Groundwater Flow Model Research Articles

Quantifying uncertainty in the temporal disposition of groundwater inundation under sea level rise projections

Over the next century, coastal regions are under threat from projected rising sea levels and the potential emergence of groundwater at the land surface (groundwater inundation). The potential economic and social damages of this largely unseen, and often poorly characterised natural hazard are substantial. To support risk-based decision making in response to this emerging hazard, we present a Bayesian modelling framework (or workflow), which maps the spatial distribution of groundwater level uncertainty and inundation under Intergovernmental Panel on Climate Change (IPCC) projections of Sea Level Rise (SLR). Such probabilistic mapping assessments, which explicitly acknowledge the spatial uncertainty of groundwater flow model predictions, and the deep uncertainty of the IPCC-SLR projections themselves, remains challenging for coastal groundwater systems. Our study, therefore, presents a generalisable workflow to support decision makers, that we demonstrate for a case study of a low-lying coastal region in Aotearoa New Zealand. Our results provide posterior predictive distributions of groundwater levels to map susceptibility to the groundwater inundation hazard, according to exceedance of specified model top elevations. We also explore the value of history matching (model calibration) in the context of reducing predictive uncertainty, and the benefits of predicting changes (rather than absolute values) in relation to a decision threshold. The latter may have profound implications for the many at-risk coastal communities and ecosystems, which are typically data poor. We conclude that history matching can indeed increase the spatial confidence of posterior groundwater inundation predictions for the 2030-2050 timeframe.

Estimation of Aquifer Storativity Using 3D Geological Modeling and the Spatial Random Bagging Simulation Method: The Saskatchewan River Basin Case Study (Central Canada)

Hydrosystems in the Saskatchewan River Basin of the Canadian Prairies are subject to natural and socioeconomic pressures. Increasingly, these strong pressures are exacerbating problems of water resource accessibility and depletion. Unfortunately, the geometric heterogeneity of the aquifers and the presence of lithologically varied layers complicate groundwater flow studies, hydrodynamic characterization, and aquifer storativity calculations. Moreover, in recent hydrogeological studies, hydraulic conductivity has been the subject of much more research than storativity. It is in this context that the present research was conducted, to establish a 3D hydrostratigraphic model that highlights the geological (lithology, thickness, and depth) and hydrodynamic characteristics of the aquifer formations and proposes a new uncertainty framework for groundwater storage estimation. The general methodology is based on collecting and processing a very fragmentary and diverse multi-source database to develop the conceptual model. Data were harmonized and entered into a common database management system. A large quantity of geological information has been implemented in a 3D hydrostratigraphic model to establish the finest geometry of the SRB aquifers. Then, the different sources of uncertainty were controlled and considered in the modeling process by developing a randomized modeling system based on spatial random bagging simulation (SRBS). The results of the research show the following: Firstly, the distribution of aquifer levels is controlled by tectonic activity and erosion, which further suggests that most buried valleys on the Prairies have filled over time, likely during multiple glaciations in several depositional environments. Secondly, the geostatistical study allowed us to choose optimal interpolation variographic parameters. Finally, the final storativity maps of the different aquifer formations showed a huge potential of groundwater in SRB. The SRBS method allowed us to calculate the optimal storativity values for each mesh and to obtain a final storativity map for each formation. For example, for the Paskapoo Formation, the distribution grid of groundwater storage shows that the east part of the aquifer can store up to 5920 × 103 m3/voxel, whereas most areas of the west aquifer part can only store less than 750 × 103 m3/voxel. The maximum storativity was attributed to the Horseshoe Canyon Formation, which contains maximal geological reserves ranging from 107 to 111 × 109 m3. The main contribution of this research is the proposed 3D geological model with hydrogeological insights into the study area, as well as the use of a new statistical method to propagate the uncertainty over the modeling domain. The next step will focus on the hydrodynamic modeling of groundwater flow to better manage water resources in the Saskatchewan River Basin.

Impacts of Subsurface Dam Construction on Downstream Groundwater Levels and Salinity in Coastal Aquifers.

