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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.

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]

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]

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]

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]

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]

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]

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.

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]

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]

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]

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]

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]

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 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 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]

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]

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]

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]