Stratified Aquifer Research Articles

Groundwater monitoring in challenging environments: an argument for the construction of observation wells based on data from Niamey, Niger

AbstractGroundwater sampling in challenging environments often leads to compromises in following best practices to obtain representative samples from aquifers. This includes collecting samples from existing production or domestic wells instead of using properly constructed monitoring wells or using a bailer instead of a submersible pump for sampling. To address unusual patterns and trends in groundwater chemistry data collected in Niamey, Niger from 2012–2021, a state-of-the-art monthly sampling routine was established for eight wells tapping the basement aquifer. This was based on the hypothesis that the observed changes in groundwater composition were mainly due to differences in sampling technique, and the aim of the study was to gain insights into possible seasonal variations in water composition, to examine if the previously observed trends could be validated and to provide baseline data for future studies. The results indicate that in most cases the long well response zones in the stratified aquifer system led to the collection of water from different strata/aquifers or of strongly mixed samples. Therefore, any sample from those wells is only of limited value for the interpretation of hydrogeological processes. To obtain sound data for the development of groundwater management strategies, the monitoring has to be shifted from existing production wells to properly constructed monitoring wells. In the complex hydrogeological setting of Niamey, with hydraulically interacting aquifers and occurrences of density layering, it is fundamental to ensure that a monitoring well taps one specific depth of one target aquifer and that well-volume purging is applied properly.

Effects of layered heterogeneity on mixed physical barrier performance to prevent seawater intrusion in coastal aquifers

Mixed physical barriers (MPB), which combine a cutoff wall and subsurface dam, were applied in heterogeneous coastal aquifer settings, and their ability to prevent seawater intrusion (SWI) was tested. The performance of MPB was examined in a typical stratified aquifer using laboratory experiments and numerical modelling. The performance of the MPB was compared to that of a single cut-off wall by measuring the percentage of reduction of the intrusion length. SEAWAT was used for validation purposes and to further evaluate the effectiveness of MPB in five additional realistic heterogeneous configurations. Experimental results show that the MPB effectively reduced the saltwater wedge length by up to 71%. The MPB outperformed the single cut-off wall by up to 55 %. Also, numerical results show that the MPB proved to be effective, achieving a SWI length reduction of up to 69% in the scenario regardless of the aquifer layering structures. Comparable performance was observed when the high K layer was confined between two low K layers or when the aquifer presented a monotonically increasing/decreasing K pattern. The findings of this study provide insights into the applicability of the MPB in realistic heterogeneous coastal aquifers.

Analytical solutions for freshwater lenses in stratified riparian aquifers

Analytical solutions for defining stable freshwater lenses within saline riparian aquifers (“riparian lenses”) are largely limited to homogenous aquifers. However, real-world riparian aquifers typically consist of a surficial layer of low-permeability soils or deposits, the impact of which on the occurrence of riparian lenses has not been investigated. This study develops steady-state, sharp-interface analytical solutions for riparian lens shape and saltwater discharge in a two-layered riparian aquifer. The developed analytical solutions are verified by sand tank experiments and numerical simulations, showing good agreement. The sensitivity analysis based on the proposed analytical solutions suggests that aquifer stratification (i.e., considering a low-permeability layer overlying a high-permeability layer) leads to larger lenses in head-controlled systems (i.e., with the inland constant-head boundary) but smaller lenses in flux-controlled systems (i.e., with the inland constant-flux boundary), compared to the scenario of a homogenous aquifer. The surficial layer with hydraulic conductivity only an order of magnitude lower than the underlying high-permeability layer would have a pronounced effect on lens extent and volume. Importantly, the results suggest that the presence of the surficial low-permeability layer may lead to the outcropping of the groundwater table, favoring the occurrence of saline groundwater seepage near the edge of the river valley (i.e., a phenomenon reported by the field survey of the Murray River floodplains in South Australia). The analytical solution presented in this study can be used to provide a preliminary assessment of riparian lenses in saline stratified aquifers, expanding the applicability of analytical methods to more realistic riparian settings.

