Heterogeneous Coastal Aquifers Research Articles

A Deep Learning‐Based Data Assimilation Approach to Characterizing Coastal Aquifers Amid Non‐Linearity and Non‐Gaussianity Challenges

AbstractSeawater intrusion (SI) poses a substantial threat to water security in coastal regions, where numerical models play a pivotal role in supporting groundwater management and protection. However, the inherent heterogeneity of coastal aquifers introduces significant uncertainties into SI predictions, potentially diminishing their effectiveness in management decisions. Data assimilation (DA) offers a solution by integrating various types of observational data with the model to characterize heterogeneous coastal aquifers. Traditional DA techniques, like ensemble smoother using the Kalman formula (ESK) and Markov chain Monte Carlo, face challenges when confronted with the non‐linearity, non‐Gaussianity, and high‐dimensionality issues commonly encountered in aquifer characterization. In this study, we introduce a novel DA approach rooted in deep learning (DL), referred to as ESDL, aimed at effectively characterizing coastal aquifers with varying levels of heterogeneity. We systematically investigate a range of factors that impact the performance of ESDL, including the number and types of observations, the degree of aquifer heterogeneity, the structure and training options of the DL models. Our findings reveal that ESDL excels in characterizing heterogeneous aquifers under non‐linear and non‐Gaussian conditions. Comparison between ESDL and ESK under different experimentation settings underscores the robustness of ESDL. Conversely, in certain scenarios, ESK displays noticeable biases in the characterization results, especially when measurement data from non‐linear and discontinuous processes are used. To optimize the efficacy of ESDL, attention must be given to the design of the DL model and the selection of observational data, which are crucial to ensure the universal applicability of this DA method.

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.

Quantifying salinity in heterogeneous coastal aquifers through ERT and IP: Insights from laboratory and field investigations

The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation in aquifers constituted by clay-rich sediments. In this study, we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using spectral IP in controlled salinity conditions to derive their formation factors, surface conductivity, and normalized chargeability. Second, the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River catchment and found that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications vary from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, while normalized chargeability above 1.0 mS.m−1 indicates the presence of clay. The clay-bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.

Time-Lapse Geophysical Measurements for Monitoring Coastal Groundwater Dynamics in an Unconfined Aquifer.

The coastal zone, which is the interface between land and sea, is hydrodynamically very active due to the complex interactions of various hydrological controls and variable-density fluids. These forces vary over time, resulting in a state of dynamic equilibrium in the system. The major hydrological processes in coastal aquifer systems are salt water intrusion and submarine groundwater discharge, which are interdependent. Monitoring these complex processes is crucial for sustainable coastal zone management but poses a significant research challenge. In this study, we demonstrate the effectiveness of non-invasive geophysical techniques, specifically the time-lapse electrical resistivity imaging method, in conjunction with groundwater monitoring, for monitoring coastal groundwater dynamics in an unconfined aquifer at varying time scales and hydrogeological settings present at formerly glaciated sites worldwide. We generated two-dimensional baseline salt water intrusion maps for the test site, located on the coast of Rhode Island, USA. The time-lapse electrical resistivity survey method enables the rapid estimation of fresh groundwater discharge. Our approach offers insight into the mechanisms and seasonably variable salt water-freshwater interactions in unconfined heterogeneous aquifers. Although the results are site-specific, their implications are broad and may stimulate other studies related to sea to land pollution (sea water intrusion) and land to sea pollution (groundwater discharge) in heterogeneous coastal aquifer settings.

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.

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.

Influence of layered heterogeneity on nitrate enrichment induced by cut-off walls in coastal aquifers