Subsurface dam is a promising engineering technology for groundwater resources development. However, the possible impacts of these dams on the groundwater environment have been a major concern. Here, we used a three-dimensional (3D), variable-density, unsaturated-saturated groundwater flow model to explore how a groundwater-storage-type subsurface dam, built in the freshwater domain of an unconfined coastal aquifer, affected groundwater levels and salinity in the downstream area. Model results suggested that, after subsurface dam construction, groundwater levels in the downstream area showed intensified fluctuations in terms of phase advances, greater amplitudes, and higher frequencies following heavy rainfall events. Numerical simulations with variable subsurface dam scenarios indicated that the groundwater level fluctuations were further intensified with a higher crest elevation or a shorter distance from the coast. Moreover, during the recharging period of the subsurface reservoir, sea water in the downstream area intruded landward from its initial location, which can at least temporarily threaten water quality near the coast. A higher dam crest elevation prolonged the duration of sea water intrusion, while a dam positioned closer to the coast induced sea water intrusion with a greater horizontal extent. General implications are discussed with respect to improving assessment methodologies and engineering designs of subsurface dams.

Development of Groundwater Flow Models for the Integrated Management of the Alluvial Aquifer Systems of Dravsko polje and Ptujsko polje, Slovenia

With increasing exploitation of groundwater resources and implementation of various activities in their recharge areas, it is vital to conduct a comprehensive assessment of aquifers to ensure their conservation and sustainable management. In the present study, we used a comprehensive approach to conceptualise and identify the functioning of two connected aquifer systems in north-eastern Slovenia: the Quaternary porous aquifers Dravsko polje and Ptujsko polje. The study presents the conceptual models of both aquifers and their interconnectedness using separate mathematical-numerical models with the aim of ensuring an integrated management of these alluvial aquifer systems. It also highlights the importance of understanding connections between such systems for simulating groundwater flow and transport of different contaminants. To describe the entire aquifer system, the study defines its three essential elements: the geometry of the aquifers, their recharge by precipitation, and other boundary conditions. The geometry of the Quaternary aquifers was defined using Sequential Indicator Simulation (SIS) with the ESRI’s ArcMap software. Next, LIDAR was used for determining their surface geometry. The hydrogeologic model was designed using the Groundwater Modelling System (GMS) developed by AQUAVEO. We used the MODFLOW 2000 calculation method based on the finite difference method (FDM). The model was calibrated with the PEST module, which was used to calibrate hydraulic conductivity and hydraulic heads between the measured and modelled data. Finally, the model was validated using the Nash–Sutcliffe (NSE) efficiency coefficient. In addition, the model results estimated using the PEST tool were validated with the hydraulic conductivities determined at the pumping sites (pumping tests), each belonging to water protection zones that define the maximum travel time of the particles. This was performed using the MODPATH method. The paper also presents the possibility of modelling heterogeneous but interdependent aquifers in a groundwater body. Modelling the connection between the two aquifers, which are the most important ones in the region, is essential for a comprehensive management of the entire system of water resources. The models allow for a better understanding of groundwater flow in both aquifers. Moreover, their interconnectedness will be used for further studies in this field, as well as for integrated water management.

Quantification of Groundwater Hazards Related to Fluvial Floods via Groundwater Flow Modelling: A Review

Flood-related issues include the impact of groundwater on flood protection measures and other subsurface structures in a protected area. At the same time, subsurface elements of flood protection structures may significantly influence the natural groundwater regime and affect existing structures during non-flood periods. The paper provides an overview of hazardous factors linked to groundwater and specifies variables for the quantification of related hazards. Appropriate hydraulic groundwater flow models are presented and discussed, and their suitability for the modelling of individual typical problems and for hazard quantification is specified. The use of models and the application of CAD and GIS tools for data pre- and post-processing is mentioned in brief and demonstrated on examples of typical practical situations.

