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Although renewed interest in flow around groundwater circulation wells is a positive development, the multiple and unacknowledged republication of previously published results is not. This is what has happened in a recent paper published in Water Resources Research by Tu et al. (2020, https://doi.org/10.1029/2020WR027529). The paper repeats results that appeared in Ni et al. (2011, https://doi.org/10.1016/j.applthermaleng.2011.04.030) as well as in the lead authors' own earlier work: Tu et al. (2019, https://doi.org/10.1016/j.applthermaleng.2018.09.029). And all three papers fail to acknowledge that the same ideas and closely related solutions were published in this Journal much earlier by Kabala (1993, https://doi.org/10.1029/92WR01820) and Zlotnik and Ledder (1996, https://doi.org/10.1029/95WR03813). Key Points: Tu et al. (2020) conducted a superficial literature review for flow around groundwater circulation well and missed numerous relevant publications, including those that previously published their resultsAll results claimed as novel by Tu et al. (2020) have been published earlier [ABSTRACT FROM AUTHOR]

The article discusses papers presented in the AGU Chapman Conference ‘The MADE Challenge for Groundwater Transport in Highly Heterogeneous Aquifers: Insights from 30 Years of Modeling and Characterization at the Field Scale and Promising Future Directions' in Valencia, Spain in October 2015.

Hydraulic connections between a river and an adjacent aquifer are controlled by the river resistance and aquifer diffusivity. In this paper, we derive a spectral solution linking the power spectrum of river stage fluctuations to that of the hydraulic head of a confined aquifer by means of a physical scaling factor. The physical scaling factor represents an algebraic expression of the river resistance and aquifer diffusivity and is included in an exact spectral solution derived herein. Statistical measures of the aquifer diffusivity and river resistance are provided by fitting the solution versus observed groundwater hydraulic head obtained at several distances and/or frequencies. At a study site in the middle reach of the Yangtze River and downstream of the Three Gorges Dam in China, we find systematic damping of the hydraulic head variations with distance from the river, which follows a fractal pattern driven by the river stage. In general, the estimated parameters are consistent with results reported in the literature, which supports the validity of the proposed spectral approach, although the paper discusses advantages and limitations due to application conditions. Plain Language Summary: The characterization of hydraulic connections between a river and an adjacent aquifer requires knowledge of two critical parameters: the river resistance and the aquifer diffusivity. In this paper, we derive a spectral solution to link the power spectrum of river stage fluctuations to that of the groundwater hydraulic head in a confined aquifer and show how such solutions can be used to simultaneously assess the two critical parameters. Parameters estimated from the field case located in the middle reach of the Yangtze River and downstream of the Three Gorges Dam in China are consistent with results reported in the literature. Application conditions, advantages, and limitations of this spectral approach are also discussed. Key Points: The power spectrum of the hydraulic head is linked to that of the river stage by a scaling factorRiver resistance and aquifer diffusivity are analyzed in spectral formsTheoretical developments are applied to a field case study of the Yangtze River, China [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]

Determining the factors that control the sensitivity of aquifer responses to earthquakes may provide insight into the interaction between hydrogeological and tectonic processes in the shallow crustal zone. Such response sensitivity varies randomly at the spatial scale and its determining factors have not been evaluated quantitatively and are still under debate. In this paper, we analyze seismically induced water level changes and then infer aquifer permeability changes following multiple earthquakes in two long‐term groundwater‐monitoring wells. While earthquake‐related water level changes were observed in one well, the second was insensitive to seismic events. These different seismic responses provide a unique opportunity to identify factors that control earthquake‐related changes. Wavelet transform, coupled with water level responses to tidal forcing, provides a methodology for comparing pre‐ and post‐seismic responses to periodic behavior. These methods found aquifer hydrogeological properties (e.g., transmissivity, storativity, degree of confinement) to be functions of their local hydrogeologic and tectonic settings. Key factors that may affect hydrogeological responses include the seismic energy arriving around the well, the degree of aquifer confinement, and the well location relative to local faults. Such factors favor seismic shaking and fracture unclogging, thus determining the sensitivity of hydrogeological responses to earthquakes. These findings are useful for designing wells for monitoring earthquakes, understanding earthquake‐inducing mechanisms, evaluating underground waste repositories, and estimating hydrogeological parameters using inversion. Plain Language Summary: The sensitivity of hydrological response to earthquakes seems varies randomly at the spatial scale or differentially from one earthquake to another. Understanding the factor and mechanism that control such phenomena provide insight into the interaction between hydrogeological and tectonic processes in the shallow crustal. In this paper, by comparing the sensitivity of hydrogeological response to different earthquakes in two long‐term groundwater monitoring wells, we evaluated the potential causal factors (i.e., seismic factor and hydrogeological factors). We proposed that factors favor seismic shaking and fracture unclogging would determine the sensitivity of hydrogeological responses to earthquakes. The finding of this study would be benefit in the field of earthquake‐monitoring wells designing, seismic triggering, underground waste repositories, etc. Key Points: Aquifer hydrogeological properties are estimated from the tidal responses of water level and used to evaluate the sensitivity to earthquakesThe responses may be controlled by the seismic energy around the well, the aquifer confinement, and the location relative to local faultsFactors that favor seismic shaking and fracture unclogging determine the sensitivity of hydrogeological responses to earthquakes [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]

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]

We consider the approach applied by Rumer and Shiau (1968) to interface flow in a layered coastal aquifer. The authors match the boundary conditions along the interfaces between layers of different hydraulic conductivities by changing the vertical scale of the layers, which causes violation of the governing equations. In particular, the mass balance equation is not met in each of the transformed layers. [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]

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]

The article presents the authors' reply to S. Neuman's comments which advocates the need for an adequate hydrogeological characterization of aquifers. It provides a brief summary of their work that was overlooked by Neuman's comment which include a critical reexamination of the field data collected by L.W. Gelhar et al especially those referred to as of medium or low reliability and the addition of new data what were again screened for reliability.

