs also provided a novel, high‐resolution data set of groundwater flow directions. Since the ultimate objective of aquifer characterization includes the definition of velocity fields, this work not only extends the 1986 work, but also demonstrates a viable alternative for characterizing flow patterns in aquifers—with the advantage that direct v measurements reflect variability in porosity and hydraulic gradient, as well as K. This could ultimately be advantageous at sites with greater heterogeneity or other dynamic hydrogeological variabilities more pronounced than those at the Borden site. Plain Language Summary: Transport of contaminants in the ground is dominated by groundwater flow. This is typically investigated using conventional methods based on water‐level measurements and other indirect techniques. This article outlines an alternative method for investigating groundwater flow by direct measurements of groundwater velocity. A chief advantage of the approach is a reduction of measurement uncertainty. Geostatistics describing aquifer structure based on the two methods were shown to be similar. This illustrates the advantages of making direct velocity measurements are not offset by a loss of data quality or geostatistical value. This new method may have application in the characterization and monitoring of contaminants transporting by groundwater or undergoing clean‐up efforts. Key Points: Geostatistics of direct groundwater velocity measurements are consistent with those from hydraulic conductivity at Canadian Forces Base BordenCorrelation lengths of groundwater velocity magnitude and direction shown to be similarHigh‐resolution velocity data set shows plume spreading (i.e., dispersion), without Fickian dispersion, based solely on advective processes [ABSTRACT FROM AUTHOR]

Dissolution trapping plays a significant role in CO2 geological storage. When CO2 dissolves in aquifer brines, the aqueous solution is heavier than the underlying resident brine. This results in convective instability in the form of CO2‐rich fingers, which accelerates dissolution. It was recently shown that background flow affects CO2 dissolution by suppressing the development of convective fingers. However, there was a discrepancy between computational results and measurements, with the computational results demonstrating a decrease in the dissolution rate and the laboratory measurements showing no significant effect. Here, we attempted to understand this difference by exploring the relations between the transport mechanisms that facilitate convective mixing. We investigated the role of background flow by using numerical simulations that were validated by laboratory measurements and analytical solutions. Simulations were generated for the same porous medium and fluid pair used in the experiments, keeping the Rayleigh number constant, while changing the background velocity and the Peclet number. Studying the flux components revealed three distinct regimes characterized by the Peclet to Rayleigh ratio Pe/Ra. When Pe/Ra < 0.77, natural convection dominated, and dissolution remained approximately constant. When the background flow was large (Pe/Ra > 2), dissolution was controlled by pure forced convection and increased with Pe/Ra. In the intermediate range (0.77 < Pe/Ra < 2), both natural convection and forced convection were important. Key Points: We investigate the effect of background flow and hydrodynamic dispersion on convective dissolution using numerical simulationsResults confirm that the dissolution flux is invariant within a certain range of velocities in accordance with previous experimental resultsWe demonstrate a tradeoff between natural convection and the sum of forced convection and the associated dispersive flux [ABSTRACT FROM AUTHOR]

The recovery efficiency of aquifer storage systems with radial flow fields are studied for heterogeneous aquifers. Macrodispersion, arising from spatially heterogeneous hydraulic conductivity, is modeled as a scale‐dependent mechanical dispersion process. Approximate solutions for the recovery efficiency as a function of local dispersion and macrodispersion parameters, the injection‐extraction rate Q $Q$ and duration T $T$, and storage cycle count, are derived and validated against numerical simulations. If macrodispersion dominates and the macrodispersion coefficient scales linearly with distance, the recovery efficiency is independent of both Q,T $Q,T$. For sublinear and superlinear scalings, recovery increases and decreases respectively if Q,T $Q,T$ increases. However, if local dispersion dominates, increasing Q,T $Q,T$ always increases recovery. As macrodispersion becomes increasingly dominant with scale, the recovery efficiency may be a nonmonotonic function of Q,T $Q,T$, with a maximum. In homogeneous aquifers, nonmonotonicity does not occur for 1D and 2D radial flow, but occurs for 3D radial flow fields only as a function of T $T$, not Q $Q$. These methods may also be used for fitting local dispersion and macrodispersion parameters with push‐pull tests using recovery data, with advantages in scope of applicability and ease of data acquisition and interpretation, compared to existing push‐pull test methods, which fit to breakthrough curves and do not consider macrodispersion. Furthermore, characterizing macrodispersion with push‐pull tests may be advantageous over methods that use observation wells, as observation well placement may be challenging in highly heterogeneous aquifers. The results show that the macrodispersion parameters are not innate aquifer hydraulic properties, as their values vary with flow field geometry. Key Points: New methods for evaluating heat (ATES) and freshwater (ASR) storage systems, and interpreting push‐pull tests, in heterogeneous aquifersApproximate solutions for recovery efficiency, where macrodispersion is modeled as a scale‐dependent dispersion processDepending on the scale‐dependence function, recovery may vary non‐monotonically with stored volume under planar and cylindrical flow [ABSTRACT FROM AUTHOR]