Cut-off walls have proven to be an effective countermeasure for mitigating seawater intrusion (SI). However, construction of cut-off walls may lead to upstream accumulation of nitrate contaminants in coastal aquifers. While stratified geological heterogeneities are widely present in coastal areas, how they may influence nitrate enrichment induced by cut-off walls remains poorly understood. In the present study, we employ variable-density coupling with reactive transport numerical models to understand and quantify the dynamic changes of salinity and NO3− in layered heterogeneous coastal aquifers in the presence of cut-off walls. Our comprehensive simulation results indicate that aquifer heterogeneities are a primary factor in regulating nitrate accumulation. For example, the presence of a middle low-permeability layer between two high-permeability layers reduces the influence of cut-off wall height on SI if the bottom of the wall is placed inside the low-permeability layer. While the thickness of the middle low-permeability layer has a minor impact on SI, a thicker middle low-permeability layer leads significantly aggravated nitrate contamination. Moreover, our simulations reveal that the degree of nitrate accumulation changes non-monotonically with changes in the hydraulic conductivity of low-permeability layer—the total nitrate mass (TNM) first increases and then decreases as the hydraulic conductivity of the low-permeability layer increases. We also found that the impact of heterogeneities is a function of the location and depth of the cut-off wall. With the increases of the distance between wall and the ocean, the nitrate enrichment degree of nitrate pollution in the upstream aquifer decreases while seawater intrudes further in layered aquifer (i.e. case HLH). TNM increases as the wall depth increases, but the growth rates of TNM becomes lower when the bottom of the wall is placed inside the low-permeability layer. These quantitative analyses have critical implications for the planning and design of cut-off walls to effectively control SI while at the same mitigating nitrate pollution.

Along‐Shore Movement of Groundwater and Its Effects on Seawater‐Groundwater Interactions in Heterogeneous Coastal Aquifers

AbstractStudies of coastal groundwater dynamics often assume two‐dimensional (2D) flow and transport along a shore‐perpendicular cross‐section. We show that along‐shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three‐dimensional (3D) preferential flow paths. The along‐shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit‐type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three‐dimensionality in systems with connected high‐permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land‐sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS‐type systems, less‐connected high‐permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit‐type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean.

A multi-tracer approach for understanding the functioning of heterogeneous phreatic coastal aquifers in humid tropical zones

ABSTRACT In many areas around the world, phreatic coastal aquifers are vulnerable to contamination and saltwater intrusion. This study aims to explore the hydrodynamic functioning of the coastal phreatic aquifer of the Urabá region, Colombia, based on the identification of hydrogeochemical processes and groundwater residence and transit times. Hydrodynamic data showed a fast response of the piezometric levels to precipitation with a maximum time lag of 48 h. Mean annual recharge was estimated at 19% with respect to precipitation. Another recharge mode is linked to infiltration from surface water, according to the hydrodynamic results. The main geochemical process identified is hydrolysis due to dissolved carbon dioxide, inducing silicate weathering and resulting in predominant HCO3-Ca-Mg facies. Monitoring of δ18O and δ2H compositions combined with 3H, 14C, chlorofluorocarbons (CFCs) and SF6 contents showed a fast recharge in the aquifer. Groundwater dating indicates recent waters mixed with older (>40 years) waters.

Multi-Objective Management of Heterogeneous Coastal Aquifers.(Dept.C)

A nonlinear optimization problem based on both single and multiple objective management functions for sustainable utilization of heterogeneous coastal aquifers was formulated and solved. The sharp interface approach was assumed within this work. Finite Element Method (FEM) was applied on Sirack's formulation to simulate the hydraulic response of the steady slate heterogeneous aquifer under different objectives. The LU-decomposition method was exploited to inverse the conductance matrix, that enhanced the computation time. The constraint technique was used to simplify, without any approximation, the multi objective management problem to single objective management problem. Genetic Algorithm (GA) was used to solve the nonlinear optimization problem. A Fortran program was written and verified against a problem that has an analytical solution. A hypothetical example was solved for one and three objective functions. These func tions are maximum magnitude of extracted freshwater, minimum drop in water table level, and minimum increment in volume of intruded saltwater.

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.

Relative importance of conceptual and computational errors when delineating saltwater intrusion from resistivity inverse models in heterogeneous coastal aquifers

In coastal areas, geological heterogeneity influences the geometry and characteristics of the freshwater-saltwater mixing zone. Geophysical electrical and electromagnetic methods are increasingly used to delineate the freshwater-saltwater mixing zone or for groundwater model calibration. However, in practical applications, it is common when assessing pore water salinity from resistivity models to disregard the spatial variability of the aquifer properties assuming pore salinity is the dominant control on bulk electrical conductivity. In this paper we use a coupled hydrogeophysical model to assess the discrepancies in recovered salinity caused by the geophysical inversion and the application of the chosen petrophysical model when heterogeneity is ignored. We find that errors made when neglecting heterogeneity during petrophysical transformation are equivalent or higher in magnitude than inversion errors, more so when an inappropriate petrophysical model is used, but exhibit different spatial distribution. The results show that their combined effect results in significant overestimation of the spreading of the mixing zone. The analysis provides new insights to the groundwater community to better evaluate the reliability of geophysically-derived information when delineating the freshwater-saltwater interface or when geophysics is used quantitatively as part of model scenario simulation, uncertainty analysis or optimisation strategies.

