Heterogeneous Aquifers Research Articles

Computation of submarine groundwater discharge from geomorphologically different coastal catchments of SW India using numerical modeling

In this study, we investigate submarine groundwater discharge (SGD) and the influence which coastal geomorphology and geology can have on its behavior. Numerical models have been developed to estimate the magnitude of submarine groundwater discharge (SGD) from catchments with different geomorphology on the Southwest coast of India. Two coastal catchments of Thiruvananthapuram district, Kerala, India namely Varkala (184 km2) and Kadinamkulam (186 km2), have been selected to compute SGD. The results of the study confirmed that the SGD flux from the catchments differs in a great way across its multiple layers as well as in comparison to one another. The fresh SGD flux from Varkala catchment is found to be 0.1 m3/day/m and from Kadinamkulam catchment is 6 m3/m/day. In the present study, it is concluded that SGD is reflected in the heterogeneity of aquifers owing to the differences in geomorphology, land use-land cover, hydrogeology, and lithology. A numerical modeling approach that can yield reliable results with low computational cost and time is a potent method for quantifying SGD. The inferences drawn will be useful for designing aquifer recharge options and optimum pumping quantities for the region.

Stochastic nitrate simulation under hydraulic conductivity uncertainty of an agricultural basin aquifer at Eastern Thessaly, Greece.

Lake Karla Watershed is an agricultural basin characterized by intense agricultural activities, which lead to quantitative and qualitative aquifer degradation. The exploitation of non-renewable groundwater resources and nitrate contamination are the major threats to aquifer sustainability. Groundwater resources cover mainly irrigation and domestic water needs. Hence, the simulation of groundwater resources is necessary to (a) determine the quantity available for water supply and (b) estimate probable nitrate contamination to propose subsequent remediation techniques to protect public health. Furthermore, the aquifer's heterogeneity as well as the lack of hydraulic conductivity data, in a large-scale study area, creates uncertainty regarding groundwater flow and nitrate transport simulation. Deterministic modelling approaches for spatially distributed nitrate concentration simulation could not estimate the contamination risk, since it can address only one realization of the aquifer. This study estimates the effect of hydraulic conductivity uncertainty on the simulation of groundwater nitrate concentration. The proposed framework uses Geostatistical Sequential Gaussian Simulation (SGSIM) for the generation of equally probable realizations of the spatial distribution of hydraulic conductivity. It includes the application of a modelling system based on the following inter-linked models: a rainfall-runoff model, a reservoir operation model, a lake-aquifer interaction model, a groundwater flow model, and a nitrate transport and dispersion model. The last two models simulate the multiple realizations of aquifer's groundwater flow and nitrate concentration. Furthermore, two analyses (a statistical and a threshold analysis) are employed to estimate the exceedance probability of the nitrate concentrations and their spatial extent. The results indicate that hydraulic conductivity uncertainty does affect the simulation of nitrate concentration and that nitrate concentrations will most probably exceed the thresholds in areas where groundwater is extracted for domestic use.

Including Vertical Fault Structures in Layered Groundwater Flow Models.

Accurate representation of groundwater flow and solute transport requires a sound representation of the underlying geometry of aquifers. Faults can have a significant influence on the structure and connectivity of aquifers, which may allow permeable units to connect, and aquifers to seal when juxtaposed against lower permeability units. Robust representation of groundwater flow around faults remains challenging despite the significance of faults for flow and transport. We present a methodology for the inclusion of faults utilizing the unstructured grid features of MODFLOW-USG and MODFLOW 6. The method focuses on the representation of fault geometries using non-neighbor connections between juxtaposed layers. We present an illustration of the method for a synthetic fluvial aquifer. The combined impact of the heterogeneous aquifer and fault offset is clearly visible where channel features at different depths in the aquifer were connected at the fault. These results highlight the importance of representing fault features in groundwater flow models.

Drainage Capillary Pressure Distribution and Fluid Displacement in a Heterogeneous Laminated Sandstone

AbstractWe applied three‐dimensional X‐ray microtomography to image a capillary drainage process (0–1,000 kPa) in a cm‐scale heterogeneous laminated sandstone containing three distinct regions with different pore sizes to study the capillary pressure. We used differential imaging to distinguish solid, macropore, and five levels of subresolution pore phases associated with each region. The brine saturation distribution was computed based on average CT values. The nonwetting phase displaced the wetting phase in order of pore size and connectivity. The drainage capillary pressure in the highly heterogeneous rock was dependent on the capillary pressures in the individual regions as well as distance to the boundary between regions. The complex capillary pressure distribution has important implications for accurate water saturation estimation, gas and/or oil migration and the capillary rise of water in heterogeneous aquifers.