Foster et al. (2020) use the pumping data from a heavily monitored area in the state of Nebraska to assess the effectiveness of remote-sensing methods for estimating irrigation water use and to explore the policy implications of adopting those methods. Aquifer modeling cannot be done in isolation, as expertise from multiple disciplines is required to improve the reliability of predictions of what the future holds for the world's aquifers. Rateb et al. (2020) compare the GRACE water-storage estimates with those derived from water-level monitoring data and regional and global models for 14 major aquifers in the US. Groundwater is a critical resource for drinking water and food production, yet limited management amid intensive use has led to aquifer depletion across the globe (Alley & Alley, 2017; Bierkens & Wada, 2019). [Extracted from the article]

In recent years, higher-order geostatistical methods have been used for modeling of a wide variety of large-scale porous media, such as groundwater aquifers and oil reservoirs. Their popularity stems from their ability to account for qualitative data and the great flexibility that they offer for conditioning the models to hard (quantitative) data, which endow them with the capability for generating realistic realizations of porous formations with very complex channels, as well as features that are mainly a barrier to fluid flow. One group of such models consists of pattern-based methods that use a set of data points for generating stochastic realizations by which the large-scale structure and highly-connected features are reproduced accurately. The cross correlation-based simulation (CCSIM) algorithm, proposed previously by the authors, is a member of this group that has been shown to be capable of simulating multimillion cell models in a matter of a few CPU seconds. The method is, however, sensitive to pattern's specifications, such as boundaries and the number of replicates. In this paper the original CCSIM algorithm is reconsidered and two significant improvements are proposed for accurately reproducing large-scale patterns of heterogeneities in porous media. First, an effective boundary-correction method based on the graph theory is presented by which one identifies the optimal cutting path/surface for removing the patchiness and discontinuities in the realization of a porous medium. Next, a new pattern adjustment method is proposed that automatically transfers the features in a pattern to one that seamlessly matches the surrounding patterns. The original CCSIM algorithm is then combined with the two methods and is tested using various complex two- and three-dimensional examples. It should, however, be emphasized that the methods that we propose in this paper are applicable to other pattern-based geostatistical simulation methods. [ABSTRACT FROM AUTHOR]

Stochastic hydrological process models have two conceptual advantages over deterministic models. First, even though water flow in a well‐defined environment is governed by deterministic differential equations, a hydrological system, at the level we can observe it, does not behave deterministically. Reasons for this behavior are unobserved spatial heterogeneity and fluctuations of input, unobserved influence factors, heterogeneity and variability in soil and aquifer properties, and an imprecisely known initial state. A stochastic model provides thus a more realistic description of the system than a deterministic model. Second, hydrological models simplify real processes. The resulting structural deficits can better be accounted for by stochastic than by deterministic models because they, even for given parameters and input, allow for a probability distribution of different system evolutions rather than a single trajectory. On the other hand, stochastic process models are more susceptible to identifiability problems and Bayesian inference is computationally much more demanding. In this paper, we review the use of stochastic, time‐dependent parameters to make deterministic models stochastic, discuss options for the numerical implementation of Bayesian inference, and investigate the potential and challenges of this approach with a case study. We demonstrate how model deficits can be identified and reduced, and how the suggested approach leads to a more realistic description of the uncertainty of internal and output variables of the model compared to a deterministic model. In addition, multiple stochastic parameters with different correlation times could explain the variability in the time scale of output error fluctuations identified in an earlier study. Key Points: Stochastic, time‐dependent parameters consider intrinsic model uncertainty and propagate this uncertainty to the model outputCross‐validation sensitively identifies model structure deficits and the time‐course of the identified parameters gives hints for model improvementsPosterior uncertainty naturally gets autocorrelated and reflects the difference in knowledge between calibration and validation or extrapolation periods [ABSTRACT FROM AUTHOR]

Karstified carbonate aquifers are highly heterogeneous systems characterized by multiple recharge, flow, and discharge components. The quantification of the relative contribution of these components, as well as their numerical representation, remains a challenge. This paper identifies three recharge components in the time and frequency domain. While the analysis in the time domain follows traditional approaches, the analysis of the power spectrum allows frequencies associated with specific spectral coefficients and noise types to be distinguished more objectively. The analysis follows the presented hypothesis that the different frequency‐noise components are the result of aquifer heterogeneity transforming the random rainfall input into a sequence of non‐Gaussian signals. The distinct signals are then numerically represented in the context of a semidistributed pipe network model in order to simulate recharge, flow, and discharge of an Irish karst catchment more realistically. By linking the power spectra of the modeled recharge components with the spectra of the spring discharge, the information usually gained by classical performance indicators is significantly widened. The modeled spring discharge is well matched in the time and frequency domain, yet the different recharge dynamics explain the signal of the aquifer outlet in different noise domains across the spectrum. This study demonstrates the conjunctive use of frequency analysis in conceptualization of a hydrological system together with modeling and evaluation. Key Points: Hydrograph analysis was coupled with frequency and noise analysis to identify recharge dynamics in a karst spring discharge time seriesRecharge, flow, and discharge were modeled in a semidistributed pipe network model considering aquifer heterogeneities and noise domainsA new model evaluation approach using noise analysis provided information on the individual contribution of the internal flows [ABSTRACT FROM AUTHOR]

The slug test is a common field technique for obtaining local hydraulic parameters near wells, applied, for example, for the hydrogeological investigation at contaminated sites. Although many slug test models have been developed for interpretation of measurements, only a few of them have considered heterogeneous conditions, and water column inertial effects are usually neglected. In this paper, we propose a novel three‐dimensional slug test model (3DHIM) for application in heterogeneous aquifers, considering inertial effects associated with skin effects and linear friction forces. After comparison with existing analytical and numerical solutions of slug tests, the model is applied to an aquifer analog to simulate a series of slug tests. The results from single‐well slug tests show that the well geometry (i.e., the well radius, well depth, and screen length) has an impact on the water level response. For cross‐well slug tests, the results indicate that the water level fluctuations not only include information on the hydraulic signal propagation process in the aquifer but also on well characteristics, such as wellbore storage and inertial effects. These effects cause a phase shift and amplitude change of the water level fluctuation. As the observation and test wells have a good hydraulic connection and similar well geometry, the water level amplitude could be amplified relative to aquifer pressure at the measured position. Therefore, we suggest considering wellbore storage and in‐well inertial effects in slug test‐based subsurface investigations, otherwise the parameter estimates based on well water levels may include errors, particularly in highly permeable layers. Key Points: We establish a three‐dimensional slug test model that accounts for aquifer heterogeneity and inertial effects in wellsThe new 3DHIM model is demonstrated in an aquifer analog studyThe impact of aquifer heterogeneity, in‐well inertial, and wellbore storage effects on simulation results is evaluated [ABSTRACT FROM AUTHOR]

