Heterogeneous Aquifers Research Articles

Effects of airflow rate distribution and nitrobenzene removal in an aquifer with a low-permeability lens during surfactant-enhanced air sparging

The application of surfactant-enhanced air sparging (SEAS) in heterogeneous aquifers has received increasing attention. In this study, a two-dimensional laboratory visualization device was used to study the migration and distribution mechanism of airflow and the nitrobenzene removal effect in an aquifer with a low-permeability lens during AS and SEAS. Experimental results showed that the surfactant significantly reduced the blocking effect of the geological interface on airflow, and the ΔPe (the air entry pressure difference between the background media and the lens) value of the geological interface decreased from 1.1 kPa to 0.3 kPa when the surfactant concentration was 800 mg/L. When the surfactant injection location was at the center of the lens and the injection volume was 1 PV (pore volume of the lens), part of the airflow entered the lens through its below interface, which clearly improved the nitrobenzene removal inside and above the lens compared with AS remediation. However, when SEAS remediation was 24 h, the surfactant redistribution caused by air sparging resulted in the airflow entering the lens to bypass the lens again, which changed the spatial distribution of airflow rate and was not conducive to the continuous removal of nitrobenzene inside the lens.

VisU-HydRA: A Computational Toolbox for Groundwater Contaminant Transport to Support Risk-Based Decision Making

Obtaining accurate and deterministic predictions of the risks associated with the presence of contaminants in aquifers is an illusive goal given the presence of heterogeneity in hydrological properties and limited site characterization data. For such reasons, a probabilistic framework is needed to quantify the risks in groundwater systems. In this work, we present a computational toolbox VisU-HydRA that aims to statistically characterize and visualize metrics that are relevant in risk analysis with the ultimate goal of supporting decision making. The VisU-HydRA computational toolbox is an open-source Python package that can be linked to a series of existing codes such as MODFLOW and PAR2, a GPU-accelerated transport simulator. To illustrate the capabilities of the computational toolbox, we simulate flow and transport in a heterogeneous aquifer within a Monte Carlo framework. The computational toolbox allows to compute the probability of a contaminant’s concentration exceeding a safe threshold value as well as the uncertainty associated with the loss of resilience of the aquifer. To ensure consistency and a reproducible workflow, a step-by-step tutorial is provided and available on a GitHub repository.

Characterization of Shallow Groundwater Circulation Based on Chemical Kinetics: A Case Study of Xiong’an New Area, China

Xiong’an New Area, located in the middle of the North China Plain, will have been built as a “city of the future.” Urban planning and construction need to comprehensively consider the constraints of hydrogeological conditions such as aquifer structure and parameters. As the main aquifer in this area, the paleo-channel is heterogeneous and anisotropic, and the two-dimensional hydraulic conductivity in each horizontal direction cannot be obtained from aquifer tests. Therefore, this study adopts a chemical kinetics method to calculate the ionic activity and mineral saturation indices of shallow groundwater, determine the groundwater chemical potential field, and construct a horizontal two-dimensional groundwater chemical kinetics model. This model is used to calculate the hydraulic conductivity, flow rate and retention time of groundwater in areas of different chemical kinetics, as well as evaluate horizontal heterogeneity of the Quaternary paleo-channel aquifer. The results indicate that the groundwater chemical potential field can reflect the characteristics of the groundwater seepage field in each horizontal direction. The paleo-channel is the main channel of groundwater circulation, which shows the statistical difference of its permeability. Alluvial and lacustrine strata affect groundwater circulation due to their different hydrogeological structures and permeability. The groundwater chemical kinetics results of hydraulic conductivity along the paleo-channel are approximately consistent with traditional hydrogeological calculation results derived from aquifer test data. Hydraulic conductivity is higher in the extension direction of the paleo-channel, and lower if the path crosscuts multiple paleo-channels. This feature can be used to build a hydrogeological structure model combined with drilling data. Furthermore, excessive groundwater exploitation will change the actual flow rate and retention time of groundwater, thereby affecting the groundwater circulation conditions. This study of groundwater circulation in Xiong’an New Area by means of chemical kinetics makes up for the deficiency in the study of the unconsolidated sedimentary aquifer anisotropy within the paleo-channel.