How to Find Aquifer Statistics Utilizing Pumping Tests? Two Field Studies Using welltestpy.

We present a workflow to estimate geostatistical aquifer parameters from pumping test data using the Python package welltestpy. The procedure of pumping test analysis is exemplified for two data sets from the Horkheimer Insel site and from the Lauswiesen site, Germany. The analysis is based on a semi-analytical drawdown solution from the upscaling approach Radial Coarse Graining, which enables to infer log-transmissivity variance and horizontal correlation length, beside mean transmissivity, and storativity, from pumping test data. We estimate these parameters of aquifer heterogeneity from type-curve analysis and determine their sensitivity. This procedure, implemented in welltestpy, is a template for analyzing any pumping test. It goes beyond the possibilities of standard methods, for example, based on Theis' equation, which are limited to mean transmissivity and storativity. A sensitivity study showed the impact of observation well positions on the parameter estimation quality. The insights of this study help to optimize future test setups for geostatistical aquifer analysis and provides guidance for investigating pumping tests with regard to aquifer statistics using the open-source software package welltestpy.

Spatio-temporal estimation of monthly groundwater levels from GPS-based land deformation

Significant subsidence is susceptible to groundwater level variations in aquifer systems. The relation between groundwater level change and global positioning system (GPS) estimated subsidence is spatially variable. Time-dependent spatial regression can be used for the estimation of groundwater level changes using GPS based deformation data. Furthermore, the model can be validated using observed hydraulic head data from available monitoring stations. This study uses GPS station data to estimate the monthly groundwater levels in the west-central Taiwan for the period: 2016–17.Time-dependent spatial regression provides a more realistic estimation of groundwater level changes in response to highly heterogeneous aquifer properties than other methods. The high correlation (r = 0.95) between observed and estimated groundwater levels shows that GPS estimated deformations represent an alternative approach for estimating seasonal groundwater changes. Due to availability of spatially broad/low cost GPS data (compared to the sparse availability groundwater monitoring stations), the use of GPS data represents a powerful solution for future monitoring of estimated seasonal groundwater level changes in areas where only few groundwater observations are available.

Integrating deep learning-based data assimilation and hydrogeophysical data for improved monitoring of DNAPL source zones during remediation

Data assimilation techniques allow the integration of multi-source data with physical modeling of dense nonaqueous phase liquid (DNAPL) source zones to monitor their evolution in time and thereby improve remediation efforts. The DNAPL source zone is governed by multiphase physics and its architecture is usually highly irregular and nonstationary. Traditional data assimilation methods use stationary models to describe these nonstationary DNAPL saturation fields, leading to significant errors in the estimation of unknown source zone architectures (SZAs). In this study, we propose a geostatistical data assimilation method to improve performance monitoring of DNAPL remediation. We estimate the DNAPL SZA after parameterizing the DNAPL saturation field using a deep learning method instead of a stationary statistical model. First, we train a convolutional variational autoencoder (CVAE) network to capture the physics of DNAPL infiltration and depletion. Then, we incorporate the CVAE network within the Ensemble Kalman filter (EnKF) to resolve the evolving DNAPL SZA by assimilating time-lapse hydrogeophysical measurements (i.e., oscillatory hydraulic tomography, downgradient dissolved DNAPL concentration, and electrical resistivity tomography). To assess the proposed CVAE-EnKF framework's performance, we conducted numerical experiments with evolving DNAPL SZAs in a 2D heterogeneous aquifer. Results show that the proposed framework significantly improved DNAPL saturation estimates throughout the source zone over time, compared to the standard EnKF method. CVAE-EnKF reduced the estimation error of DNAPL mass remediated by 51% from EnKF, along with better predictions of source zone longevity. It also better captured the morphology and center/spread of mass of the evolving remediated portions of the source zone and provided better estimates of pooled-DNAPL mass. Furthermore, the CVAE-EnKF method is more stable and converges faster than EnKF alone. Overall, the proposed data assimilation framework can improve real-time monitoring of DNAPL remediation and risk analysis.