A Borehole-Based Approach for Seawater Intrusion in Heterogeneous Coastal Aquifers, Eastern Part of Jeju Island, Korea

Understanding the basaltic aquifer system and seawater intrusion on the volcanic island of Jeju, Korea, has received significant attention over the years, and various methodologies have been suggested in the contributions. Nevertheless, it is still difficult to effectively characterize groundwater systems due to the long period of volcanic activity and the lithological variability of basalt. In this study, geophysical well logging in seawater intrusion monitoring boreholes detected a sudden decrease of electrical conductivity within the saltwater zone in the eastern part of Jeju Island. This anomalous condition cannot be explained by the Ghyben-Herzberg model, which has historically been considered as the basic groundwater model of Jeju Island. This paper focuses on fine-scale temporal and spatial variability of groundwater flow using electrical conductivity and temperature logs and borehole temperature monitoring by a thermal line sensor. On the basis of the results, we evaluate an alternative model to replace the traditional conceptual model in the eastern part of Jeju Island. It is revealed that the area consists of heterogeneous aquifer systems, and the behavior of freshwater and saltwater is understood by temperature monitoring over the entire depth of boreholes. Coastal aquifers flow through two or more independent channels with weak vertical connections. In addition, seawater intrusion does not occur continuously in the vertical direction from the bottom depth, but instead occurs through these multilayered aquifers. In particular, the multilayered aquifers that are responsible for flow pathway, as well as the freshwater–saltwater interface form mainly at lithological boundaries. Our preliminary conceptual model is expected to be improved and revised by various measurements of hydrodynamic parameters such as flowmeter or packer test.

Probabilistic numerical assessment of seawater intrusion overshoot in heterogeneous coastal aquifers

Seawater intrusion is considered as one of the main hazards to coastal aquifers. In coastal aquifers, an overshoot occurs when the freshwater–saltwater interface exceeds the steady state position due to sea level rise (SLR). Hence, it is considered a more critical state than the terminal state. In the present study, overshoot is characterized in an unconfined, heterogeneous two-dimensional aquifer. For a more accurate evaluation, overshoot is investigated using three indicators, seawater toe, salinized volume, and effective dispersivity. In combination with the associated land surface inundation (LSI) impact, two types of SLR are assumed, gradual SLR (GSLR) and instantaneous SLR (ISLR). For addressing the heterogeneity of the aquifer, 50 sets of log-normally-distributed conductivity fields using a spherical correlation function are generated for each of the scenarios. Heterogeneity of the aquifer is modeled using the variance of conductivity field ($$\sigma_{{{ \ln }k}}^{2}$$) and the longitudinal correlation length ($$\lambda_{x}$$). Three different values of 0.5, 1, and 2 are assumed for $$\sigma_{{{ \ln }k}}^{2}$$ where two values of 20 m and 40 m are assigned to $$\lambda_{x}$$. Using Monte-Carlo simulations, it is shown that (1) in both GSLR and ISLR scenarios, the overshoot is observed for both the seawater toe and the salinized volume where LSI is not assumed; (2) the SLR impact is overshadowed by the significance of conductivity field properties in heterogeneous scenarios; (3) $$\sigma_{{{ \ln }k}}^{2}$$ plays a more discernible role in the overshoot characteristics compared to $$\lambda_{x}$$; (4) a realistic assumption of GSLR results in lower overshoot occurrence probability. These observations are interpreted using the associated behavior of the flow field in the aquifer and the time needed for hydraulic pressures in this field to re-equilibrate after SLR.