Efficient compositional models are required to simulate underground gas storage in porous formations where, for example, gas quality (such as purity) and loss of gas due to dissolution are of interest. We first extend the concept of vertical equilibrium (VE) to compositional flow, and derive a compositional VE model by vertical integration. Second, we present a hybrid model that couples the efficient compositional VE model to a compositional full‐dimensional model. Subdomains, where the compositional VE model is valid, are identified during simulation based on a VE criterion that compares the vertical profiles of relative permeability at equilibrium to the ones simulated by the full‐dimensional model. We demonstrate the applicability of the hybrid model by simulating hydrogen storage in a radially symmetric, heterogeneous porous aquifer. The hybrid model shows excellent adaptivity over space and time for different permeability values in the heterogeneous region, and compares well to the full‐dimensional model while being computationally efficient, resulting in a runtime of roughly one‐third of the full‐dimensional model. Based on the results, we assume that for larger simulation scales, the efficiency of this new model will increase even more. Key Points: A compositional vertical equilibrium model is coupled to its full‐dimensional counterpartA criterion is developed to adaptively identify and assign regions where the vertical equilibrium model is applicable during simulationA test case of hydrogen storage in a heterogeneous porous aquifer demonstrates efficiency and accuracy of the hybrid model [ABSTRACT FROM AUTHOR]

Numerical models of variable‐density groundwater flow and salt transport are a primary tool for predicting salinity distributions in coastal aquifers and estimating submarine groundwater discharge (SGD). Models are particularly useful to estimate the saline component of SGD, which can occur far offshore and is difficult to measure directly. Depending on the system and application, the level of geologic detail represented can range from homogeneous or layered to fully heterogeneous hydraulic conductivity fields. These features strongly affect model results, limiting understanding of subsurface salinity distributions and associated density‐driven saltwater circulation along coasts worldwide. In this study, the impact of the scale of representation of heterogeneity on salinity distributions and SGD was investigated using numerical simulations. Upscaling hydraulic conductivity can significantly modify salinity distributions and flow paths, resulting in unpredictable variations in simulated SGD, though the values for homogeneous fields with equivalent hydraulic conductivity show consistent trends. Simulated density distributions control both the rate and direction of subsurface saltwater circulation. The length of the mixing zone perimeter, a measure of salinity distribution complexity, is shown to correlate with both the rate of subsurface saltwater circulation and the amount of groundwater circulating in the reverse direction from homogeneous cases. Overall, the results demonstrate a strong dependence of salinity distributions and saltwater circulation on the scale and distribution of geologic heterogeneity represented in numerical models. This suggests that numerical models with simplified geologic structure may substantially underestimate saltwater circulation, and attempts to calibrate them using salinity distributions or SGD measurements may be problematic. Plain Language Summary: Groundwater flow and solute transport models that simulate how groundwater moves across the land‐ocean interface and seabed are important to science and engineering because these processes affect our coastal water resources as well as the chemical composition of the ocean. These models have been applied over different domain sizes, simulation grid resolutions, and geologic representations. We show that simulation results are strongly scale‐dependent, which makes it difficult to develop general conclusions that are independent of model setup. We simulated saltwater distributions and groundwater flow across a range of scales and detail of geologic representation. This work improves the understanding of scale‐dependent heterogeneity in simulations of saltwater circulation and cautions those modeling these processes to carefully consider how results relate to the model itself. Key Points: Upscaling hydraulic conductivity alters circulation direction and affects geometry of mixing zone where saltwater circulation occursFresh submarine groundwater discharge (SGD) tends to increase and saline SGD tends to decrease with upscaling; mixing zone area and perimeter do not vary systematicallyModels with simplified geologic structure may underestimate density‐driven saltwater circulation [ABSTRACT FROM AUTHOR]