Synthesizing multifaceted characterization techniques to refine a conceptual model of groundwater sources to springs in valley settings (Minnesota, USA)

Springs are commonly used as low-cost monitoring locations to assess groundwater quality and long-term trends. However, spring waters in many settings are a mixture of groundwater sources that range in physical properties and water chemistry. The objective of this work was to determine water sources of springs emerging from the North American midcontinent Cambrian-Ordovician aquifer system at a fish hatchery near Lanesboro, Minnesota (USA), and compare and contrast the sources to shallower and deeper sources. The hydrology of the Lanesboro State Fish Hatchery has been studied for decades using a combination of dye tracing, thermal monitoring, geochemical sampling, and nearby borehole and outcrop observations. Previous studies are integrated with recently collected dye tracing results and geochemical data to develop a comprehensive conceptual model of groundwater flow. Dye trace findings and geochemistry indicate well-developed karst and bedrock fractures in shallowly buried unconfined carbonate formations are important transport pathways to convey anthropogenically influenced waters from the land surface to the hatchery springs. However, borehole dye traces, thermal monitoring, continuous nitrate monitoring, and mixing calculations show that a deeper confined siliciclastic aquifer is responsible for delivering relatively pristine water that accounts for about half of hatchery spring flux. Characterization of the hatchery’s groundwater systems provides fishery managers with information to protect this vital resource and improved context to interpret water-quality-monitoring data that track agricultural contaminants. The methods and results of this study may be widely applicable across a large extent of the Cambrian-Ordovician aquifer system, and to multiaquifer sedimentary bedrock systems elsewhere.

Effect of unsaturated flow on groundwater-river interactions induced by flood event in riparian zone

A coupled unsaturated–saturated model for groundwater flow in a riparian zone is established to evaluate the impacts of unsaturated zone on groundwater flow and aquifer-river interactions during a flood event. The semi-analytical solutions for hydraulic head and discharge are obtained by the Laplace-Fourier transform and verified with COMSOL Multiphysics. The results show that the groundwater flow induced by a flood wave is significantly influenced by the dimensionless constitutive exponent κD in the moisture characteristic curve: the larger the value of κD, the stronger the water storage capacity of unsaturated zones, leading to smaller fluctuations of the hydraulic head and higher water amount exchanging between an aquifer and river. The present solution approaches to the solution for an unconfined aquifer when κD=100, and approaches to the solution of confined aquifers when κD=0.01. A thicker unsaturated zone causes less hydraulic head change and slower head recession after a flood wave passes. The traditional solution for an unconfined aquifer tends to underestimate the hydraulic head and overestimate the water exchange between the aquifer and river because it neglects the water retention of the unsaturated zone. The solution is applied to field data, and it improves the prediction of hydraulic heads over a previous solution.

Hydrogen sulphide (H2S) migration in groundwater of the Zechstein strata in the Legnica-Głogów Copper Basin and its vicinity, SW Poland

Hydrogen sulphide (H2S) occurs in groundwater in various lithostratigraphic units of the Zechstein Basin in the Legnica-Głogów Copper Basin (SW Poland). This region is located in the Fore-Sudetic Monocline within which, several tens of kilometres NE of the study area, at greater depths, natural gas fields with hydrogen sulphide (H2S) occur. The Main Dolomite (Ca2), in which H2S-containing natural gas has accumulated, is younger than the Zechstein Limestone (Ca1), which is actively mined. The Ca2 and Ca1 formations are separated by a thick anhydrite succession including a wedge-shaped salt body. Hydrochemical analyses of 18 groundwater samples taken from different horizons within the Zechstein strata showed spatial variability ofH2S and chloride concentrations. A conceptual model of groundwater flow with dissolved H2S in the Zechstein formations was developed. H2S migration is associated with groundwater flow between the Ca2 and Ca1 aquifers through fissures in the anhydrite strata that separate them. Hydraulic contact through fissures in the anhydrite layers is the result of long-term exploitation of the underground copper deposit. Groundwater flow between the layers is influenced by a large change in the piezometric pressure of the groundwater in the depression cone caused by mining drainage.