Three-Dimensional Modelling of Heterogeneous Coastal Aquifer: Upscaling from Local Scale

The aquifer heterogeneity is often simplified while conceptualizing numerical model due to lack of field data. Conducting field measurements to estimate all the parameters at the aquifer scale may not be feasible. Therefore, it is essential to determine the most significant parameters which require field characterization. For this purpose, the sensitivity analysis is performed on aquifer parameters, viz., anisotropic hydraulic conductivity, effective porosity and longitudinal dispersivity. The results of the sensitivity index and root mean square deviation indicated, that the longitudinal dispersivity and anisotropic hydraulic conductivity are the sensitive aquifer parameters to evaluate seawater intrusion in the study area. The sensitive parameters are further characterized at discrete points or at local scale by using regression analysis. The longitudinal dispersivity is estimated at discrete well points based on Xu and Eckstein regression formula. The anisotropic hydraulic conductivity is estimated based on established regression relationship between hydraulic conductivity and electrical resistivity with R2 of 0.924. The estimated hydraulic conductivity in x and y-direction are upscaled by considering the heterogeneous medium as statistically homogeneous at each layer. The upscaled model output is compared with the transversely isotropic model output. The bias error and root mean square error indicated that the upscaled model performed better than the transversely isotropic model. Thus, this investigation demonstrates the necessity of considering spatial heterogeneous parameters for effective modelling of the seawater intrusion in a layered coastal aquifer.

Spatio-temporal mapping of salinity in the heterogeneous coastal aquifer

The interface between freshwater and saltwater in the coastal aquifer is a main factor that is affected by environmental threat and excessive pumping. In this study, a density-dependent numerical model was utilized to simulate the salt distribution of a coastal aquifer in the Talar region near the Caspian Sea, Mazandaran, Iran. To simulate the aquifer salinity and head contour over 2-year stress period, SEAWAT model in groundwater modeling system (GMS) package was used. Moreover, different variogram models based on 21 observation points were tested to find a best fit between observed and simulated total dissolved solids (TDS). A genetic algorithm was coupled with GMS model to identify the optimal sill ratio, range, and nugget value of each variogram. The interface between saltwater and freshwater estimated by the optimal variogram model and compared with the result of sharp interface and variable density flow assumption. The result indicates that the optimal Gaussian variogram as best estimation of spatial TDS can be used for estimation of temporal salinity with a coefficient determination about 0.91. Also, an integration of spherical and exponential variogram shows the high performance for mapping spatial TDS and neutralizes the underestimation behavior of single variogram. A comparison between variogram model and analytical solution indicates that the interface length in the eastern part of aquifer for Gaussian model is relatively small than sharp interface relation. The maximum saltwater penetration based on sharp interface and SEAWAT results is about 1600 m and 1750 m, respectively. Additionally, maximum saltwater intrusion is observed in the western part of aquifer which has a large number of active wells.

Data-Worth Assessment for a Three-Dimensional Optimal Design in Nonlinear Groundwater Systems.

Groundwater model predictions are often uncertain due to inherent uncertainties in model input data. Monitored field data are commonly used to assess the performance of a model and reduce its prediction uncertainty. Given the high cost of data collection, it is imperative to identify the minimum number of required observation wells and to define the optimal locations of sampling points in space and depth. This study proposes a design methodology to optimize the number and location of additional observation wells that will effectively measure multiple hydrogeological parameters at different depths. For this purpose, we incorporated Bayesian model averaging and genetic algorithms into a linear data-worth analysis in order to conduct a three-dimensional location search for new sampling locations. We evaluated the methodology by applying it along a heterogeneous coastal aquifer with limited hydrogeological data that is experiencing salt water intrusion (SWI). The aim of the model was to identify the best locations for sampling head and salinity data, while reducing uncertainty when predicting multiple variables of SWI. The resulting optimal locations for new observation wells varied with the defined design constraints. The optimal design (OD) depended on the ratio of the start-up cost of the monitoring program and the installation cost of the first observation well. The proposed methodology can contribute toward reducing the uncertainties associated with predicting multiple variables in a groundwater system.