Efficiently measuring groundwater flow in bedrock aquifers is inherently challenging due to the irregular distribution and fine scale of fractures. Recent advances in Active Distributed Temperature Sensing (A‐DTS) in boreholes temporarily sealed with liners have made it possible to quantify flow rates in such aquifers at many different depths using heat as a tracer, but until now only data collected under a single hydraulic condition have been published. This paper presents the first field data from multiple A‐DTS field tests conducted under different hydraulic conditions to quantify groundwater flow redistribution within a bedrock aquifer. Three separate quasi steady state A‐DTS tests were collected in a sealed borehole: (1) natural gradient condition where all boreholes were sealed with flexible and impermeable liners, (2) cross‐connected condition where a nearby borehole was open allowing vertical flow within the borehole, and (3) forced gradient condition where the nearby open borehole was pumped at a constant rate of 54 L/min. The depth‐discrete hydraulic head responses were also measured during the three tests using a string of transducers in a sealed borehole. Results provide quantifiable insights as to how the bedrock aquifer responds, including A‐DTS‐derived measurements of flow changes in fractures at multiple depths driven by changes in gradients. The results confirm that a single open borehole or long‐screened well can significantly alter the site hydraulics and demonstrate that not all large or transmissive fractures show evidence of active flow and thus, transmissivity and aperture should not be used alone to infer active flow zones. Plain Language Summary: Measuring groundwater flow in fractured bedrock aquifers is difficult because flow is primarily controlled by small and irregularly spaced fractures. Very few tools exist to measure the natural flow through fractures in these aquifers, which is essential for understanding contaminant transport flow paths. One emerging technique, called Active Distributed Temperature Sensing (A‐DTS), uses a type of fiber optic sensor that can measure temperature at many different intervals along a fiber optic cable. This cable is lowered into a borehole, and a flexible inflatable liner is installed to prevent vertical flow within the borehole. The cable is then heated using integrated heating wires for an extended period, and the temperature response can be used to locate and estimate groundwater flow rates. This study collects field data under three different flow conditions at a site to demonstrate how the flow in a bedrock aquifer responds when it is stressed and the sensitivity of the A‐DTS technique. Results demonstrate highly variable flow with depth and that having a single open borehole on a site can strongly affect the natural flow system. A‐DTS allows efficient measurement of this variable flow with depth and provides a better understanding of these complex bedrock groundwater systems. Key Points: A‐DTS in sealed boreholes can effectively quantify changes in fracture flow with depth in a bedrock aquiferAn open and cross‐connected borehole can significantly affect the site hydraulicsUnder natural gradient conditions, transmissive fractures are not always hydraulically active [ABSTRACT FROM AUTHOR]

This paper develops a coupled hydrologic‐economic model that estimates the effects of six tax institutions that theory predicts will lead to equal amounts of aquifer withdrawal. That said, the distributive effects of the tax institutions are expected to differ because each involves different combinations of tax thresholds and side payments (returned tax revenue). The tax policies can lead to groundwater users being worse off than they would be using an unmanaged aquifer. This study explores whether the distributive impacts of specific policies that have equal marginal incentives lead to differences in the behavior of participants in an experiment involving a common pool groundwater resource. The results reveal that each of the tax policies results in approximately the same reduction in resource use but affects participants' earnings and opinions regarding the policies differently. A tax imposed on groundwater use above a threshold and without a side payment is most effective in increasing the net social benefit associated with using the aquifer; participant earnings under that scheme are almost equal to earnings from an unmanaged aquifer (1.04% less), and overall social efficiency is greater (4.34%). Unfortunately, participants tend to prefer an unmanaged aquifer and tax policies with high side‐payments—treatments that led to lower overall social efficiency. The evidence suggests that aquifer management may require a two‐fold approach: (1) a carefully selected threshold that can make water users financially indifferent between a managed and unmanaged aquifer and (2) education to increase the political acceptability of the managed aquifer policy. Key Points: Six aquifer withdrawal taxes result in similar groundwater use but different user earnings and opinionsThe most effective policy is a groundwater use tax with a threshold and without an upfront side paymentGroundwater users prefer an unmanaged aquifer and tax policies with high side‐payments irrespective of overall aquifer use efficiency [ABSTRACT FROM AUTHOR]

We present an approach for reducing saltwater intrusion in coastal aquifers by artificially reducing the hydraulic conductivity in the upper part of selected areas by using a precipitate. We apply a previously presented analytical approach to develop formulas useful for the design of artificial barriers. Equations for the location of the tip of the saltwater wedge are presented and verified through a sand-tank experiment. The analysis is capable of computing discharges exactly, but requires the Dupuit-Forchheimer approximation to compute points of the interface between flowing fresh and stationary saltwater. We consider a vertical coastline and boundaries in the freshwater zone of either given discharge or given head. We demonstrate in the paper that reduction of the hydraulic conductivity in the upper part of a coastal aquifer will result in a decrease of saltwater intrusion, and present analytic expressions that can be used for design purposes. The previously presented analytical approach can be applied to design systems to reduce saltwater intrusion caused by pumping inland from the zone that contains saline groundwater. [ABSTRACT FROM AUTHOR]

This paper presents the fundamental theory and laboratory test results on a new device that is deployed in boreholes in fractured rock aquifers to characterize vertical distributions of water and contaminant fluxes, aquifer hydraulic properties, and fracture network properties (e.g., active fracture density and orientation). The device, a fractured rock passive flux meter (FRPFM), consists of an inflatable core assembled with upper and lower packers that isolate the zone of interest from vertical gradients within the borehole. The outer layer of the core consists of an elastic fabric mesh equilibrated with a visible dye which is used to provide visual indications of active fractures and measures of fracture location, orientation, groundwater flux, and the direction of that flux. Beneath the outer layer is a permeable sorbent that is preloaded with known amounts of water soluble tracers which are eluted at rates proportional to groundwater flow. This sorbent also captures target contaminants present in intercepted groundwater. The mass of contaminant sorbed is used to quantify cumulative contaminant flux; whereas, the mass fractions of resident tracers lost are used to provide measures of water flux. In this paper, the FRPFM is bench tested over a range of fracture velocities (2-20 m/day) using a single fracture flow apparatus (fracture aperture = 0.5 mm). Test results show a discoloration in visible dye corresponding to the location of the active fracture. The geometry of the discoloration can be used to discern fracture orientation as well as direction and magnitude of flow in the fracture. Average contaminant fluxes were measured within 16% and water fluxes within 25% of known imposed fluxes. [ABSTRACT FROM AUTHOR]