Data and insights to advance characterization of groundwater resources in the Plaine des Gonaïves, Haiti

The Plaine des Gonaïves houses a regionally significant yet data-scarce coastal aquifer in one of the driest regions of Haiti. It is the primary source of water for the region, including Haiti’s third largest city, Gonaïves. Pressure on groundwater resources will be compounded as municipal water services modernize and energy infrastructure enables the resurgence of commercial agriculture. In anticipation of increased stress and greater impacts, this study was undertaken (including data discovery, reconnaissance, and analysis) to gain insights and to advance understanding of the groundwater resources. The alluvial aquifer covers >115 km2, exceeds 100 m thickness, and supports pumping yields as high as 532 m3/h. The heterogeneous multilayer aquifer exhibits hydraulic conductivities that range by an order of magnitude, influencing groundwater flow, residence times, and chemistry. Significant recharge occurs in the central and upgradient portions of the plain; river infiltration appears to be a more significant recharge source than direct infiltration. In the downgradient portion of the plain, confined aquifer pressure increases and discharge areas include the Quinte River, springs, wetlands, ditches and canals. Underlying the alluvium, discontinuous groundwater in bedrock with structural influence may mix with the alluvial aquifer, affecting isotopic composition and water chemistry. Increased abstraction, climate change, and intensification of agriculture and urban development could have consequential impacts on water quantity, water quality and ecosystems. The results, insights, and supporting datasets are a baseline to guide planning and to advance a hydrogeological conceptual model that supports sustainable and informed groundwater management.

Numerical Simulation of the Enrichment of Chemotactic Bacteria in Oil-Water Two-Phase Transfer Fields of Heterogeneous Porous Media

Oil pollution in soil-groundwater systems is difficult to remove, and a large amount of residual oil is trapped in the low permeable layer of the heterogeneous aquifer. Aromatic hydrocarbons in oil have high toxicity and low solubility in water, which are harmful to the ecological environment. Chemotactic degrading bacteria can perceive the concentration gradient of non-aqueous phase liquid (NAPL) pollutants in the groundwater environment, and enrich and proliferate around the pollutants, thus achieving a more efficient and thorough remediation effect. However, the existing theoretical models are relatively simple. The physical fields of oil–water two-phase flow and oil-phase solute convection and diffusion in water are not coupled, which further restricts the accuracy of studies on bacterial chemotaxis to NAPL. In this study, geometric models based on the actual microfluidic experimental study were constructed. Based on the phase field model, diffusion convection equation and chemotaxis velocity equation, the effects of heterogeneity of porous media, wall wettability and groundwater flow rate on the residual oil and the concentration distribution of chemotaxis bacteria were studied. Under all of the simulation conditions, the residual oil in the high permeable area was significantly lower than that in the low permeable area, and the wall hydrophilicity enhanced the water flooding effect. Chemotactic bacteria could react to the concentration gradient of pollutants dissolved into water in the oil phase, and enrich near the oil–water interface with high concentration of NAPL, and the density of chemotactic bacteria at the oil–water interface can be up to 1.8–2 times higher than that in the water phase at flow rates from 1.13 to 6.78 m/d.

Spatial correlation analysis between hydraulic conductivity and specific storage in a heterogeneous sandbox by hydraulic tomography

Hydraulic conductivity (K) and specific storage (Ss) are closely related to porosity, the compressibility of aquifer skeleton, connectivity of pores, sorting of the grains, and many others. These well-known relationships promote the investigation of the correlation between spatially varying K and Ss values and their sensitivity to the overlying materials stresses in heterogeneous aquifers. However, the costs of the field experiments for this investigation limit us to a laboratory heterogeneous sandbox aquifer. Before the laboratory experiments, we conducted numerical experiments to verify our tools and approaches. Specifically, we tested the ability of the recently developed hydraulic tomography (HT) for mapping K and Ss fields in two cases. One case in which Ss field was correlated with the K field, and the other was uncorrelated. With the assurance of the ability of our approaches, the laboratory experiments mimicking the numerical ones followed. Accurate drawdown predictions of different flow events in the laboratory experiments further guarantee the reliability of the estimated detailed K and Ss fields. The estimated K and Ss fields reveal that the Ss field's variability is much less significant than K and has much longer correlation scales. Further, the K distribution is closely related to the distribution of sand grain sizes in the sandbox, while Ss is not. Moreover, Ss estimates show a decreasing trend with depth. This trend does not exist in K estimates, indicating that Ss is highly sensitive to overburden stresses and K is not. Lastly, there is no clear spatial correlation between K and Ss, and the variability of Ss is critical in predicting the transient behavior of groundwater flow.