Contaminant transport in heterogeneous aquifers: A critical review of mechanisms and numerical methods of non-Fickian dispersion

Contaminant transport in heterogeneous aquifers: A critical review of mechanisms and numerical methods of non-Fickian dispersion

Improving our ability to model crystalline aquifers using field data combined with a regionalized approach for estimating the hydraulic conductivity field

Modelling of heterogeneous aquifers, such as crystalline aquifers, is often difficult and, flow and transport predictions are always uncertain, suffering of our imperfect knowledge of the spatial distribution of aquifer parameters. This paper aims to test the robustness of a first-order hydraulic conductivity map estimated from both a detailed water-table map and hydraulic-conductivity statistics on a granitic watershed in Brittany (57 km2), compare it to local estimates and assess it through various numerical models. Map values range on four orders of magnitude (10-7 to 10-4 m/s, mean: 3.9x10-6 m/s) and their comparison to local estimates from various sources (pumping tests, MRS measurements, streamflow) gives satisfactory results.Four hydraulic conductivity fields were assessed through numerical modelling under steady-state condition. Model 1 used the regionalized hydraulic conductivity field directly, Model 2 used a uniform value (average of Model 1), Model 3 used a hydraulic conductivity field obtained by inverse modelling of the water table and Model 4 used two zones of uniform values based on the analysis of Model 1 and Model 3 fields. Model results were analysed based on their ability to reproduce the observed water-table levels and the groundwater flow directions. Modelled groundwater discharges to streams at sub-catchment scale were compared to spatial streamflow measurements performed during low water condition, which help validating models. The comparison between the fields obtained from Model 3 and that from the regionalized method (Model 1) shows that they are close in terms of mean values and spatial distribution. Model 1 reproduces rather well the water-table map and the groundwater flow directions. Model 2 shows the less good results. Model 4 has led to satisfactory results and shows that the hydraulic conductivity is higher (2.1x10-6 m/s) where the water table is located in the fractured zone, and lower (3.3x10-7 m/s) where it is located in the saprolites (highly weathered rock), which is expected for such aquifer system. Modelled groundwater discharges to streams are comparable in all models to streamflow measurements in most sub-catchments, but the models overestimate them in certain places, mainly because of sub-surface drains in a forest capturing part of the groundwater that can no longer return to streams (drains were not considered in the models).In addition, experiment on a second watershed (40 km2) shows how with much less field data the methodology can already provide interesting information on the hydraulic conductivity field (values and spatial distribution).Results are very encouraging and open up prospects for using quantitative and qualitative information from the mapping of hydraulic conductivity to constrain the spatialization of hydrodynamic parameters on models and thus our ability to model such complex aquifers.

Laboratory testing of real-time flux measurements in fractured media

The In-Well Point Velocity Probe (IWPVP) was originally developed for use in screened porous media wells. The tool can characterize small-scale aquifer heterogeneity from wells that have been carefully developed to ensure an open screen and good hydraulic connection with the surrounding aquifer. This capability led to the hypothesis that the IWPVP could be adapted to characterize flow in fractured media, which features small-scale openings (i.e. fractures) that resemble screen slots. To test this hypothesis, a lab-scale fracture flow apparatus (FFA) was fabricated. The FFA consisted of two parallel acrylic plates that could be separated by a fixed distance, simulating a fracture with a known aperture. Two lengths of PVC well casing served as a cylindrical access port and were centrally placed in the plates. This mock-well left the interface with the fracture open – analogous to a fracture intersecting an uncased borehole. The probe was calibrated experimentally by relating known fluxes through the FFA with those measured inside the IWPVP. The ratio of these fluxes was found to be predictable for two aperture sizes (500 μm and 1290 μm), based on a flux balance and the Cubic Law. Additionally, the ability of the probe to discern flow direction in a fracture was tested and found to be accurate within ±20° under the conditions of the laboratory testing. The results suggest that an IWPVP could be reliably used in fractured media to directly measure horizontal flux and flow direction in a single fracture without reference to Darcy’s Law.