Modeling the river-aquifer via a new exact model under a more general function of river water level variation

The interaction between surface water and groundwater is a significant topic in groundwater-related problems. This study suggests an exact model based on Laplace transformation to calculate the groundwater flow in river-aquifer systems. Exact models play an important role in simulating the future behavior of river-aquifer systems. Therefore, investigation of the exact models for river-aquifer systems is a hot topic in the hydraulics of groundwater flow modeling. The objective of this research is to present new exact models for simulating the hydraulics of groundwater flow in river-aquifer systems with a more general function of river level variation under recharge by means of Laplace transform method. A general function is adopted to describe the river level variation, in which some situations such as linear, exponential and power of time variations in the river level can be treated as special cases. The effects of variations in aquifer parameters on groundwater hydraulic head are evaluated. It is shown that the groundwater hydraulic head grows slower in aquifers with a greater thickness or hydraulic conductivity. In addition, the effect of changes in specific storage is too little on the groundwater hydraulic head. The variations in hydraulic heads due to changes in recharge rate with different values of thickness, hydraulic conductivity, specific storage, and length are analyzed. It is observed that the groundwater hydraulic head in an aquifer with a lesser length, higher hydraulic conductivity or higher thickness is less sensitive to a change in the recharge rate than in an aquifer with a higher length, lesser hydraulic conductivity or lesser thickness. Furthermore, it is shown that the differences in hydraulic heads due to the increase in recharge rate are not significant for different values of specific storage. The results of the present new exact models are successfully verified by the results obtained from the analytical solution of Bansal and Das. Also, for more reliability, the results are compared with those results of MODFLOW. The results show that the presented new exact models are accurate, robust and efficient. One of the advantages of the solutions is to investigate the sensitivity analysis of aquifer parameters, which has been carried out in this paper. Furthermore, in the present research a more general function describing river level variation is considered, in which the linear, exponential and power of time variations are special cases.

Controls on flood managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

Abstract. In water-stressed regions of the world, managed aquifer recharge (MAR), the process of intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependent regions, including the Central Valley in California, USA, are underlain by thick unsaturated zones (ca. 10 to 40 m thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within the saturated zone, interconnected deposits of coarse-grained material (sands and gravel) can act as preferential recharge pathways, while fine-textured facies (silts and clays) accommodate the majority of the long-term increase in aquifer storage. However, this relationship is more complex within the vadose zone. Coarse facies can act as capillary barriers that restrict flow, and contrasts in matric potential can draw water from coarse-grained flow paths into fine-grained, low-permeability zones. To determine the impact of unsaturated-zone stratigraphic heterogeneity on MAR effectiveness, we simulate recharge at a Central Valley almond orchard surveyed with a towed transient electromagnetic system. First, we identified three outcomes of interest for MAR sites: infiltration rate at the surface, residence time of water in the root zone and saturated-zone recharge efficiency, which is defined as the increase in saturated-zone storage induced by MAR. Next, we developed a geostatistical approach for parameterizing a 3D variably saturated groundwater flow model using geophysical data. We use the resulting workflow to evaluate the three outcomes of interest and perform Monte Carlo simulations to quantify their uncertainty as a function of model input parameters and spatial uncertainty. Model results show that coarse-grained facies accommodate rapid infiltration rates and that contiguous blocks of fine-grained sediments within the root zone are >20 % likely to remain saturated longer than almond trees can tolerate. Simulations also reveal that capillary-driven flow draws recharge water into unsaturated, fine-grained sediments, limiting saturated-zone recharge efficiency. Two years after inundation, fine-grained facies within the vadose zone retain an average of 37 % of recharge water across all simulations, where it is inaccessible to either plants or pumping wells. Global sensitivity analyses demonstrate that each outcome of interest is most sensitive to parameters that describe the fine facies, implying that future work to reduce MAR uncertainty should focus on characterizing fine-grained sediments.