Numerical simulation of aquifer thermal energy storage using surface-based geologic modelling and dynamic mesh optimisation

Aquifer thermal energy storage (ATES) has significant potential to provide largescale seasonal cooling and heating in the built environment, offering a low-carbon alternative to fossil fuels. To deliver safe and sustainable ATES deployments, accurate numerical modelling tools must be used to predict flow and heat transport in the targeted aquifers. This paper presents a simulation methodology for ATES based on surface-based geologic modelling (SBGM) and dynamic mesh optimisation (DMO). DMO has been previously applied in other fields of computational fluid dynamics to reduce the cost of numerical simulations. DMO allows the resolution of the mesh to vary during a simulation to satisfy a user-defined solution precision for selected fields, refining where the solution fields are complex and coarsening elsewhere. SBGM allows accurate representation of complex geological heterogeneity and efficient application of DMO. The paper reports the first systematic convergence study for ATES simulations, and demonstrates the application of these methods in two ATES scenarios: a homogeneous aquifer, and a realistic heterogeneous fluvial aquifer containing meandering, channelised sand bodies separated by mudstones. It is demonstrated that DMO reduces the required number of mesh elements by a factor of up to 22 and simulation time by a factor of up to 15, whilst maintaining the same accuracy as an equivalent fixed mesh. DMO offers significant potential to reduce the computational cost of ATES simulations in both homogeneous and heterogeneous aquifers.

Estimation of hydraulic conductivity and porosity of a heterogeneous porous aquifer by combining transition probability geostatistical simulation, geophysical survey, and pumping test data

Estimation of hydraulic conductivity and porosity of a heterogeneous porous aquifer by combining transition probability geostatistical simulation, geophysical survey, and pumping test data

Detection of Tracer Plumes Using Full‐Waveform Inversion of Time‐Lapse Ground Penetrating Radar Data: A Numerical Study in a High‐Resolution Aquifer Model

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

Impacts of Heterogeneity on Aquifer Storage and Recovery in Saline Aquifers

AbstractAquifer storage and recovery (ASR) involves the injection, and later extraction, of freshwater into aquifers, which often contain saline groundwater. Mixing between the fresh injectant and native saltwater often leads to part of the injectant becoming unrecoverable, thereby impacting ASR performance. This study explores freshwater‐saltwater mixing within ASR operations arising from aquifer heterogeneity using Monte Carlo analysis of 2D‐axisymmetric, density‐dependent flow and transport models. Logarithmic hydraulic conductivity (lnK) distributions are generated using either two‐point statistics or higher‐order connectivity features. Results show that higher variance in lnK leads to stronger freshwater‐saltwater mixing, which lowers the recovery efficiency (RE; i.e., ratio of extracted to injected freshwater). On average, the lowest RE values were obtained from lnK fields with connected high‐K features, across all ASR cycles. In contrast, RE values from lnK fields with connected low‐K features were typically the highest, at least where buoyancy was considered. The impact of aquifer heterogeneity on RE reduces with subsequent ASR cycles. Buoyancy was a major factor in lowering RE regardless of the adopted heterogeneity model. Heterogeneity tended to mitigate the adverse impacts of buoyancy, leading to some heterogeneous cases having higher RE values than the corresponding homogeneous case. These results highlight the importance of understanding buoyancy effects and subsurface heterogeneity (including the connectivity of geological structures), and interrelationships thereof when assessing the feasibility of multi‐cycle ASR in heterogeneous saline aquifers.