The impact of the embankment and the recreational lake on capturing front with drilling wells

This paper presents the influence on the levels from the well caption front Gherăeşti-Bacău, located on the right side of the Bistrița River, through the construction, on the right shore, of a 2.25 km long perimetral embankment, and a recreational lake with a surface of 3.18 hectares and average depth of 5.5 m (Figure 1). Through numerical modelling, using the modelling software of Visual Modflow aquifer, the influence on the hydrostatic level from drilled wells was determined, as well as the influence on the phreatic flow through the making of the recreational lake, which captures a part of the underground water. The use of the mathematical model allows the determination of the optimal parameters for an extraction from the aquifer known through classical methods. To achieve the mathematical model the method of filtration resistances was used, which is applied in the case of a sudden curvature of the streamlines (in the proximity of the imperfect permeable/impervious borders: channels, drains, well systems, screens), hence they are applied during pressure/level drops. This method allows the following equalizations: the imperfect wells can be considered as perfect; the lines of perfect/imperfect wells can be considered as perfect trenches (channels) with a flow equal to the sum of all wells; a stratified aquifer (up to 3 layers) can be equivaled with a homogenous one. Aspects regarding the flow were taken into consideration: with a free level or under pressure, the dimensioning of the model, the initial conditions of the aquifer layer, the lithostratigraphic characteristics, homogeneity, anisotropy, the transfer mechanisms in the domain’s interior, the studied hydrogeological structure was schematized, having done a monolayer model. The calibration of the model consists of adjusting, in reasonable conditions, of the domain data (the permeabilities), boundary conditions, so that through the running of the model and obtaining the corresponding piezometric lines and the obtained data will overlap the levels of the existing drillings.

From Cut-in to Qanats - Ancient Groundwater Extraction Techniques

Where a hillside stratified aquifer intersects the earth surface, springs and outseeping are observed. Cutting into this zone, thus opening it by digging, allows to increase and capture water outflow. As a matter of principle this classical method for water extraction without pumping, which is still found in hilly rural regions today, was already used 3600 years ago by the Hittites to fill the ponds of their capital Hattuşa in Central Anatolia. The today sedimented reservoirs were dug downhill of groundwater bearing zones. Rising in winter, groundwater discharged into the ponds through alongside cuts. The Hittites avoided the risks of strongly varying surface flows by opening near-surface groundwater and stratum aquifers. Although hydraulic investigation based on in-situ measurement of groundwater level supports the short-term efficiency of the ponds in supplying water to the ancient city, at the long-term, the decline of the Empire was probably triggered by severe droughts expanded over years. This seems plausible as severe droughts are still being experienced. For a higher and more reliable water yield, the further development went from ’cutting’ in to ’penetrating’ into the aquifer with tunnel-like drain conduits which collected the water and conveyed it to settlements and irrigation schemes. The improved water extraction system, named qanats, appeared in Eastern Anatolia and Persia about 500 years after the abandon of Hattuşa. An example of a qanat system in western Iran is presented in this study with less emphasis compared to the cut-in yet representative enough to demonstrate its role in supplying water sustainably. We conclude that the ancient time thinking is the same as that of modern engineering, and the ancient time hydraulic works are fundamental for today's civil structures.