Specific yield (Sy) is one of the most important aquifer parameters in groundwater models with large storage changes. However, it has received limited attention from the modeling community and there are few, if any, approaches that can be widely applied for determination of Sy at the scale required for regional aquifer studies. Despite the limitations of parameter estimation methods and of direct measurements of aquifer storage properties in the field, the large uncertainty in model Sy values is typically ignored and Sy is incorrectly assumed to be a known quantity. These practices can introduce errors into model predictions of aquifer budget components, such as recharge and storage depletion. In turn, aquifer management and planning can be greatly impacted, as estimates of aquifer responses to pumping changes are highly dependent on those simulated budget components. In this work, we use a groundwater model of a portion of the High Plains aquifer in the central United States to illustrate the impacts of inappropriately large Sy values on model predictions and the related water management ramifications. Building on our recent work, we propose a data‐driven approach for determining specific yield for regional groundwater models. Results demonstrate that this new approach can improve model reliability and lead to more robust predictions of aquifer responses to management scenarios. Key Points: Specific yield values are uncertain in many models of unconsolidated aquifersModels with inappropriate specific yield values can produce unreliable aquifer budgets and responses under future management scenariosA new approach is proposed for estimating the spatial distribution of specific yield for regional groundwater models [ABSTRACT FROM AUTHOR]

Studies of coastal groundwater dynamics often assume two‐dimensional (2D) flow and transport along a shore‐perpendicular cross‐section. We show that along‐shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three‐dimensional (3D) preferential flow paths. The along‐shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit‐type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three‐dimensionality in systems with connected high‐permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land‐sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS‐type systems, less‐connected high‐permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit‐type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean. Plain Language Summary: The findings of this study provide insight into the complex patterns of groundwater flow under the influence of geologic variability in coastal aquifers. In coastal regions, studies of solute transport processes mainly rely on an assumption of 2D groundwater flow and solute transport in the shore‐perpendicular direction. Our results reveal that groundwater does not only flow toward the sea, it also can flow along‐shore, especially in aquifers with features that connect the onshore and offshore. This affects exchange and mixing between fresh and saline groundwater, which can strongly impact delivery of contaminants and nutrients to sensitive nearshore marine ecosystems. Results highlight the importance of characterizing the geology of coastal aquifers and representing it in models of groundwater flow and contaminant transport. Key Points: Geologic heterogeneity causes substantial along‐shore movement of groundwater in coastal heterogeneous aquifersAlong‐shore groundwater flow in heterogeneous aquifers increases variability of travel length and time of groundwater prior to dischargeConnected, high‐permeability features increase the rate of SGD compared to equivalent homogeneous, SIS, and corresponding 2D models [ABSTRACT FROM AUTHOR]

In this study, we developed two‐dimensional hydrodynamic age expressions for the free‐surface aquifer problem required for radiocarbon data interpretation in terms of recharge. Time‐varying recharge is accounted for in fully transient or pseudo‐steady state alternative analytical expressions applying to weakly or highly hydraulically reactive unconfined aquifers, respectively. These expressions extend the well‐known model proposed by Vogel in 1967 and represent a convenient alternative approach to the more complex numerical solution of a particle tracking problem considering climate variability. An application of the developed expressions was carried out to assess past (40 Kyr) and present recharge rates for the North Western Sahara Aquifer System (NWSAS) and the Djeffara plain (Tunisia‐Libya). Using the available 14C data at different outcrops, we obtained local recharge values ranging from 0.2 ± 1.7 to 4.5 ± 3.2 mm yr−1. Here, it is shown that using the standard purely steady‐state model for radiocarbon interpretation leads to a present recharge overestimation of up to 360%. A linear relation between the present recharge and the average annual rainfall was obtained and used to identify the regional distribution of the recharge and domain‐averaged values. Global values of 1.6 ± 2.3 mm yr−1 for the NWSAS and 3.5 ± 2.3 mm yr−1 for the Djeffara are in excellent agreement with the values obtained by hydrogeological models and derived using Satellite gravity data. The relation between recharge and rainfall obtained here can be used as a first‐order estimate for hydrogeological modeling or water management analysis for aquifers in the Saharan area. Key Points: Analytical expressions for Radiocarbon distribution in transient state and unconfined aquifersDetermination of recharge rates of North Africa aquifers by using radiocarbon tracers accounting for past climate variabilityThe use of Vogel's standard solution for North Africa aquifers leads to an average overestimation of present‐day recharge of about 70% [ABSTRACT FROM AUTHOR]

Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency's regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M2 and the O1 tides and to the barometric pressure across a large (2.046 × 106 km2) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China. Plain Language Summary: A great amount of wastewaters coproduced from hydrocarbon exploration in the mid‐continental US has been injected into deep aquifers. Such practice has been based upon an implicit assumption that deep aquifers confined by aquitards are fully confined so that the injected toxic water will not leak back to contaminate shallow groundwater or the environment. This assumption, however, has never been carefully tested. Here we test this hypothesis using groundwater data from wells over a large geological regime, the North China Platform, as a function of depth and time. Our results show no clear dependence of aquifer confinement on depth, that significant leakage occurs even in the deepest studied aquifer (>3400 m) confined by thick clay/mud aquitards, and that an initially confined aquifer became may leaky after a distant earthquake. Because the practice of hydrofracking for natural gas and deep injection for the disposal of the coproduced wastewaters may soon be adopted by other countries, such as China, our study is timely and may be of global significance. Key Points: Systematic evaluation of the confinement of aquifers in the North China Platform at different depths down to 3.4 kmNovel revealing most of the studied aquifers leak, and has no dependence on aquifer depth or thickness of the overlain geologic barrierBoth the M2 and O1 tidal waves are used in the inversion of aquifer parameters without a prior assumption [ABSTRACT FROM AUTHOR]

Uncertainty quantification of groundwater (GW) aquifer parameters is critical for efficient management and sustainable extraction of GW resources. These uncertainties are introduced by the data, model, and prior information on the parameters. Here, we develop a Bayesian inversion framework that uses Interferometric Synthetic Aperture Radar (InSAR) surface deformation data to infer the laterally heterogeneous permeability of a transient linear poroelastic model of a confined GW aquifer. The Bayesian solution of this inverse problem takes the form of a posterior probability density of the permeability. Exploring this posterior using classical Markov chain Monte Carlo (MCMC) methods is computationally prohibitive due to the large dimension of the discretized permeability field and the expense of solving the poroelastic forward problem. However, in many partial differential equation (PDE)‐based Bayesian inversion problems, the data are only informative in a few directions in parameter space. For the poroelasticity problem, we prove this property theoretically for a one‐dimensional problem and demonstrate it numerically for a three‐dimensional aquifer model. We design a generalized preconditioned Crank‐Nicolson (gpCN) MCMC method that exploits this intrinsic low dimensionality by using a low‐rank‐based Laplace approximation of the posterior as a proposal, which we build scalably. The feasibility of our approach is demonstrated through a real GW aquifer test in Nevada. The inherently two‐dimensional nature of InSAR surface deformation data informs a sufficient number of modes of the permeability field to allow detection of major structures within the aquifer, significantly reducing the uncertainty in the pressure and the displacement quantities of interest. Key Points: Using Interferometric Synthetic Aperture Radar data reduces the uncertainty in selected quantities of interest compared to using prior knowledge onlyThe preconditioned Crank‐Nicolson (pCN) Markov chain Monte Carlo (MCMC) method is extended to exploit posterior curvature and allow better chain mixingWe demonstrate the intrinsic low dimensionality of the poroelastic inverse problem that is, critical for the success of the MCMC method [ABSTRACT FROM AUTHOR]