Application of hydrochemical and isotopic data to determine the origin and circulation conditions of karst groundwater in an alpine and gorge region in the Qinghai–Xizang Plateau: a case study of Genie Mountain

Complex structural systems, lithological differences, and extreme heterogeneity of aquifers combine to create a complex karst aquifer structure in alpine and gorge areas; however, because of the topography, direct investigation of aquifer structure is difficult. In this study, field survey, hydrochemical, and isotopic data are analyzed to reveal the development of karst groundwater and to describe the karst water cycle in Genie Mountain, Qinghai–Xizang Plateau. The results show that groundwater circulation is mainly controlled by active fracture. Atmospheric precipitation and melting ice and snow are the groundwater recharge sources, and recharge areas are mostly located in high mountains above 4500 m a.s.l. The direction of groundwater flow is mostly controlled by the Jinshajiang active Fault, with drainage areas at the intersection of multiple faults. There are two regional karst water runoff conduits. One is along the Edexi-Hongjunshan Fault, where groundwater runs from south to north; the other is along Gangtonglong Fault, where groundwater runs from north to south, and is discharged at Gangtonglong gully. The groundwater cycle can be divided into three levels: epikarst water circulation; mid to deep karst water circulation; and deep geothermal water circulation. The karst springs located in the outlet of the Huolong gully contain markedly higher levels of Na+ and SO42− than other karst springs because of the leaching effect of groundwater on mirabilite. The presence of evaporites in Huolong gully indicates that the plateau planation formed by the structural uplift will change the local climatic conditions, and then affect the groundwater circulation process and the water–rock reaction in the process.

Exploring Predictive Uncertainty at a Double‐Source Managed Aquifer Recharge Site via Stochastic Modeling

AbstractIn Menashe Streams managed aquifer recharge (MAR) site (Israel) desalinated seawater (DSW) is recharged since 2015, alongside ephemeral stream flows. This study quantifies the uncertainty of predictions of the mixing of these two water sources in the aquifer. Mixing estimations are based on the significant difference in the content of stable water isotopes between the water sources. Uncertainties driven from aquifer heterogeneity and climate variability are compared. We use realistic flow and isotope‐transport models in a multiple‐realization stochastic approach considering space and time for the two drivers of uncertainty. Predictive uncertainty is evaluated via comparison of the temporal coefficient of variation of four realization ensembles. Results show that the impact of subsurface structure uncertainty on the predictions is small, compared to the uncertainty resulting from variability related to future hydrometeorological conditions. A generalized conclusion from this result is the difficulty to make long‐term predictions of the mixing ratios, regardless of the level of certainty in the subsurface structure interpretation, when one source of recharge water has significant annual fluctuations. Implication on prediction of magnesium concentration is demonstrated as an example for prediction uncertainty of concentration of a solute of interest in one of the MAR sources. Furthermore, we show that considering a single source MAR site, uncertainty in mixing of the MAR water and native groundwater in wells upgradient of the recharge location is higher than prediction uncertainty in wells located downgradient. Insights are also drawn regarding the change in uncertainty with distance from the recharge pond of the DSW.

Flow Cytometry and Fecal Indicator Bacteria Analyses for Fingerprinting Microbial Pollution in Karst Aquifer Systems.

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

Intra aquifer variations in pesticide sorption during a field injection experiment

A field injection experiment was performed in an anoxic sandy aquifer over 6 days to assess sorption characteristics of 7 commonly applied pesticides in agriculture and 2 frequently detected metabolites. Pesticide use changed considerably in the last decades, and there is insufficient knowledge of the fate of currently used pesticides in aquifers. Injected water arrival was monitored at 6 depth intervals of 1 m ranging from 11.4 to 32.2 m-below surface level with varying organic carbon contents (0.057–0.91%d.w.) to examine intra-aquifer variations in sorption. Observed pesticide concentrations were fit using a non-linear least squares routine to an advection-dispersion equation, from which retardation factors (R) were obtained. Pesticide degradation did not significantly influence the simulated R during the experiment. We observed that bentazon and cycloxydim were most mobile with R < 1.1 at all depths. Desphenyl chloridazon, methyl desphenyl chloridazon, and imidacloprid were, on average, less mobile, with maximum R of 1.5. Boscalid, chloridazon, fluopyram, and flutolanil showed a larger range of R, and R > 2.0 were observed in the shallowest part of the aquifer. Largest R were observed at the top of the aquifer and decreased with depth. Koc values varied similarly, which indicates that sorption is not only influenced by sedimentary organic matter (SOM) content but also by its sorption reactivity. Obtained sorption parameters were substantially lower than reported in a widely used pesticide sorption database, which suggests that sorption parameters are influenced by methodological differences and variations in the sorption reactivity of SOM. The large intra-aquifer variations in pesticide sorption highlights that aquifer heterogeneity should be considered in groundwater risk assessments.