Numerical investigation of mixed physical barriers for saltwater removal in coastal heterogeneous aquifers

Saltwater intrusion is a prevalent global environmental issue that detrimentally impacts coastal groundwater aquifers. This problem is exacerbated by climate change and increased groundwater abstraction. Employing physical barriers proves effective in mitigating saline water intrusion. In this study, a validated numerical simulation model is utilized to assess the impact of aquifer stratification on the effectiveness of mixed physical barriers (MPBs) and their response to structural variations. Additionally, the performance of MPBs was compared with that of single physical barriers in a laboratory-scale aquifer. Three different configurations were replicated, comprising two stratified aquifers (HLH and LHL) and a homogenous reference aquifer (H). The results demonstrate that MPBs are efficient in decreasing the saltwater penetration length in the investigated cases. The reductions in penetration length were up to 65% in all cases. The removal efficacy of residual saline water for MPBs exceeded that of the subsurface dam by 2.1–3.3 times for H, 2.1–3.6 times for HLH, and 8.3 times for LHL conditions, while outperforming the cutoff wall by 38–100% for H, 39–44% for HLH, and 2.7–75% for LHL. These findings are of importance for decision-makers in choosing the most appropriate technique for mitigating saline water intrusion in heterogeneous coastal aquifers.

Combined effects of aquifer heterogeneity and subsurface dam on nitrate contamination in coastal aquifers

Subsurface dams are effective for seawater intrusion mitigation, yet they can cause upstream nitrate accumulation. This research examines the interplay between subsurface dam construction and aquifer layering on nitrate pollution in coastal settings, employing numerical models to simulate density-driven flow and reactive transport. The study reveals that while subsurface dams are adept at curbing seawater intrusion, they inadvertently broaden the nitrate accumulation zone, especially when a low-K layer is present. Heterogeneous aquifers see more pronounced nitrate accumulation from subsurface dams. This effect is pronounced as it influences dissolved organic carbon dynamics, with a notable retreat inland correlating with the expansion of the nitrate pollution plume. A critical finding is that controlling seawater intrusion via dam height adjustment within the Effective Damming Region effectively reduces nitrate levels and bolsters freshwater output. However, exceeding the critical threshold—where the dam surpasses the low-K layer's bottom—results in a substantial shift in nitrate concentration, underscoring the need for precise dam height calibration to avoid aggravating nitrate pollution. This study's innovative contribution lies in its quantification of the nuanced effects of subsurface dams in stratified aquifers, providing an empirical basis for dam design that considers the layered complexities of coastal aquifers. The insights offer a valuable framework for managing nitrate contamination, thus informing sustainable coastal groundwater management and protection strategies.

Evaluating the Impact of Inclined Cutoff-Wall to Control Seawater Intrusion in Heterogeneous Coastal Aquifers

Subsurface physical barriers have been effectively used to mitigate seawater intrusion (SWI). Traditionally, the primary emphasis in both numerical studies and practical implementations has been on vertical barriers. The current research aims to explore the dynamics of SWI under various cutoff-wall inclination angles and depths, as well as aquifer heterogeneity using both experimental and numerical simulations. The impact of aquifer characteristics was assessed by utilizing a low hydraulic conductivity (K) aquifer (case L), a high hydraulic conductivity aquifer (case H), and two stratified aquifers. The stratified aquifers were created by grouping different hydraulic conductivity layers into two cases: high K above low K (case H/L) and low K above high K (case L/H). The model simulations covered seven different cutoff-wall inclination angles: 45.0°, 63.4°, 76.0°, 90.0°, 104.0°, 116.6°, and 135.0°. The maximum repulsion ratio of SWI wedge length was observed at an inclination angle of 76.0° for cutoff-wall depth ratios up to 0.623. However, as the depth ratio increased to 0.811, the maximum repulsion ratio shifted to an angle of 63.4° for all aquifers studied. At an inclined cutoff depth ratio of 0.811, the cutoff-wall inclination angle of 45.0° had the most significant impact on the saltwater wedge area. This results in SWI area reductions of 74.9%, 79.8%, 74.7%, and 62.6% for case L, case H, case H/L, and case L/H, respectively. This study provides practical insights into the prevention of SWI. Nevertheless, a thorough cost–benefit analysis is necessary to assess the feasibility of constructing inclined cutoff-walls.