Geological and hydrogeological assessment of the Brito Formation: Municipio de Tola, Nicaragua

There are sparse hydrogeological data and insufficient hydrogeological knowledge in many areas of the world reliant on groundwater. Nicaragua’s Pacific coast is one such region that is also experiencing water scarcity resulting from increasing demand on groundwater resources and climate change. The primary source of water in the region is the aquifer system associated with the Brito Formation, which is a marine sedimentary stratum of mostly sandstone that blankets 75 km of coastline in southwest Nicaragua. This study focused on the Tola municipality with the objective to advance a conceptual understanding of the hydrogeology and to support sustainable water development. Results demonstrate a heterogeneous aquifer system with regional flow characteristics and other factors that influence groundwater availability and water quality. Primary porosity is low, and secondary porosity is the primary mechanism of aquifer storage and is influenced by geological structure and diagenesis processes. Groundwater recharge is spatially and temporally heterogeneous and direct recharge is low. Infiltration of streamflow and runoff, especially early in the rainy season, is thought to be a large component of groundwater recharge. Climate, flow and recharge dynamics, and low storage capacity make the Brito Formation a sensitive resource and vulnerable to drought, increased abstraction, and climate change. This assessment provides data and insights useful for informing future studies and investments within the region and may be applicable in other Central American and Caribbean nations with coastal sandstone aquifers.

Depth–Sequential Investigation of Major Ions, δ18O, δ2H and δ13C in Fractured Aquifers of the St. Lawrence Lowlands (Quebec, Canada) Using Passive Samplers

General and isotopic geochemistry of groundwater is an essential tool to decipher hydrogeological contexts and flow paths. Different hydrogeochemical patterns may result from the inherent physical aquifer heterogeneity, which may go unnoticed without detailed investigations gathered from multilevel or multiple observation wells. An alternative to overcome the frequent unavailability of multiple wellbores at sites is to perform a detailed investigation on the single wellbore available. In this perspective, the aim of this study is to use passive samplers to sequentially collect groundwater at depths in long–screened wellbores. Such investigation is carried out for major ions and stable isotopes compositions (δ2H, δ18O, δ13C) at ten sites in the context of fractured carbonate aquifers of the St. Lawrence Lowlands (Quebec, Canada). The information gathered from the calco–carbonic system, major ions and stable isotopes report poorly stratified and evolved groundwater bodies. Contribution of water impacted by anthropogenic activities, such as road salts pollution and carbon sources from C4 vegetation, when they occur, are even observed at the greatest depths. Such observations suggest quick flow paths and efficient mixing conditions, which leads to significant contributions of contemporary groundwater bodies in the fractured aquifers investigated down to depths of about 100 m. Although physical aquifer investigation reported few and heterogeneously distributed fractures per wellbore, hydrogeochemical findings point to at overall well interconnected fracture networks in the aquifer and high vulnerability of groundwater, even at significant depths.

Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography

With the growing population and the adverse effects of climate change, the pressure on coastal aquifers is increasing, leading to a larger risk of saltwater intrusion (SI). SI is often complex and difficult to characterize from well data only. In this context, electrical resistivity tomography (ERT) can provide high-resolution qualitative information on the lateral and vertical distribution of salinity. However, the quantitative interpretation of ERT remains difficult because of the uncertainty of petrophysical relationships, the limitations of inversion, and the heterogeneity of aquifers. In this contribution, we propose a methodology for the semiquantitative interpretation of ERT when colocated well data are not available. We first use existing wells to identify freshwater zones and characterize the resistivity response of clayey deposits. Then, we approximate the formation factor from water samples collected in the vicinity of ERT data to derive a resistivity threshold to interpret the saline boundary. We applied the methodology in the shallow aquifers of the Luy River in the Binh Thuan province, Vietnam, where water resources are under pressure due to agricultural, aquacultural, and industrial production. Twenty-one ERT profiles were collected and revealed a much larger intrusion zone, compared to the previous study. Saltwater is present in lowland areas of the left bank over almost the whole thickness of the aquifer, while the right bank is constituted of sand dunes that are filled with freshwater. At a larger distance from the sea, a complex distribution between fresh and saltwater is observed. Our methodology could be applied to other heterogeneous aquifers in the absence of a dense monitoring network.