The aim of this paper is to improve semiseasonal forecast of groundwater availability in response to climate variables, surface water availability, groundwater level variations, and human water management using a two‐step data‐driven modeling approach. First, we implement an ensemble of artificial neural networks (ANNs) for the 300 wells across the High Plains aquifer (USA). The modeling framework includes a method to choose the most relevant input variables and time lags; an assessment of the effect of exogenous variables on the predictive capabilities of models; and the estimation of the forecast skill based on the Nash‐Sutcliffe efficiency (NSE) index, the normalized root mean square error, and the coefficient of determination (R2). Then, for the ANNs with low‐ accuracy, a MultiModel Combination (MuMoC) based on a hybrid of ANN and an instance‐based learning method is applied. MuMoC uses forecasts from neighboring wells to improve the accuracy of ANNs. An exhaustive‐search optimization algorithm is employed to select the best neighboring wells based on the cross correlation and predictive accuracy criteria. The results show high average ANN forecasting skills across the aquifer (average NSE > 0.9). Spatially distributed metrics of performance showed also higher error in areas of strong interaction between hydrometeorological forcings, irrigation intensity, and the aquifer. In those areas, the integration of the spatial information into MuMoC leads to an improvement of the model accuracy (NSE increased by 0.12), with peaks higher than 0.3 when the optimization objectives for selecting the neighbors were maximized.tT Key Points: Artificial neural networks can accurately forecast semiseasonal groundwater level changesMuMoC improved the groundwater well‐level forecasting skill for 1‐ to 4‐month lead times with respect to a single ANN model by 25% in NSEThe implementation of MuMoC is recommended in case of densely gauged areas [ABSTRACT FROM AUTHOR]

Numerous analytical solutions have been developed for modeling thermal perturbations to underground formations caused by deep‐well injection of fluids. Each solution has been derived for a specific boundary value problem and a simplified flow network with one set of parallel fractures. In this paper, new generalized solutions G*(x, s) are developed using (existing) global transfer functions G0*xs and a new memory function g*(s), where x and s are the space and Laplace variable. The memory function represents the solutions of conductive heat exchange between fractures and matrix blocks and between fractured aquifers and unfractured acquitards. The memory function is developed to account for multirate exchange induced by different shapes, sizes, properties, and volumetric fractions of matrix blocks bounded by multiple sets of orthogonal fractures with different spacing. The global transfer functions represent the fundamental solutions to convective, convective‐conductive, and convective‐dispersive heat transport in fractures (or aquifers) without exchange and are available for various (1‐D linear, 1‐D radial, 2‐D dipole, and single‐well injection‐withdrawal) flow fields. The new solutions with exchange are developed using G*xs=B*sG0*xs1+ϑg*s, thereby greatly simplifying solution development in a novel way, where ϑ and B*(s) are a fracture‐matrix scaling factor and the boundary condition function. The new solutions are applied to several example problems, showing that heat transport in fractured aquifers is significantly impacted by (1) thermal dispersion in fractures that is rarely considered, (2) multirate heat exchange with a wide range of size and anisotropy of rectangular matrix blocks, and (3) heat exchange between aquifers and acquitards. Key Points: A generalized memory function was developed for calculating heat exchange between fractures and matrix blocks of various shapes and sizesNew solutions were developed by plugging the memory function into existing global transfer functions of convective‐dispersive heat transportExample results show the effects of thermal dispersion and multirate heat exchange between 3‐D orthogonal fractures and matrix blocks [ABSTRACT FROM AUTHOR]

The objective of this paper is to study the impact of immobile water, its fraction, and its mass transfer with the flowing region on efficiency of CO2 dissolution in aquifers with an immobile water zone. A continuum scale code is developed with underlying assumptions of spatially homogeneous and temporally invariable partitioning fraction of the porous media, first‐order mass transfer between the mobile and immobile zones, and simplified reaction of CO2 aqueous solution with calcium carbonate rock. Using ranges of values for Damköhler number (Da), fraction of the total pore volume, and mass transfer coefficient rate (α), 96 simulations are conducted. It is shown that due to a lower intensity of reaction in the mobile region, intermediate values of α serve as a threshold below which the mass transfer coefficient is not affecting the overall CO2 storage and above which overall CO2 storage increases as a function of mass transfer coefficient. Additionally, we found that (i) when α is high and geochemistry is intensive (high Da), the overall CO2 storage decreases with increase in fraction of mobile water. This is because CO2 storage through consumption of rock in immobile water with higher geochemistry is reduced. (ii) When α is high but Da is low, the system is effectively a single porosity medium with no chemistry‐influenced discrimination between mobile and immobile zones, and therefore, overall CO2 storage increases with fraction of mobile water. (iii) When α is low, the magnitude of Da does not influence the overall CO2 storage. Key Points: Presence of immobile water in aquifers is studied for CO2 density‐driven convectionMass transfer coefficient is a key parameter to determine overall efficiency of carbon storage through dissolutionFor intermediate mass transfer coefficients, contribution of calcite dissolution into overall CO2 storage as a function of mobile region fraction is reversed [ABSTRACT FROM AUTHOR]

In subsurface aquifers, dispersion of contaminants is highly affected by the heterogeneity of the hydraulic conductivity field. As an alternative to Monte Carlo simulations on probable conductivity fields, stochastic velocity processes have been introduced to assess the uncertainty in the transport of contaminants. In continuum‐scale simulations, discrete velocity models (such as correlated continuous time random walk) focus on modeling plume dispersion in the longitudinal direction. There are alternative continuous velocity processes (such as the polar Markovian velocity process [PMVP]) that are able to accurately model transport in both longitudinal and transverse directions. Importantly, the PMVP model correctly predicts the limited spreading of the ensemble contaminant plume in the transverse direction. However, the stochastic differential equations used in the PMVP model have specific drift and diffusion functions that are designed for the exponential correlation structure. In this paper, a new discrete velocity process is described that is applicable to modeling transport in two‐dimensional conductivity fields for both Gaussian and exponential correlation structures. This method is simple, in a sense that it does not require modeling the functional form of the drift and diffusion functions. The new method is validated against Monte Carlo simulations for both correlation structures with high variances of log conductivity. Key Points: A discrete temporal Markov model is proposed for modeling transport in correlated porous mediaThe proposed model is significantly more flexible compared to its continuous counterpart based on SDEsThe proposed model can correctly predict the spreading of the ensemble plume in both longitudinal and transverse directions [ABSTRACT FROM AUTHOR]