Analytical solutions for maximum fresh groundwater extraction in stratified coastal aquifers

Existing analytical solutions for defining maximum freshwater extraction in coastal aquifers mostly presume a homogeneous aquifer. In this study, analytical solutions for the pumping-induced interface toe position and the maximum pumping rate of a single well in stratified coastal aquifers are developed, based on the potential theory and the analytical solution proposed by Rathore et al. (2020), where aquifer stratification is concisely characterized by the total transmissivity and the transmissivity centroid elevation (hereafter “TCE”). Three typical aquifer domain scenarios are examined, including semi-infinite, infinite-strip, and rectangular coastal aquifers. The sensitivity analysis based on the proposed analytical solutions demonstrates that in all aquifer domain scenarios, a lower TCE leads to a more seaward interface toe and a larger maximum pumping rate, while the impact of total transmissivity is complicated, depending on the type of the inland boundary condition. The effect of inland and lateral boundary conditions on the maximum pumping rate is enhanced in the stratified aquifers of lower total transmissivity and TCE. Importantly, the boundary effects can be eliminated by properly defining the size of the stratified aquifer domain, providing valuable guidance for the design of numerical models and laboratory experiments that are often of finite sizes. Our analytical solutions can serve as a simple tool for the first-order assessment of allowable freshwater extraction in stratified coastal aquifers.

Approximate analytical solution for seepage field of drained tunnel in vertically stratified phreatic aquifer

Approximate analytical solution for seepage field of drained tunnel in vertically stratified phreatic aquifer

Hydrodynamic model for independent cold and thermo-mineral twin springs in a stratified continental karst aquifer, Camou, Arbailles Massif, Pyrénées, France

The Camou springs (Arbailles Massif, French Western Pyrenees) display an unusual close association of a typically cold karstic spring that drains the Urgonian western limb of the Arbailles, and a thermo-mineral spring (33.5°C; salinity 17.7 g/L). The latter gains its mineralization at the contact of Triassic evaporites mainly through a deep loop in the Apanicé syncline. The fast upflow of this deep water occurs at the cross of large active lines (the North-Pyrenean thrust located at depth, and the Saison transverse fault). Cave diving in the nearby Maddalen Cave allowed reaching the phreatic passage at the origin of the cold spring, which however also crosses the thermal body in the third sump (S3). Both water bodies are separated by a sharp thermocline. 6 pressure-temperature dataloggers were placed in both water bodies along the thermocline for 6 months. The dataloggers located downstream on either side of the thermocline show at the beginning of flood first a rise of the thermal body, then an invasion of the whole phreatic passage by the cold floodwater, controlled by head pressure changes in the karst aquifer. From observation of these mechanisms, we deduce a hydrodynamic model with a warm plume rising into the cold aquifer, without significant mixing. Such independence of water bodies is explained by the decrease of turbulent rate at the interface, due to the sharp density gradient. The relative absence of mixing does not actually require independent “watertight” routes, both water bodies can thus coexist even in the same conduit. This model locally implies the existence of unknown secondary passages close to the spring, which allow an independent draining of each water body toward separate outlets during low stage. Such type of stratified aquifer linked to density differences is common in coastal karst (Florida, French Calanques…), in the continental evaporite karst (Schlotten of the Harz in Germany, Kungur karst in Ural…), but remains poorly identified in continental carbonate karst areas, mainly because of the difficulty of access. Together with the Mescla spring in French Alpes-Maritimes, the Camou twin springs discharging in the same porch are an outstanding example, allowing a direct study of the stratification and the dynamic of highly contrasted water bodies.