Parameterization of a channel network model for groundwater flow in crystalline rock using geological and hydraulic test data

Groundwater flow in sparsely fractured crystalline rocks is highly channelized due to the existing complex hydrogeological heterogeneity in fracture networks. The impacts of multiscale hydraulic heterogeneity on channelized flow in a block of fractured rock under the unidirectional flow condition are investigated by three-dimensional (3D) discrete fracture network (DFN) modeling. A channel network (CN) model generated from DFN is used to model the channelized groundwater flow in fractured crystalline rock. An approach for parameterizing the channel conductance is proposed which uses information from the hydrogeological characterization data. The results show that the network scale hydrogeological heterogeneity dominates the distribution variability of flowrates. The proposed parameterization approach for the channel conductance is effective and robust. Based on the available hydrogeological characterization data, it is possible to compensate for the neglected heterogeneity in the CN model by enhancing the variability of assigned channel conductance. The findings from this work are useful for model simplification from 3D DFN to CN, and for overcoming the difficulty in the parameterization of CN models in applications.

Quantifying the impacts of a proposed hydraulic dam on groundwater flow behaviors and its eco-environmental implications in the large Poyang Lake-floodplain system

Dam-induced hydrological alterations and eco-environmental impacts have significant implications, however, these concern issues in large floodplain systems are less well understood. The present study shows a first attempt to adopt a quasi-three-dimensional groundwater flow modeling FEFLOW (Finite Element subsurface FLOW system) to investigate the influences of a proposed hydraulic dam on groundwater dynamics in the largest floodplain lake of the Yangtze River basin (Poyang Lake, China). The FEFLOW model was successfully constructed and has the ability to represent the hydrodynamics of floodplain groundwater flow. Model simulations indicate that, in general, the dam is likely to increase the groundwater levels across the floodplain during different hydrological phases. The responses of floodplain groundwater levels to the dam during the dry and recession phases are stronger (∼2–3 m) than the rising and flooding phases (<2 m). Under the natural condition, the floodplain groundwater may recharge the lake during the dry and recession phases, and discharge the lake during the rising and flooding phases. However, the dam regulation may alter the natural recharge-discharge patterns, forming a generally gaining condition of the floodplain groundwater. The proposed dam is most likely to reduce the groundwater flow velocity (∼<1 m/d) relative to the natural condition (up to 2 m/d) during different hydrological phases, and it may also alter the floodplain groundwater flow direction during the dry and recession phases. Additionally, the floodplain groundwater system is mainly characterized by losing state (−4.5 × 106 m3/yr) under the natural condition, while the dam-induced groundwater system exhibits an overall gaining state (9.8 × 106 m3/yr). The current research findings contribute to future water resources assessment and management by providing a foundation for assessing associated eco-environmental changes of the large lake-floodplain system.

Simulating spatial variability of groundwater table in England and Wales

AbstractGroundwater is a vital natural resource that affects human life and the environment. Groundwater supplies drinking water, supports irrigation for food production, and helps to sustain flow of water in streams and wetlands during dry periods. Therefore, it is important to assess the variability of groundwater resources and consider it in freshwater management. Numerical modelling is widely used for understanding the variability of groundwater table. However, there is still a lack of large‐scale groundwater flow models. For instance, in the United Kingdom, groundwater modelling efforts so far have focused on aquifer to regional scales. A national scale model would aid the quantification of the available groundwater storage and the assessment of the impacts of climate change and anthropogenic activities on this crucial resource. In this study, we evaluate a numerical groundwater flow modelling approach for entire England and Wales. This model operates at 1 km spatial and 1 day temporal resolution. We force the model with precipitation and potential evapotranspiration, parameterise the subsurface using our estimation of transmissivity and specific yield based on available information, and simulate steady‐state groundwater heads over England and Wales. The results demonstrate that the model can reproduce spatial variability of groundwater table in the region. Because of a two‐dimensional numerical representation of groundwater flow, the computational demand of this model is low, facilitating multiple simulations required for calibration, sensitivity analysis, and quantification of the impacts of environmental changes on groundwater resources.