This paper contains a new formulation for infiltration inside domains bounded by polygons and its application to problems of steady multiaquifer flow, using the Dupuit-Forchheimer approximation and assuming vertical flow in the separating layers. An alternative formulation is presented for leaky aquifer systems where infiltration or extraction is given. Existing formulations of multiaquifer flow involve a system of equations that must be solved for the heads in the aquifers. These formulations are abstract, and the relation between the parameters in the solution and physical quantities is hidden. The formulation in the paper aims at linking the system of equations to physical quantities; we have done this in two ways. First, we formulate the problem in terms of leakage potentials, related directly to the leakage through the leaky layers. Second, we introduce the concept of 'equilibrated leakage,' leakage that is either the result of infiltration or of some disturbance in the flow pattern, such as that caused by a well. The leakage through the leaky layers tends to some constant value far from a disturbance, e.g., a well, or the boundary of an area of constant infiltration. This concept of equilibrated leakage is useful in practice and helps in understanding the distribution of leakage; we explain this in detail in the paper. The study of problems of steady flow in leaky aquifer systems is inspired by problems of groundwater sustainability, where the overall distribution of flow over long periods of time is important, rather than detailed information. [ABSTRACT FROM AUTHOR]

While in catchment and hillslope hydrology a more nuanced approach is now taken to streamflow recession analysis, in the context of major aquifers it is commonly still assumed that the groundwater head recession rate will take exponential form, an idea originally proposed in the 19th Century. However it is shown here that, in early times, the groundwater head recession in a major aquifer should take an almost straight line form with a rate approximately equal to the long-term recharge rate divided by the aquifer storage coefficient. The length of this phase can be estimated from an analytical expression derived in the paper which depends on the aquifer diffusivity, length scale, and the position of the monitoring point. A transitional phase then leads to an exponential phase after some critical time which is independent of the position of the monitoring point. Major aquifers in a state of periodic quasi-steady state are expected to have rates of groundwater flux recession which deviate little from the average rate of groundwater recharge. Where quasi-exponential groundwater declines are observed in nature, their form may be diagnostic of particular types of aquifer properties and/or boundary effects, such as proximity to drainage boundaries, variations in transmissivity with hydraulic head, storage changes due to pumping, nonequilibrium flow at a range of spatial and temporal scales, and variations in specific yield with depth. Recession analysis has applicability to a range of groundwater problems and is powerful way of gaining insight into the hydrologic functioning of an aquifer. [ABSTRACT FROM AUTHOR]

This paper introduces the special section on 'new modeling approaches and novel experimental technologies for improved understanding of process dynamics at aquifer-surface water interfaces.' It is contextualizing the framework for the 27 research papers of the special section by firth identifying research gaps and imminent challenges for ecohydrological research at aquifer-surface water interfaces and then discussing the specific paper contributions on (i) new developments in temperature/heat tracing at GW-SW interfaces, (ii) new methods to capture the temporal and spatial variability of groundwater-surface water exchange, (iii) new approaches in modeling aquifer-river exchange flow, and (iv) new concepts and advanced theory of groundwater-surface water exchange. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a methodology to predict the shape of solute breakthrough curves in heterogeneous aquifers at early times and/or under high degrees of heterogeneity, both cases in which the classical macrodispersion theory may not be applicable. The methodology relies on the observation that breakthrough curves in heterogeneous media are generally well described by lognormal distributions, and mean breakthrough times can be predicted analytically. The log‐variance of solute arrival is thus sufficient to completely specify the breakthrough curves, and this is calibrated as a function of aquifer heterogeneity and dimensionless distance from a source plane by means of Monte Carlo analysis and statistical regression. Using the ensemble of simulated groundwater flow and solute transport realizations employed to calibrate the predictive regression, reliability estimates for the prediction are also developed. Additional theoretical contributions include heuristics for the time until an effective macrodispersion coefficient becomes applicable, and also an expression for its magnitude that applies in highly heterogeneous systems. It is seen that the results here represent a way to derive continuous time random walk transition distributions from physical considerations rather than from empirical field calibration. [ABSTRACT FROM AUTHOR]

We analyze solute transport in a radially converging 3-D flow field in a porous medium with spatially heterogeneous hydraulic conductivity ( K). The aim of the paper is to analyze the impact of heterogeneity and the mode of injection on BreakThrough Curves (BTCs) detected at a well pumping a contaminated aquifer. The aquifer is conceptualized as an ensemble of blocks of uniform but contrasting K and the analysis makes use of the travel time approach. Despite the approximations introduced, the model reproduces a laboratory experiment without calibration of transport parameters. Our results also show excellent agreement with numerical simulations for different levels of heterogeneity. We focus on the impact on the BTC of both heterogeneity in K and solute release conditions. It is shown that the injection mode matters, and the differences in the BTCs between uniform and flux-proportional injection increase with the heterogeneity of the K-field. Furthermore, we study the effect of heterogeneity and mode of injection on early and late arrivals at the well. [ABSTRACT FROM AUTHOR]

The proportion of an aquifer with constituent concentrations above a specified threshold (high concentrations) is taken as a nondimensional measure of regional scale water quality. If computed on the basis of area, it can be referred to as the aquifer scale proportion. A spatially unbiased estimate of aquifer scale proportion and a confidence interval for that estimate are obtained through the use of equal area grids and the binomial distribution. Traditionally, the confidence interval for a binomial proportion is computed using either the standard interval or the exact interval. Research from the statistics literature has shown that the standard interval should not be used and that the exact interval is overly conservative. On the basis of coverage probability and interval width, the Jeffreys interval is preferred. If more than one sample per cell is available, cell declustering is used to estimate the aquifer scale proportion, and Kish's design effect may be useful for estimating an effective number of samples. The binomial distribution is also used to quantify the adequacy of a grid with a given number of cells for identifying a small target, defined as a constituent that is present at high concentrations in a small proportion of the aquifer. Case studies illustrate a consistency between approaches that use one well per grid cell and many wells per cell. The methods presented in this paper provide a quantitative basis for designing a sampling program and for utilizing existing data. [ABSTRACT FROM AUTHOR]

Many environmental systems are driven by complex gas-water processes in fractured aquifers. One example is degassing processes occurring in fractures in the vicinity of radioactive waste disposal sites. These fractures can represent a potential fast track for radioactive substances to reach the surface: due to buoyancy effects, the created gas phase can move relatively quickly to the surface. The aim of this paper is to investigate and model the involved processes. First, a model for a single fracture is developed. Based on statistical properties of real fractures, an aperture distribution (raster element model) is created. A percolation-renormalization model yields effective properties, such as relative permeabilities and capillary pressure, and provides the size of a representative elementary volume. These effective relationships represent the basis for modeling the migration of the created gas phase through the fractured aquifer of the subsurface. An example modeling degassing and flow in a single fracture is shown and compared to experimental data. The presented model represents a conceptual method for degassing processes and the subsequent migration of the gas phase in fracture-matrix systems, improving predictions on the fate of radioactive substances. [ABSTRACT FROM AUTHOR]