Airborne electromagnetic (AEM) data can be inverted to recover models of the electrical resistivity of the subsurface; these, in turn, can be transformed to obtain models of sediment type. AEM data were acquired in Butte and Glenn Counties, California, USA to improve the understanding of the aquifer system. Around 800 line‐kilometers of high‐quality data were acquired, imaging to a depth of ∼300 m. We developed a workflow designed to obtain, from the AEM data, information about the large‐scale structure and heterogeneity of the aquifer system to better understand the vertical connectivity. Using six different inversions incorporating various forms of available information and posterior sampling of the recovered resistivity models, we produced 6,006 resistivity models. These models were transformed to models of sediment type and estimates of percentage of sand/gravel. Exploring the model space, containing the resistivity models and the derived models, allowed us to delineate the large‐scale structure of the aquifer system in a way that captures and communicates the uncertainty in the identified sediment type. The uncertainty increased, as expected, with depth, but also served to indicate, as areas of high uncertainty in sediment type, the location of both large‐scale and small‐scale interfaces between sediment types. A plan view map of the integrated percentage of sand/gravel, when compared to existing hydrographs, revealed the extent of lateral changes in vertical connectivity within the aquifer system throughout the study area. Plain Language Summary: In studying and managing groundwater systems, it can be very difficult to get the information needed about the subsurface. The airborne electromagnetic (AEM) method uses a helicopter to move a geophysical system over the land surface to collect this needed information. In this study we acquired ∼800 line‐kilometers of high‐quality AEM data in an area of Butte and Glenn Counties in the Central Valley of California, USA. Acquisition of these data allowed us to obtain three‐dimensional (3D) resistivity models covering the region from the ground surface to a depth of about 300 m. Working with descriptions from wells, we were able to transform the 3D resistivity models into 3D sediment‐type models. These models allowed us to map out the large‐scale structure of the groundwater system and better understand the vertical connectivity within the system. Because of fundamental limitations in the AEM method, we obtained many different resistivity models and corresponding sediment‐type models. Exploring these models allowed us to quantify the uncertainty in our interpretation of the data. This not only assisted in our interpretation, but it also communicated, to the local water agency, our confidence in our interpretation. Key Points: By incorporating various forms of available information in the inversion of airborne electromagnetic data, we designed a workflow that reduces and quantifies uncertainty in the derived sediment‐type modelsModels of sediment type reveal the large‐scale architecture of the aquifer systemModels of the percentage of sand/gravel, when combined with hydrographs, provide information about the lateral variability in vertical connectivity within the aquifer system [ABSTRACT FROM AUTHOR]

We characterized horizontal hydraulic conductivity (K) of a fractured granitic aquifer using single‐ and cross‐hole hydraulic tests to evaluate "scale effect." For selected boreholes, K estimates were obtained using single‐hole FLUTe liner and slug tests. Several cross‐hole pumping tests were carried out at various durations. Drawdown responses were first interpreted using analytical well‐test solutions to obtain an effective horizontal conductivity (Keff) assuming a homogeneous and infinite aquifer. The same drawdowns were then numerically inverted using transient hydraulic tomography (THT) to delineate spatial distributions of K and storativity in the area encompassing the boreholes. Papadopulos (1965) and a nonlinear least squares minimization method produced a similar principal Keff direction that is consistent with the dominant fracture strike observed from outcrop and borehole televiewer data. However, principal direction and magnitude of this Keff depend on the pumping test duration and the number of monitoring boreholes used in the interpretation. As a group, K obtained from cross‐hole tests is larger than that obtained from single‐hole tests. However, because cross‐hole tests stressed the aquifer at both interwell and larger scales, Keff obtained from interpreting cross‐hole data is observed to decrease with pumping time, likely due to the dominance of less permeable fractures at larger scale. This lateral reduction of mean K is also revealed by THT as low K zones surrounding the test boreholes. Overall, K is found to increase from single‐hole to the interwell scale and then decrease at larger scale, exhibiting a non‐monotonic scale effect. Plain Language Summary: Knowledge of hydrological properties, particularly hydraulic conductivity (K), which controls groundwater flow in aquifers, is limited by the lack of subsurface data. Scientists have long observed that K estimated using field‐testing methods that stimulate different volumes of aquifers can vary by several orders of magnitude, which creates a conundrum for how groundwater flow should be analyzed. To determine if scale effect is apparent in a fractured granitic aquifer in the Laramie Range, Wyoming, testing methods that stressed the aquifer with different water volumes were employed. Our results indicate that K increases from single‐hole to the interwell scale but decreases at larger scale, with an overall nonmonotonic, albeit mild, scale effect. Compared to similar studies performed at other fractured rock sites, scale effect appears to be site‐specific and is not generally transferable across different aquifers. Key Points: In a granitic aquifer, K anisotropy identified from cross‐hole drawdown data is consistent with site geological dataAnalytical and hydraulic tomography interpretations of cross‐hole data suggest a reduced mean K at large scaleK increases from single‐hole to the interwell scale and then decreases at a larger scale, exhibiting a nonmonotonic scale effect [ABSTRACT FROM AUTHOR]