Unstructured-grid approach to develop high-fidelity groundwater model to understand groundwater flow and storage responses to excessive groundwater withdrawals in the Southern Hills aquifer system in southeastern Louisiana (USA)

Study regionThe Southern Hills aquifer system (SHAS) in the Louisiana Capital Area Groundwater Conservation District (CAGCD), USA. Study focusThe SHAS provides abundant groundwater for public and industrial supplies in the CAGCD. Groundwater depletion, saltwater intrusion, and land subsidence are potential concerns due to prolonged excessive groundwater withdrawals. This study develops a high-fidelity groundwater flow model utilizing a complex unstructured grid to investigate groundwater flow and storage responses to excessive groundwater withdrawals for the SHAS in the CAGCD. The groundwater model incorporates the Mississippi River alluvial aquifer down to the Miocene sands extending to depths around 1 km. New hydrological insightsGroundwater modeling results indicate large cones of depression in the Evangeline and Jasper formations in the Baton Rouge area due to prolonged groundwater withdrawals. Low-permeability faults are inferred by significant groundwater level difference across the faults. While local groundwater storage depletion in deeper aquifers is evident, overall estimated groundwater storage changes of the SHAS in the CAGCD are close to zero in the past two decades, indicating insignificant groundwater storage changes. This is attributed to dominant interactions between the major rivers and the shallower alluvial aquifer. In addition, the simulated groundwater storage changes exhibit patterns similar to those derived by the Gravity Recovery and Climate Experiment (GRACE) model that has been used in evaluation of groundwater depletion in many regional studies.

ELM Lateral Groundwater Flow model documents

ELM Lateral Groundwater Flow model documents

Groundwater levels forecasting using machine learning models: A case study of the groundwater region 10 at Karst Belt, South Africa

The crucial role which groundwater resource plays in our environment and the overall well-being of all living things can not be underestimated. Nonetheless, mismanagement of resources, over-exploitation, inadequate supply of surface water and pollution have led to severe drought and an overall drop in groundwater resources’ levels over the past decades. To address this, a groundwater flow model and several mathematical data-driven models have been developed for forecasting groundwater levels. However, there is a problem of unavailability and scarcity of the on-site input data needed by the data-driven models to forecast the groundwater level. Furthermore, as a result of the dynamics and stochastic characteristics of groundwater, there is a need for an appropriate, accurate and reliable forecasting model to solve these challenges. Over the years, the broad application of Machine Learning (ML) and Artificial Intelligence (AI) models are gaining attraction as an alternative solution for forecasting groundwater levels. Against this background, this article provides an overview of forecasting methods for predicting groundwater levels. Also, this article uses ML models such as Regressions Models, Deep Auto-Regressive models, and Nonlinear Autoregressive Neural Networks with External Input (NARX) to forecast groundwater levels using the groundwater region 10 at Karst belt in South Africa as a case study. This was done using Python Mx. Version 1.9.1., and MATLAB R2022a machine learning toolboxes. Moreover, the Coefficient of Determination (R2);, Root Mean Square Error (RMSE), Mutual Information gain, Mean Absolute Percentage Error (MAPE), Mean Squared Error (MSE), Mean Absolute Error (MAE), and the Mean Absolute Scaled Error (MASE)) models were the forecasting statistical performance metrics used to assess the predictive performance of these models. The results obtained showed that NARX and Support Vector Machine (SVM) have higher performance metrics and accuracy compared to other models used in this study.

The role of Canadian research in advancing groundwater hydrology: historical sketches from the past 75 years

Canadian hydrologists have played important roles in advancing the scientific understanding of groundwater flow in the hydrologic cycle. This article highlights some of the major episodes that made fundamental contributions to groundwater hydrology, rather than attempt a comprehensive review of all Canadian advances in groundwater research. The early years of groundwater research were driven by the need to find water supplies and to evaluate the capacity of wells and aquifers using the well hydraulics theories. The research focus shifted to the understanding of groundwater flow in drainage basins in the late 1950s and the 1960s through field observations using piezometer networks, rigorous mathematical analysis, and the development of numerical groundwater flow models. These researchers established a new conceptual framework and mathematical tools to describe groundwater flow systems in the context of topographic features and geological heterogeneity. In the 1970s, Canadian researchers made important contributions in establishing the new paradigm of hillslope hydrology by field observations and numerical modelling of storm runoff generation, development of tracer-based hydrograph separation techniques, and the demonstration of the role of the capillary fringe. The groundwater research group at the University of Waterloo pioneered contaminant hydrogeology research in the 1970s and 1980s, which generated new knowledge of groundwater flow and transport processes and developed increasingly sophisticated numerical models capable of simulating coupled surface and subsurface flow and transport. The scope of groundwater hydrology diversified in the 1990s onward including the interaction of surface water and groundwater as well as the interaction between hydrological and ecological systems. Climate-change impacts on groundwater are a major topic of current and future research. To address this and other challenging problems, it is important to establish and maintain long-term monitoring sites. The history has shown that the scientific advances presented in this article would have been impossible without high-quality field data.