The occurrence of freshwater lenses in saline aquifers adjoining gaining rivers has recently been demonstrated as being theoretically possible by way of analytical solution. However, physical evidence for freshwater lenses near gaining rivers is limited largely to airborne geophysical surveys. This paper presents the first direct observations of freshwater lenses adjacent to gaining rivers, albeit at the laboratory-scale, as validation of their plausibility. The experimental conditions are consistent with the available analytical solution, which is compared with laboratory observations of lens extent and the saltwater flow rate, for various hydraulic gradients. Numerical simulation shows that dispersion can account for the small amount of mismatch between the sharp-interface analytical solution and laboratory measurements. Calibration and uncertainty analysis demonstrate that accurate mathematical predictions require calibration to laboratory measurements of the lens. The results provide unequivocal proof that freshwater lenses can persist despite gaining river conditions concordant with theoretical lenses predicted by the analytical solution, at least within the constraints of the experimental setup. [ABSTRACT FROM AUTHOR]

The article aims to respond a comment made on our paper about appropriate boundary condition for the original Dupuit‐Boussinesq theory for two‐dimensional steady groundwater flow in an unconfined sloping aquifer with uniform rainfall recharge. To respond to the comments arguing the existence of lateral groundwater flows and negative groundwater table, clarifications are made for our analysis focusing on two‐dimensional groundwater flow without considering lateral effects by using the original and classical approximate theory. Key Points: For the original Dupuit‐Boussinesq theory, zero volumetric discharge at the upstream yields zero groundwater table or zero seepage velocityGroundwater table can be of physical significance only upon a nonnegative valueA constant rainfall recharge determines a linear discharge distribution [ABSTRACT FROM AUTHOR]

Rivers in arid and semiarid regions often traverse saline aquifers, creating buoyant freshwater lenses in the adjoining riparian and floodplain zones. The occurrence of freshwater lenses where the river is otherwise gaining saline groundwater appears counterintuitive, given that both hydraulic and density forces act toward the river. In this paper, an analytical solution is presented that defines the extent of a stable, sharp-interface terrestrial freshwater lens (in cross section) in a riverine environment that otherwise contains saline groundwater moving toward the river. The method is analogous to the situation of an island freshwater lens, except in the riverine setting, the saltwater is mobile and the lens is assumed to be stagnant. The solution characterizes the primary controlling factors of riverine freshwater lenses, which are larger for situations involving lower hydraulic conductivities and rates of saltwater discharge to the river. Deeper aquifers, more transmissive riverbeds, and larger freshwater-saltwater density differences produce more extensive lenses. The analytical solution predicts the parameter combinations that preclude the occurrence of freshwater lenses. The utility of the solution as a screening method to predict the occurrence of terrestrial freshwater lenses is demonstrated by application to parameter ranges typical of the South Australian portion of the River Murray, where freshwater lenses occur in only a portion of the neighboring floodplains. Despite assumptions of equilibrium conditions and a sharp freshwater-saltwater interface, the solution for predicting the occurrence of riverine freshwater lenses presented in this study has immediate relevance to the management of floodplains in which freshwater lenses are integral to biophysical conditions. [ABSTRACT FROM AUTHOR]

The paper aims to solve a robust optimization problem (optimization in presence of uncertainty) for finding the optimal locations of a number of CO2 injection wells for geological sequestration of carbon dioxide in a saline aquifer. The parametric uncertainties are the interfacial tension between CO2 and aquifer brine, the Land's trapping coefficient and the boundary aquifer's absolute permeability. The spatial uncertainties are due to the channelized permeability field which exhibits a binary channel-non-channel system. The objective function of the optimization is the amount of residually trapped CO2 due to the hysteresis of the relative permeability curves. A risk-averse value derived from the cumulative density function of the distribution of the amount of trapped gas is chosen as the objective function value. In order to ensure that the uncertainties are effectively taken into account, Monte Carlo simulation and Polynomial Chaos Expansion (PCE)-based methods are used and compared with each other. For different cases of parametric and spatial uncertainties, the most accurate uncertainty quantification (UQ) method is chosen to be integrated within the optimization algorithm. While for parametric uncertainty cases of up to two uncertain variables, PCE-based methods computationally outperform Monte Carlo simulations, it is shown that for the multimodal distributions of the function of trapped gas occurring for the spatial uncertainty case, Monte Carlo simulations are more reliable than PCE-based UQ methods. For the discrete (integer) optimization problem, various mixed response surface surrogate models are tested and the robust optimization resulted in optimal CO2 injection well locations. [ABSTRACT FROM AUTHOR]

The diffusivity test considered here consists of injecting (or pumping) a volume of water through short segments of a well for a short time and measuring the travel time of the peak of the head signal at different points in the surrounding aquifer volume. The specific storage is assumed to be constant, while the hydraulic conductivity of the heterogeneous aquifer is modeled as a random lognormal field. The axi-symmetric anisotropic structure is characterized by a few parameters (logconductivity mean and variance and horizontal and vertical integral scales). The paper determines the mean and variance of the peak travel time as function of distance from an instantaneous source by solving the flow equation using a first-order approximation in the logconductivity variance. The mean travel time is recast in terms of the equivalent conductivity, which decreases from the harmonic mean near the source to the effective conductivity in uniform flow for a sufficiently large distance. Similarly, the variance drops from its maximum near the source to a small value. Application to field test is discussed and topics of future investigations are suggested. [ABSTRACT FROM AUTHOR]

The spatial heterogeneity of hydraulic conductivity ( K) exerts a major control on groundwater flow and solute transport. The heterogeneous spatial distribution of K can be imaged using indirect geophysical data as long as reliable relations exist to link geophysical data to K. This paper presents a nonparametric learning machine approach to predict aquifer K from cone penetrometer tests (CPT) coupled with a soil moisture and resistivity probe (SMR) using relevance vector machines (RVMs). The learning machine approach is demonstrated with an application to a heterogeneous unconsolidated littoral aquifer in a 12 km2 subwatershed, where relations between K and multiparameters CPT/SMR soundings appear complex. Our approach involved fuzzy clustering to define hydrofacies (HF) on the basis of CPT/SMR and K data prior to the training of RVMs for HFs recognition and K prediction on the basis of CPT/SMR data alone. The learning machine was built from a colocated training data set representative of the study area that includes K data from slug tests and CPT/SMR data up-scaled at a common vertical resolution of 15 cm with K data. After training, the predictive capabilities of the learning machine were assessed through cross validation with data withheld from the training data set and with K data from flowmeter tests not used during the training process. Results show that HF and K predictions from the learning machine are consistent with hydraulic tests. The combined use of CPT/SMR data and RVM-based learning machine proved to be powerful and efficient for the characterization of high-resolution K heterogeneity for unconsolidated aquifers. [ABSTRACT FROM AUTHOR]