Investigating the migration of immiscible contaminant fluid flow in homogeneous and heterogeneous aquifers with high-precision numerical simulations.

Numerical modeling of the migration of three-phase immiscible fluid flow in variably saturated zones is challenging due to the different behavior of the system between unsaturated and saturated zones. This behavior results in the use of different numerical methods for the numerical simulation of the fluid flow depending on whether it is in the unsaturated or saturated zones. This paper shows that using a high-resolution shock-capturing conservative method to resolve the nonlinear governing coupled partial differential equations of a three-phase immiscible fluid flow allows the numerical simulation of the system through both zones providing a unitary vision (and resolution) of the migration of an immiscible contaminant problem within a porous medium. In particular, using different initial scenarios (including impermeable “lenses” in heterogeneous aquifers), three-dimensional numerical simulation results are presented on the temporal evolution of the contaminant migration following the saturation profiles of the three-phases fluids flow in variably saturated zones. It is considered either light nonaqueous phase liquid with a density less than the water, or dense nonaqueous phase liquid, which has densities greater than the water initially released in unsaturated dry soil. Our study shows that the fate of the migration of immiscible contaminants in variably saturated zones can be accurately described, using a unique mathematical conservative model, with different evolution depending on the value of the system’s physical parameters, including the contaminant density, and accurately tracking the evolution of the sharp (shock) contaminant front.

An integrated inversion framework for heterogeneous aquifer structure identification with single-sample generative adversarial network

Generating reasonable heterogeneous aquifer structures is essential for understanding the physicochemical processes controlling groundwater flow and solute transport better. The inversion process of aquifer structure identification is usually time-consuming. This study develops an integrated inversion framework, which combines the geological single-sample generative adversarial network (GeoSinGAN), the deep octave convolution dense residual network (DOCRN), and the iterative local updating ensemble smoother (ILUES), named GeoSinGAN-DOCRN-ILUES, for more efficiently generating heterogeneous aquifer structures. The performance of the integrated framework is illustrated by two synthetic contaminant experiments. We show that GeoSinGAN can generate heterogeneous aquifer structures with geostatistical characteristics similar to those of the training sample, while its training time is at least 10 times faster than that of typical approaches (e.g., multi-sample-based GAN). The octave convolution layer and multi-residual connection enable the DOCRN to map the heterogeneity structures to the state variable fields (e.g., hydraulic head, concentration distributions) while reducing the computational cost. The results show that the integrated inversion framework of GeoSinGAN and DOCRN can effectively and reasonably generate the heterogeneous aquifer structures.

Aquifer heterogeneity controls to quality monitoring network performance for the protection of groundwater production wells

A groundwater monitoring network surrounding a pumping well (such as a public water supply) allows for early contaminant detection and mitigation where possible contaminant source locations are often unknown. This numerical study investigates how the contaminant detection probability of a hypothetical sentinel-well monitoring network consisting of one to four monitoring wells is affected by aquifer spatial heterogeneity and dispersion characteristics, where the contaminant source location is randomized. This is achieved through a stochastic framework using a Monte Carlo approach. A single production well is considered that results in converging non-uniform flow close to the well. Optimal network arrangements are obtained by maximizing a weighted risk function that considers true and false positive detection rates, sampling frequency, early detection, and contaminant travel time uncertainty. Aquifer dispersivity is found to be the dominant parameter for the quantification of network performance. For the range of parameters considered, a single monitoring well screening the full aquifer thickness is expected to correctly and timely identify at least 12% of all incidents resulting in contaminants reaching the production well. This proportion increases to a global maximum of 96% for a network consisting of four wells and very dispersive transport conditions. Irrespective of network size and sampling frequency, more dispersive transport conditions result in higher detection rates. Increasing aquifer heterogeneity and decreasing aquifer spatial continuity also lead to higher detection rates, though these effects are diminished for networks of 3 or more wells. Statistical anisotropy has no effect on the network performance. Earlier detection, which is critical for remedial action and supply safety, comes with a significant cost in terms of detection rate, and should be carefully considered when a monitoring network is being designed.