An integrative re-evaluation of Typhlatya shrimp within the karst aquifer of the Yucat\xe1n Peninsula, Mexico

The Yucatán Peninsula, Mexico is a carbonate platform well-known for extensive karst networks of densely stratified aquifer ecosystems. This aquifer supports diverse anchialine fauna, including species of the globally distributed anchialine shrimp genus Typhlatya (Atyidae). Four species (T. campecheae, T. pearsei, T. dzilamensis and T. mitchelli) are endemic to the Peninsula, of which three are federally listed in Mexico. This first integrative evaluation (i.e., molecular, morphological, broad geographic and type locality sampling, and environmental data) of Yucatán Typhlatya reveals considerable species identity conflict in prior phylogenetic assessments, broad species ranges, syntopy within cave systems and five genetic lineages (of which two are new to science). Despite sampling from the type locality of endangered T. campecheae, specimens (and molecular data) were indistinguishable from vulnerable T. pearsei. Ancestral/divergence reconstructions support convergent evolution of a low-salinity ancestor for a post-Paleogene arc Yucatán + Cuba Typhlatya clade within the anchialine Atyidae clade. A secondary adaptation for the coastal-restricted euryhaline (2–37 psu), Typhlatya dzilamensis (unknown conservation status) was identified, while remaining species lineages were low-salinity (< 5 psu) adapted and found within the meteoric lens of inland and coastal caves. This study demonstrates the need for integrative/interdisciplinary approaches when conducting biodiversity assessments in complex and poorly studied aquifers.

Revealing vertical aquifer heterogeneity and hydraulic anisotropy by pumping partially penetrating wells

The stratification of sedimentary aquifers introduces spatial variability in hydraulic conductivity, primarily between individual horizontal layers. On larger scales, the vertical heterogeneity enhances hydraulic anisotropy, with the horizontal conductivity typically exceeding the vertical one. In this study, the hydraulic anisotropy of a stratified aquifer is estimated from data of hydraulic tests in which water is sequentially extracted from well sections screened at different depths, and the hydraulic response is measured at various multilevel observation wells. The applicability of the method is demonstrated by field tests in a fluvial gravel aquifer in the Upper Rhine Valley, Germany. A homogeneous anisotropic model, and models with three and five anisotropic layers, are fitted to the measured drawdowns in the steady-shape regime, in which differences in hydraulic head between observation locations do not change over time even though the head values themselves change. The position of the five horizontal layers is based on the lithology of the drilling profile at the pumping-well location. The three-layer model is achieved by merging insensitive or similar layers with sensitive layers. The fits result in estimates of the radial and vertical hydraulic conductivities for all layers of the respective models, which are upscaled to effective parameters over the entire depth in the case of the multilayer models. The homogeneous model shows significantly higher errors than those of the heterogeneous models. The heterogeneous locally anisotropic models not only reveal vertical variability of hydraulic conductivity, but also lead to a three-times larger anisotropy ratio upon upscaling.

Haline Convection within a Fresh-Saline Water Interface in a Stratified Coastal Aquifer Induced by Tide

Sea-tide effects on the fresh-saline water interface (FSI) in a stratified coastal aquifer are examined through laboratory experiments. The physical model, a two-dimensional rectangular flow tank, is filled with layered aquifers and aquitards. The aquifers serve as the main entrances/exits of water to/from the system through significant horizontal flows, creating unstable conditions of heavier saline water above lighter freshwater for short periods of time. Several processes create mixing; this instability results in haline convection, creating downward fingering, stable rising of horizontal saltwater front, and unstable upward fingerings of flushing freshwater. The time lag between the sea tide fluctuations and the emergence of adequate fresh- and saltwater is higher in a stratified system compared to a homogeneous system. Furthermore, longer tide cycles lead to the enlargement of the FSI’s toe horizontal movement range. The combination of tidal forcing with a layering aquifer structure leads to a wider FSI by creating a significant salt- and freshwater mixing inside each layer, vertical flows between the layers, and saltwater bodies at isolated areas. Haline convection within the FSI might be the reason for the wider fresh-saline interfaces that are found in field studies.