This paper assesses the effectiveness of aquifer monitoring information in achieving more sustainable use of a groundwater resource in the absence of management policy. Groundwater user behavior in the face of an irreversible contamination threat is studied by applying methods of experimental economics to scenarios that combine a physics-based, spatially explicit, numerical groundwater model with different representations of information about an aquifer and its risk of contamination. The results suggest that the threat of catastrophic contamination affects pumping decisions: pumping is significantly reduced in experiments where contamination is possible compared to those where pumping cost is the only factor discouraging groundwater use. The level of information about the state of the aquifer also affects extraction behavior. Pumping rates differ when information that synthesizes data on aquifer conditions (a 'risk gauge') is provided, despite invariant underlying economic incentives, and this result does not depend on whether the risk information is location-specific or from a whole aquifer perspective. Interestingly, users increase pumping when the risk gauge signals good aquifer status compared to a no-gauge treatment. When the gauge suggests impending contamination, however, pumping declines significantly, resulting in a lower probability of contamination. The study suggests that providing relatively simple aquifer condition guidance derived from monitoring data can lead to more sustainable use of groundwater resources. [ABSTRACT FROM AUTHOR]

Oscillatory well pumping was proposed recently as a tool for hydraulic tomography. Periodic pumping at a few frequencies is carried out through vertical intervals along the pumping well and the periodic head is measured along a few piezometers. The paper presents an analytical solution for the head field in an unconfined aquifer of finite depth under the common assumptions of a linearized water table condition, different horizontal and vertical constant permeabilities, constant specific storativity and water table drainable porosity, and small well radius to length ratio. The solution provides the expressions of the amplitude and phase of the head as a function of coordinates, frequency, and the problem parameters. The solution simplifies to one pertaining to an upper constant head condition and a rigid aquifer for a wide range of the dimensionless frequency values. [ABSTRACT FROM AUTHOR]

Strategic management of degrading coastal aquifers in eastern North Carolina (USA) became imperative after a severe imbalance occurred between withdrawal and recharge rates. To ameliorate this growing problem, an aggressive water policy was developed through public input by creating the Central Coastal Plain Capacity Use Area (CCPCUA) to maintain beneficial use of groundwater resources. Insights from social psychology, and socio-legal studies are used to evaluate how procedural justice and public participation played major roles to resolving groundwater resource management problems. A mixed methods approach uses archival data and interviews with various rule-making participants to assess the process of stakeholder involvement that led to creation of the policy. In addition, data analysis techniques are utilized to evaluate the effects of the policy on aquifer health (through water levels) over a ∼10 year period. Results suggest that not only did a stakeholder group participate in a process that was deemed fair, understandable, and relatively easy to administer for users and regulators, but public participation resulted in an effective plan that ensures the long-term sustainable use of groundwater. Declining groundwater withdrawals and recovering water levels suggest that the rule is achieving its intended goal of protecting the aquifers from depletion and degradation. This paper touches on global themes that are essential to water demand and consumption, water management techniques, and water resources protection. [ABSTRACT FROM AUTHOR]

This paper is intended to present a newly developed, comprehensive model to understand hydrochemical behaviors related to dissolutional growth of complex fractures underground. Under a fully implicit solution framework, the finite volume approach is employed to solve the reactive-convectivedispersive system. Based on the embedded discrete fracture model, a novel modeling approach is proposed to describe dissolutional fractures with various apertures in three dimensions (3-D). This model is verified against preexisting numerical models. Then, a 3-D field case study regarding hypogene speleogenesis in a deep-seated, artesian setting is carried out, based on field data concerning karst terrain compiled from Western Ukraine. Finally, we perform a case study to elucidate the finger-like dissolution process related to a synthetic 3-D fracture network in gypsum and limestone with heterogeneous aperture fields. Simulation results suggest that (a) the combination of the kinetic trigger mechanism and the transverse speleogenesis concept favors the generation of hierarchical caves in homogeneous fractures; (b) the mechanism of reactive infiltration instabilities determines dissolutional propagation in gypsum with rough fractures; and (c) in limestone, nonlinear kinetics matters for both uniform- and variable-aperture fractures. [ABSTRACT FROM AUTHOR]

In a recent paper (Trefry et al., 2019, https://doi.org/10.1029/2018wr023864), we showed that the interplay of aquifer heterogeneity and poroelasticity can produce complex transport in tidally forced aquifers, with significant implications for solute transport, mixing, and reaction. However, what was unknown was how broadly these transport dynamics can arise in natural groundwater systems and how these dynamics depend upon the aquifer properties and tidal and regional flow characteristics. In this study we answer these questions through parametric studies of these governing properties. We uncover the mechanisms that govern complex transport dynamics and the bifurcations between transport structures that depend upon changes in the governing parameters, and we determine the propensity for complex dynamics to occur in natural aquifer systems. These results clearly demonstrate that complex transport structures and dynamics may arise in natural tidally forced aquifers around the world, producing solute transport and mixing behavior that is very different to that of the conventional Darcy flow picture. [ABSTRACT FROM AUTHOR]

This paper presents two methods for joint inversion of aquifer test data, magnetic resonance sounding (MRS) data, and transient electromagnetic data acquired from a multilayer hydrogeological system. The link between the MRS model and the groundwater model is created by tying hydraulic conductivities ( k) derived from MRS parameters to those of the groundwater model. Method 1 applies k estimated from MRS directly in the groundwater model, during the inversion. Method 2 on the other hand uses the petrophysical relation as a regularization constraint that only enforces k estimated for the groundwater model to be equal to MRS derived k to the extent that data can be fitted. Both methodologies can jointly calibrate parameters pertaining to the individual models as well as a parameter pertaining to the petrophysical relation. This allows the petrophysical relation to adapt to the local conditions during the inversion. The methods are tested using a synthetic data set as well as a field data set. In combination, the two case studies show that the joint methods can constrain the inversion to achieve estimates of k, decay times, and water contents for a leaky confined aquifer system. We show that the geophysical data can assist in determining otherwise insensitive k, and vice versa. Based on our experiments and results, we mainly advocate the future application of method 2 since this seems to produce the most reliable results, has a faster inversion runtime, and is applicable also for linking k of 3-D groundwater flow models to multiple MRS soundings. [ABSTRACT FROM AUTHOR]