Seawater intrusion in heterogeneous coastal aquifers under flooding events

Coastal flooding due to storm surge and sea-level rise (SLR) affects seawater intrusion (SWI) in aquifers. Under coastal flooding events, SWI from both sea boundary and land-surface is expected to increase. SWI in coastal aquifers is also strongly influenced by geological heterogeneity characteristics. In this study, the combined impacts of fully geological heterogeneity and coastal flooding on SWI is quantified in a two-dimensional unconfined coastal aquifer. Three indices including the seawater toe location (xtoe), the mixing volume (vmv) and the SWI volume (vswiv) are used to assess the vulnerability to the intrusion of the seawater. A series of 50 log-normally distributed random hydraulic conductivity with a spherical correlation function is generated using the GCOSIM3D code. Then, numerical simulations of transient, saturated-unsaturated variable-density flow and solute transport, SUTRA, are performed in a stochastic Monte Carlo framework to understand how coastal flooding and geological heterogeneity effects can change the groundwater salinity.During storm surge events, seawater penetrates into the aquifer and leads to reduce temporary of the freshwater quality. The results indicate that the storm surge has a short-term impact on fresh groundwater. The aquifer is significantly influenced by SLR as a long-term effect while it causes a larger xtoe in comparison with pre-flooding events’ conditions. Also, the results obtained under gradual and instantaneous assumption of SLR show that the gradual assumption leads to a smaller xtoe which is more realistic. Comparison of the homogeneous and heterogeneous scenarios indicates the heterogeneity affects SWI status both in the presence and the absence of coastal flooding events. It causes reduction in xtoe and vswiv as well as increase in vmv. This study shows the significant value of considering a wide range of variations due to heterogeneity, in order to better manage the coastal fresh groundwater aquifers.

Groundwater withdrawal in randomly heterogeneous coastal aquifers

Abstract. We analyze the combined effects of aquifer heterogeneity and pumping operations on seawater intrusion (SWI), a phenomenon which is threatening coastal aquifers worldwide. Our investigation is set within a probabilistic framework and relies on a numerical Monte Carlo approach targeting transient variable-density flow and solute transport in a three-dimensional randomly heterogeneous porous domain. The geological setting is patterned after the Argentona river basin, in the Maresme region of Catalonia (Spain). Our numerical study is concerned with exploring the effects of (a) random heterogeneity of the domain on SWI in combination with (b) a variety of groundwater withdrawal schemes. The latter have been designed by varying the screen location along the vertical direction and the distance of the wellbore from the coastline and from the location of the freshwater–saltwater mixing zone which is in place prior to pumping. For each random realization of the aquifer permeability field and for each pumping scheme, a quantitative depiction of SWI phenomena is inferred from an original set of metrics characterizing (a) the inland penetration of the saltwater wedge and (b) the width of the mixing zone across the whole three-dimensional system. Our results indicate that the stochastic nature of the system heterogeneity significantly affects the statistical description of the main features of the seawater wedge either in the presence or in the absence of pumping, yielding a general reduction of toe penetration and an increase of the width of the mixing zone. Simultaneous extraction of fresh and saltwater from two screens along the same wellbore located, prior to pumping, within the freshwater–saltwater mixing zone is effective in limiting SWI in the context of groundwater resources exploitation.

Analysis of Tide and Offshore Storm‐Induced Water Table Fluctuations for Structural Characterization of a Coastal Island Aquifer

AbstractAnalysis of water table fluctuations can provide important insight into the hydraulic properties and structure of a coastal aquifer system including the connectivity between the aquifer and ocean. This study presents an improved approach for characterizing a permeable heterogeneous coastal aquifer system through analysis of the propagation of the tidal signal, as well as offshore storm pulse signals through a coastal aquifer. Offshore storms produce high wave activity, but are not necessarily linked to significant onshore precipitation. In this study, we focused on offshore storm events during which no onshore precipitation occurred. Extensive groundwater level data collected on a sand barrier island (Sable Island, NS, Canada) show nonuniform discontinuous propagation of the tide and offshore storm pulse signals through the aquifer with isolated inland areas showing enhanced response to both oceanic forcing signals. Propagation analysis suggests that isolated inland water table fluctuations may be caused by localized leakage from a confined aquifer that is connected to the ocean offshore but within the wave setup zone. Two‐dimensional groundwater flow simulations were conducted to test the leaky confined‐unconfined aquifer conceptualization and to identify the effect of key parameters on tidal signal propagation in leaky confined‐unconfined coastal aquifers. This study illustrates that analysis of offshore storm signal propagation, in addition to tidal signal propagation, provides a valuable and low resource approach for large‐scale characterization of permeable heterogeneous coastal aquifers. Such an approach is needed for the effective management of coastal environments where water resources are threatened by human activities and the changing climate.