Model-Based Analysis of the Link between Groundwater Table Rising and the Formation of Solute Plumes in a Shallow Stratified Aquifer

Groundwater table rising (GTR) represents a well-known issue that affects several urban and agricultural areas of the world. This work addresses the link between GTR and the formation of solute plumes from contaminant sources that are located in the vadose zone, and that water table rising may help mobilize with time. A case study is analyzed in the stratified pyroclastic-alluvial aquifer near Naples (Italy), which is notoriously affected by GTR. A dismissed chemical factory generated a solute plume, which was hydraulically confined by a pump-and-treat (P&amp;T) system. Since 2011, aqueous concentrations of 1,1-dichloroethene (1,1-DCE) have been found to exceed regulatory maximum concentration levels in monitoring wells. It has been hypothesized that a 1,1-DCE source may occur as buried waste that has been flushed with time under GTR. To elucidate this hypothesis and reoptimize the P&amp;T system, flow and transport numerical modeling analysis was developed using site-specific data. The results indicated that the formulated hypothesis is indeed plausible. The model shows that water table peaks were reached in 2011 and 2017, which agree with the 1,1-DCE concentration peaks observed in the site. The model was also able to capture the simultaneous decrease in the water table levels and concentrations between 2011 and 2014. Scenario-based analysis suggests that lowering the water table below the elevation of the hypothesized source is potentially a cost-effective strategy to reschedule the pumping rates of the P&amp;T system.

The impact of aquifer stratification on saltwater intrusion characteristics. Comprehensive laboratory and numerical study

AbstractLaboratory experiments and numerical simulations were utilized in this study to assess the impact of aquifer stratification on saltwater intrusion. Three homogeneous and six layered aquifers were investigated. Image processing algorithms facilitated the precise calculation of saltwater wedge toe length, width of the mixing zone, and angle of intrusion. It was concluded that the length of intrusion in stratified aquifers is predominantly a function of permeability contrast, total aquifer transmissivity and the number of heterogeneous layers, being positively correlated to all three. When a lower permeability layer overlays or underlays more permeable zones its mixing zone widens, while it becomes thinner for the higher permeability strata. The change in the width of the mixing zone (WMZ) is positively correlated to permeability contrast, while it applies to all strata irrespectively of their relative vertical position in the aquifer. Variations in the applied hydraulic head causes the transient widening of WMZ. These peak WMZ values are larger during saltwater retreat and are negatively correlated to the layer's permeability and distance from the aquifer's bottom. Moreover, steeper angles of intrusion are observed in cases where low permeability layers overlay more permeable strata, and milder ones in the inverse aquifer setups. The presence of a low permeability upper layer results in the confinement of the saltwater wedge in the lower part of the stratified aquifer. This occurs until a critical hydraulic head difference is applied to the system. This hydraulic gradient value was found to be a function of layer width and permeability contrast alike.

A Semianalytical Method to Fast Delineate Seawater‐Freshwater Interface in Two‐Dimensional Heterogeneous Coastal Aquifers

AbstractThe freshwater‐seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable‐density flow and transport are the only option to characterize the interface in a two‐ or three‐dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater‐freshwater interface in a two‐dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near‐coast, near‐top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.

Effect of anisotropy and depth-dependent hydraulic conductivity on concentration curve response to nonpoint-source pollution

In this paper, we introduce 2D profile analytical models describing advection solute transport induced by a recharge boundary condition in a catchment-scale aquifer represented by a sediment or rock with permeability changing with depth. The suggested mathematical formulation is based on the concept of transit time distribution (TTD), according to which the concentration trend can be derived directly from the cumulative travel time frequency or from the convolution of TTD (considered as a transfer function) with a solute input function. This approach is shown to be applicable to water flow and solute transport in an aquifer characterized by a vertical hydraulic conductivity profile, k(z), which can be described by either continuous (e.g., massive rock exhibiting an exponential or power-law dependence of k on depth) or discrete (a stratified aquifer) analytical functions. A special focus is on the problem related to advection solute transport in a rectangular anisotropic flow domain bounded by a shallow stream of finite width (downflow-converging area). The suggested analytical models and solutions, utilizing new transfer functions and accounting for different inflow and solute flux conditions, can be involved in several applications, including the assessment of the impacts of spatially variable nonpoint contamination sources on groundwater quality; groundwater dating based on the observation of environmental tracers; and the study of groundwater recharge itself. The models were shown to be applicable not only to simulating the impact on groundwater due to the contaminated recharge, but also to predicting aquifer contamination caused by a source located below the water table within the aquifer domain. Such source can be associated with a radioactive material leachate generated within a deep geological repository of nuclear waste. A relevant modeling example is given.