A key aspect of CO2 storage is the injection rate into the subsurface, which is limited by the pressure at which formation starts to fracture. Hence, it is vital to assess all of the relevant processes that may contribute to the pressure increase in the aquifer during CO2 injection. Building on an existing analytical solution for immiscible and spatially varying non-Darcy flow, this paper presents a mathematical model that accounts for combined effects of non-Darcy flow, phase miscibility, and gas compressibility in radial two-phase displacements. Results show that in low-permeability formations when CO2 is injected at high rates, non-Darcy simulations forecast better displacement efficiency compared to flow under Darcy conditions. This will have a positive effect on the formation CO2 storage capacity. This, however, comes at the cost of increased well pressures. More favorable estimations of the pressure buildup are obtained when CO2 compressibility is taken into account because reservoir pressures are reduced due to the change in the gas phase properties. Also, non-Darcy flow results in a significant reduction in halite precipitation in the near-well region, with a positive effect on CO2 injectivity. In the examples shown, non-Darcy flow conditions may lead to significantly different pressure and saturation distributions in the near-well region, with potentially important implications for CO2 injectivity. [ABSTRACT FROM AUTHOR]

Under transient flow conditions, the groundwater age distribution (GAD) is affected by both the physical characteristics of the aquifer, such as the hydraulic conductivity field, and the past temporal flow variations. Therefore, measuring the concentration of single or multiple tracers in a water sample collected at one time does not provide sufficient information on the transient nature of groundwater flows. However, obtaining multiple tracers at different times may contain such information. To find the temporal relationship between the GADs at a certain location, and to gain understanding about the current characteristics of an aquifer using environmental tracers, it is useful to separate the effects of temporal flow variations from that of the aquifer structure. In this paper, a new approach is presented, aiming at establishing a link between a reference residence time distribution based on steady-state flow and the transient age distributions at different times. Such information allows us to look collectively at sets of tracers measured at different times, when inferring aquifer structure and temporal trends in flow. The potential of using multiple tracers collected at different times to infer the GAD and its temporal variations is shown through two demonstration cases where measurements of CFCs, SF6, and 85Kr in hypothetical water samples are used to infer the overall temporal flow pattern in the aquifer. Bayesian inverse modeling via Markov Chain Monte Carlo method was used to estimate lumped-parameter groundwater ages and temporal flow patterns. [ABSTRACT FROM AUTHOR]

Based on a literature overview, this paper summarizes the impact and legacy of the contributions of Wilfried Brutsaert and Jean-Yves Parlange (Cornell University) with respect to the current state-of-the-art understanding in hydraulic groundwater theory. Forming the basis of many applications in catchment hydrology, ranging from drought flow analysis to surface water-groundwater interactions, hydraulic groundwater theory simplifies the description of water flow in unconfined riparian and perched aquifers through assumptions attributed to Dupuit and Forchheimer. Boussinesq (1877) derived a general equation to study flow dynamics of unconfined aquifers in uniformly sloping hillslopes, resulting in a remarkably accurate and applicable family of results, though often challenging to solve due to its nonlinear form. Under certain conditions, the Boussinesq equation can be solved analytically allowing compact representation of soil and geomorphological controls on unconfined aquifer storage and release dynamics. The Boussinesq equation has been extended to account for flow divergence/convergence as well as for nonuniform bedrock slope (concave/convex). The extended Boussinesq equation has been favorably compared to numerical solutions of the three-dimensional Richards equation, confirming its validity under certain geometric conditions. Analytical solutions of the linearized original and extended Boussinesq equations led to the formulation of similarity indices for baseflow recession analysis, including scaling rules, to predict the moments of baseflow response. Validation of theoretical recession parameters on real-world streamflow data is complicated due to limited measurement accuracy, changing boundary conditions, and the strong coupling between the saturated aquifer with the overlying unsaturated zone. However, recent advances are shown to have mitigated several of these issues. The extended Boussinesq equation has been successfully applied to represent baseflow dynamics in catchment-scale hydrological models, and it is currently considered to represent lateral redistribution of groundwater in land surface schemes applied in global circulation models. From the review, it is clear that Wilfried Brutsaert and Jean-Yves Parlange stimulated a body of research that has led to several fundamental discoveries and practical applications with important contributions in hydrological modeling. [ABSTRACT FROM AUTHOR]

The Hanford 300 Area is located adjacent to the Columbia River in south-central Washington State, USA, and was a former site for nuclear fuel processing operations. Waste disposal practices resulted in persistent unsaturated zone and groundwater contamination, the primary contaminant of concern being uranium. Uranium behavior at the site is intimately linked with river stage driven groundwater-river water exchange such that understanding the nature of river water intrusion into the 300 Area is critical for predicting uranium desorption and transport. In this paper, we use 2-D surface-based time-lapse electrical resistivity tomography (ERT) to image the inland intrusion of river water during high stage conditions. We inverted approximately 1200 data sets (400 per line over three lines) using high performance computing resources to produce a time-lapse sequence of changes in bulk conductivity caused by river water intrusion during the 2011 spring runoff cycle over approximately 125 days. To invert the data, we use an image differencing approach that does not require regularization in the time dimension, enabling the inversion to accommodate the sharp, time varying contrasts in conductivity imposed by the moving water table. The resulting time series for each mesh element was then analyzed using common time series analysis to reveal the timing and location of river water intrusion beneath each line. The results reveal nonuniform flows characterized by preferred flow zones where river water enters and exits quickly with stage increase and decrease, and low permeability zones with broader bulk conductivity 'break through' curves and longer river water residence times. [ABSTRACT FROM AUTHOR]

Using a first-order cross-correlation analysis, this paper investigates the relationship between observed heads and hydraulic properties in the saturated and vadose zones at different times and locations of three-dimensional unconfined aquifers during pumping tests. Cross-correlation analysis is a weighted sensitivity analysis casted into a stochastic framework. It determines the relative impact of each parameter with respect to others in time and space on the observed heads according to uncertainty or spatial variability of each parameter. It reveals the information content in measured drawdowns about heterogeneity during a pumping test in an unconfined aquifer, which is critical for aquifer parameter estimation. Based on a synthetic, numerical example, our cross-correlation analysis reveals that heads in the saturated zone at late times carry the greatest nonsymmetrically weighted information content about the hydraulic conductivity (KS) distribution within the cone of depression. On the other hand, heads in the saturated zone at early times contain the most information about the specific storage (SS) heterogeneity in a narrow region between the observation and pumping locations. During intermediate and late times, heads in the saturated zone largely reflect the effects of saturated water content (θS) and pore-size parameter (α) in the thin unsaturated region near the water table above the pumping and observation locations. At last, heads in the vadose zone at late times carry the greatest information about θS and α around the observation point. [ABSTRACT FROM AUTHOR]