Modelling saline intrusion using dynamic mesh optimization with parallel processing

Saline intrusion (SI) in coastal aquifers is a global problem with the potential to contaminate groundwater used by over a billion people. Numerical modelling of SI in coastal aquifers is a key tool for risk assessment, aquifer management and resource regulation, but is extremely challenging because the mixing zone across the saline front is often very narrow, extending over metres or 10’s metres, yet the saline front itself may extend laterally over a large (i.e. many km) three-dimensional (3D) domain. Moreover, the aquifer may be highly heterogeneous, further complicating the movement and geometry of the front. We test here the use of dynamic mesh optimization (DMO) in a parallel computational framework to simulate SI with higher accuracy and lower computational cost compared to fixed-mesh approaches. The framework uses a double control-volume-finite-element (DCVFE) method and is implemented in the open-source Imperial College Finite Element Reservoir SimulaTor (IC-FERST), but could be implemented in other FE-based simulators. We confirm accuracy and convergence using test cases based on the classic ’Henry’ SI problem, demonstrating that solutions obtained using DMO require significantly fewer elements and therefore have much lower computational cost compared to equivalent fixed mesh solutions. We apply the framework to a realistic 3D case study simulating saline intrusion in a heterogeneous chalk aquifer, demonstrating simulation speed-up in excess of 120×. We suggest that parallelized DMO offers significant advantages over existing methods to simulate SI.

Impact of Withdrawals on Karst Watershed Water Supply

Karst systems consist of heterogeneous aquifers characterized by non-linear hydrogeological behavior. This is intrinsically linked to the coexistence of saturated versus unsaturated, open-channel versus closed-conduit flow and laminar versus turbulent flows within these aquifers. These multiple dualities together with a lack of knowledge of their internal structure lead to increasing difficulties in the management of groundwater resources related to karst aquifers. However, karst aquifers constitute strategic fresh water resources and many stakeholders carry out withdrawals sometimes continuously or centered on the period of low water level as for the irrigation. These withdrawals generate discharge decreases that can constitute sources of conflicts between upstream and downstream users of the resource. In this study, we propose a methodology to assess the impact of withdrawals on the spring discharge of a karst aquifer based on a conceptual non-linear reservoirs model. This methodology is applied to the second largest karst system in France: The Touvre karst system (La Rochefoucauld). The simulated influence of the total withdrawals on runoff deficit is larger than the total withdrawals in relationship with the non-linearity of the conceptual models. Globally, this impact is about one and a half of the total withdrawal depending of the year under consideration. Moreover, we show that it also implies a decrease from 10% to 20% of the low-flow annual discharge. The next step will be to also address the issues raised by the climate change projections.

The coastal aquifer recovery subject to storm surge: Effects of connected heterogeneity, physical barrier and surge frequency

Storm surge, a worldwide phenomenon triggering the vertical saltwater infiltration, is likely to exacerbate coastal groundwater salinization due to geologic heterogeneity, anthropogenic engineering and climate change. This study analyzed the combined effects of connected heterogeneity, physical barrier and surge frequency on the coastal aquifer recovery. A series of modeling cases were investigated using HydroGeoSphere in the heterogeneous and equivalent homogeneous aquifer. The heterogeneity setting is composed of different connectivity level of hydraulic conductivity field. The simulation results of single storm surge event demonstrate that the connected heterogeneity elevates the salinized extent and reduces the aquifer recovery time due to a number of preferential flow paths. In comparison to the equivalent homogeneous aquifer, heterogeneity alleviates the maximum salinized extent and vertical intrusion distance due to the accelerated mixing of salinized groundwater with fresh groundwater. Physical barrier, classified as subsurface dam and cutoff wall, leading to different groundwater discharge pattern is tailored to investigate the influences of the permanent subsurface engineering on the aquifer recovery. Our results show that the connectivity level controls the salinization pattern subject to physical barrier. Then, the repetitive storm surge events were simulated to investigate the effects of surge frequency. For the low-frequency surge event, the variation of salinized metric is the repetition of the unimodal curve in the single surge event. Nevertheless, for the high-frequency surge event, the residual salt mass cannot be flushed out over the simulation period, especially for the low-connectivity aquifer. Meanwhile, the high-frequency surge event broadens the differences of aquifer recovery process due to physical barrier. These findings have critical implications for coastal groundwater management which is facing the substantial environmental risks of surge-induced vertical saltwater intrusion derived from geologic heterogeneity and climate changes.