Joint Optimization of Measurement and Modeling Strategies With Application to Radial Flow in Stratified Aquifers

AbstractWhen applying environmental models, the choice of model complexity and the design of field campaigns depend on each other and on the modeling/prediction goal. We propose jointly optimizing model complexity and data collection (design) by maximizing the expected performance for the modeling goal. We use ensembles of highly resolved virtual realities and of less complex modeling variants that differ in their degrees of upscaling and simplified parameterization. For each design under consideration, we simulate hypothetical measurement data (subject to noise) with all realizations of all models. To mimic model calibration with hypothetical data, we identify pairs of best fitting realizations between virtual reality and each model variant for each design. Then, we emulate model choice by selecting (across the model variants, for each design and for each virtual reality) the pair that shows the best predictive match in the modeling goal. Finally, we identify the model/design combination that offers, on average over all virtual realities, the best predictive match. As a test application, we consider a heterogeneous, stratified aquifer, in which the stratification enhances hydraulic anisotropy on the macroscale. We define two different modeling goals: (a) estimating the hydraulic conductivity tensor upscaled to the full aquifer thickness and (b) predicting the pumping rate needed to dewater a construction pit. Our results indicate that jointly optimizing observation networks and model selection can reduce the prediction uncertainty of parameters at lower experimental costs. We also show that the involved trade‐offs between model complexity and required design depend on the target quantity.

Paleoclimatic investigations using isotopic signatures of the Late Pleistocene-Holocene groundwater of the stratified aquifers in Kuwait

The study attempts to determine paleo recharge sources for two major aquifer units of Kuwait, namely, Kuwait Group Aquifer (KGA), and Dammam Formation (DFm) were collected and analyzed for major ions and isotopes. The obtained results of 14C ages indicated that the groundwater samples ranged from 31.9 Ky to 3.9 Ky in DFm and 23.3 Ky to 0.8 Ky in KGA. The variations in δ18O signatures with respect to age reflected that there were two major sources of rainwater such as enriched southern Indian Ocean monsoon and the depleted northwesterly (Mediterranean) vapor source. The samples found to show variations during the three marine isotopic stages (MIS 1–3), Heinrich events, Bolling-Allerod, and Dryas time periods in both aquifers. A shift in the Intertropical convergence zone (ITCZ) was witnessed in the groundwater of both aquifers from 7.8 Ky to 4.9 Ky. The variation in the Holocene climate is mainly due to the influence of tropospheric easterly jet governed by the swing in ITCZ , as a result of Holocene volcanism. The mean δ18O values of DFm samples (-3.81‰) reflect depletion than KGA samples (-2.31‰), and the d-excess values were −0.95‰ and 2.05‰ respectively. Thus it could be inferred that the DFm was predominantly recharged during Late Pleistocene period and it was in a relatively cooler climate than recharge environment of KGA. The study infers that KGA was recharged between 26oN and 26.8oN latitudes, and that of the DFm was noted to be recharged (south west of Kuwait) in two different patches between 25.8oN and 26.2oN and 26.7oN-27.4oN latitudes during Late Pleistocene. The KGA was concluded to be recharged between 25.5oN and 27.5oN latitudes, south of Kuwait, during Holocene.