Remediation scenario of the abandoned Kettara mine site (Morocco): acid mine drainage (AMD) transport modeling

In this study, we present the efficiency of remediation scenario to attenuate the impact of acid mine drainage (AMD) contamination in the Kettara abandoned mine site. The study focuses on the AMD groundwater contamination of the Sarhlef shists aquifer. To predict the evolution of AMD groundwater contamination in the Kettara mine site under remediation scenario, a model of groundwater flow and AMD transport was performed.Piezometric heads were measured at the dry and wet periods from eleven wells located downstream of mine wastes. To elaborate a conceptual groundwater flow model, we faced with to the heterogeneity and anisotropy of fractured Sarhlef shists aquifer. Consequently, the study focused on the use of various approaches: 1. The inverse modeling by the CMA-ES algorithm is adopted as an alternative approach to determine hydraulic parameters indirectly, and 2. the model is treated as an equivalent porous media (EPM). The groundwater flow model was carried out in steady-state and transient conditions in the dry and wet periods using the PMWIN interface. The obtained results are satisfactory and show an excellent correlation between measured and computed heads. Contaminant transport model is used to solve the advection–dispersion equation and to generate the AMD concentration by MT3D via the PMWIN interface. A sensitivity analysis of the dispersivity coefficient is carried out. The AMD transport simulation was computed during periods of 1, 5 and 10 years, and the performed model indicates that the simulated concentrations under remediation scenario are reduced 1000 times comparing to the current concentrations. The study revealed a necessary approach in addressing an environmental issue for the AMD contamination. The results of the study will be a start-up for further research work in the study area and implementing it for the prevention of AMD propagation plume.

Impact of multiple uncertainties on gravimetric variations across randomly heterogeneous aquifers during pumping

We evaluate the relative importance of the uncertainty related to parameters characterizing partially saturated groundwater flow on head and gravity changes associated with pumping tests taking place in homogeneous and heterogeneous porous media. We frame our study in a Global Sensitivity Analysis setting and assess the way imperfect knowledge of such parameters influences the probability distribution (pdf) of head and gravimetric variations recorded during well operation. We rely on a set of detailed computational analyses and conceptualize uncertain model parameters as random quantities. Randomly heterogeneous domains are treated by considering main geostatistical descriptors (i.e., variance and correlation scale) of three-dimensional spatial distributions of system properties as affected by uncertainty. We quantify the effects of the latter on the resulting pdf of (ensemble) mean and variance of head and gravity changes through a numerical Monte Carlo approach. While all uncertain parameters are influential to gravity changes in both homogeneous and randomly heterogeneous scenarios, consistent with the integral nature of gravity observations, our study enables us to quantify their relative importance. Values of Ensemble mean and variance of head and gravity changes associated with randomly heterogeneous fields are generally more influenced by the variance rather than by the correlation scale of the spatially heterogeneous parameters considered. Uncertainty in the correlation scale is more influential to the shape, and hence on extreme values, of the probability distribution of these moments.

Water Resources Studies in Headwaters of the Blue Nile Basin: A Review with Emphasis on Lake Water Balance and Hydrogeological Characterization

The Lake Tana Basin, comprising the largest natural lake in Ethiopia, is the source and the uppermost part of the Upper Blue Nile Basin. In this review paper, research papers, mainly on the rainfall-runoff modeling and lake water balance, and on the hydrogeology, have been reviewed. The earlier water balance estimation attempts used simple conceptual and statistical approaches and calculate on a monthly timescale. More recent research has been using advanced semi-physically or physically based distributed hydrological models. Accordingly, mean annual precipitation over the lake was estimated in the range 36.1–53.1%; lake evaporation at 45.3–57.5%; river inflow (all gauged and estimated ungauged) at 43.6–63.9%; and river (lake) water outflow at 0–9.2%. With the few isotope studies, groundwater inflow and outflow are found insignificant. Different studies had estimated groundwater recharge, ranging from 57 mm to 850 mm. The basin has a heterogenous aquifer system consisting of different volcanic rocks and alluvio-lacustrine sediments. Generally, groundwater with low TDS, Ca–Mg–HCO3 type, isotopically relatively enriched, and high TDS, Na–HCO3 type, isotopically relatively depleted, water types have been identified. In this paper, major research gaps such as aquifer hydraulic characterization, surface-groundwater interaction, groundwater flow and groundwater balance have been identified. Hence, future research shall focus on the groundwater resources, so that existing surface water studies are updated and future water usage options are explored.

A Comparison of Six Transport Models of the MADE-1 Experiment Implemented With Different Types of Hydraulic Data.

Six conceptually different transport models were applied to the macrodispersion experiment (MADE)‐1 field tracer experiment as a first major attempt for model comparison. The objective was to show that complex mass distributions in heterogeneous aquifers can be predicted without calibration of transport parameters, solely making use of structural and flow data. The models differ in their conceptualization of the heterogeneous aquifer structure, computational complexity, and use of conductivity data obtained from various observation methods (direct push injection logging, DPIL, grain size analysis, pumping tests and flowmeter). They share the same underlying physical transport process of advection by the velocity field solely. Predictive capability is assessed by comparing results to observed longitudinal mass distributions of the MADE‐1 experiment. The decreasing mass recovery of the observed plume is attributed to sampling and no physical process like mass transfer is invoked by the models. Measures like peak location and strength are used in comparing the modeled and measured plume mass distribution. Comparison of models reveals that the predictions of the solute plume agree reasonably well with observations, if the models are underlain by a few parameters of close values: mean velocity, a parameter reflecting log‐conductivity variability, and a horizontal length scale related to conductivity spatial correlation. The robustness of the results implies that conservative transport models with appropriate conductivity upscaling strategies of various observation data provide reasonable predictions of plumes longitudinal mass distribution, as long as key features are taken into account.

Time-lag characteristics of the response of karst springs to precipitation in the northern China

Atmospheric precipitation is the main source of replenishment for large karst springs in the northern China. The extensive spring area and the high heterogeneity and anisotropy of the karst aquifer can cause a significant lag in the response of spring water level to precipitation. To explore the response characteristics of different flows of groundwater to precipitation, this study took the Jinan spring area as an example and used time series analysis to investigate the response characteristics of spring to rainfall. This study showed that atmospheric precipitation remains the single most important factor affecting spring discharge at present. According to the characteristics of spring discharge recession, the groundwater flows in the Jinan spring area can be divided into conduit-major fissure flow and diffuse-minor fissure flow. This study examined the response characteristics of different groundwater flows to precipitation through correlation and spectral analyses. The response time of conduit-major fissure flow to precipitation ranged 7–16 days and the duration ranged 12–24 days. The response time of diffuse-minor fissure flow to precipitation ranged 36–56 days, and the duration ranged 1–3 hydrological years. Compared with the karst water system in the southern China, the karst water system in the northern China has a stronger regulating effect. On this basis, this study proposed a correlation and spectral analyses and evaluation method suitable for understanding the dynamics of large karst springs in the northern China.

When perchlorate degradation in the riverbank cannot impede the contamination of drinking water wells

Health issues related to aquifer contamination with perchlorate are a growing concern in drinking water management. This study describes perchlorate transport and degradation processes from a contaminated stream toward drinking water pumping wells. Investigations are based on laboratory experiments and field measurements conducted at a well field near Bordeaux (France) in a heterogeneous carbonate aquifer interacting with a stream. Field measurements facilitated the characterization of perchlorate contamination and stream-to-aquifer flow. Experiments on columns of streambed sediments conducted in the laboratory confirmed that perchlorate had been degraded in the hyporheic zone. A one-dimensional reactive transport model was implemented to estimate Monod kinetic rates, which account for the inhibition of perchlorate degradation by nitrate. The estimated half-saturation constant for perchlorate $$ \Big({k}_{1/2}^{\mathrm{Pcl}} $$ ) is 6.93 10−9 mol L−1 and the estimated maximum specific degradation rate ( $$ {k}_{\mathrm{max}}^{\mathrm{Pcl}} $$ ) ranges between 10−5 and 4.0 10−3 mol L−1 day−1. Despite degradation in the hyporheic zone, perchlorate-contaminated stream water reaches drinking-water-production units. Such contamination highlights the effects of preferential flow paths between the stream and the pumping wells and significant hydraulic gradients caused by drawdowns. In such contexts, in spite of a good potential for degradation, riverbank filtration may not be effective for the protection of drinking water wells. Lessons from this study also reveal that contamination monitoring can be misleading: low concentrations can be reported in monitoring wells between the contaminant source and the production wells, but the latter may yet be contaminated.

Supporting Information: Mapping karst conduits in a heterogeneous aquifer using hydraulic tomography: the first two-dimensional sandbox validation

Supporting Information: Mapping karst conduits in a heterogeneous aquifer using hydraulic tomography: the first two-dimensional sandbox validation