Acesulfame allows the tracing of multiple sources of wastewater and riverbank filtration

Aquifers providing drinking water are increasingly threatened by emerging contaminants due to wastewater inputs from multiple sources. These inputs have to be identified, differentiated, and characterized to allow an accurate risk assessment and thus ensure the safety of drinking water through appropriate management. We hypothesize, that in climates with seasonal temperature variations, the sweetener acesulfame potassium (ACE) provides new pathways to study wastewater inputs to aquifers. Specifically, this study investigates the temperature-driven seasonal oscillation of ACE to assess multiple sources of wastewater inputs at a riverbank filtration site. ACE concentrations in the river water varied from 0.2 to 1 μg L−1 in the cold season (T < 10 °C) to 0–0.1 μg L−1 in the warm season (T > 10 °C), due to temperature-dependent biodegradation during wastewater treatment. This oscillating signal could be traced throughout the aquifer over distances up to 3250 m from two different infiltration sources. A transient numerical model of groundwater flow and ACE transport was calibrated over hydraulic heads and ACE concentrations, allowing the accurate calculation of mixing ratios, travel times, and flow-path directions for each of the two infiltration sources. The calculated travel time from the distant infiltration source was of 67 days, while that from the near source was of 20 days. The difference in travel times leads to different potential degradation of contaminants flowing into the aquifer from the river, thus demonstrating the importance of individually assessing the locations of riverbank infiltration. The calibrated ACE transport model allowed calculating transient mixing ratios, which confirmed the impact of river stage and groundwater levels on the mixing ratio of the original groundwater and the bank filtrate. Therefore, continuous monitoring of ACE concentrations can help to optimize the management of the water works with the aim to avoid collection of water with very short travel times, which has important regulative aspects. Our findings demonstrate the suitability of ACE as a transient tracer for identifying multiple sources of wastewater, including riverbank filtration sites affected by wastewater treatment plant effluents. ACE seasonal oscillation tracking thus provides a new tool to be used in climates with pronounced seasonal temperature variations to assess the origins of contamination in aquifers, with time and cost advantages over multi-tracer approaches.

A data-driven approach to exploring the causal relationships between distributed pumping activities and aquifer drawdown

Groundwater depletion, typically caused by the distributed pumping activities of multiple stakeholders (i.e., water users) that share a hydrologically connected aquifer, has led to severe environmental and ecological problems in many river basins worldwide. Conventionally, the effects of pumping on aquifer depletion are quantified using well hydraulics or physically based hydrological models in groundwater management. However, the derivation of well hydraulics-based analytical solutions requires numerous simplifying assumptions, while the construction and calibration of a physically based groundwater flow model require detailed information about the subsurface properties, which are subject to large uncertainties. In this study, we develop a novel modeling framework that does not rely on well hydraulics or groundwater flow models. The proposed framework integrates (1) a deep learning model that captures the spatiotemporal variations in the aquifer in response to distributed pumping activities in multiple well fields and (2) a statistical causal inference model that identifies the causal networks among stakeholders to quantify the causal effects of individual pumping activities on aquifer depletion. The proposed framework is tested on a synthetic case study site with well fields that have various spatial distributions and pumping rates. The modeling results show that the deep learning method can effectively capture the water table dynamics influenced by distributed pumping activities with R2 >90 % for all observation data. More importantly, our model is capable of assessing the causal networks between the drawdown of water table and the pumping activities of multiple well fields and quantifying their causal strengths. These results suggest that our modeling framework can be used to explicitly assess the extent to which each individual stakeholder's pumping activities contribute to aquifer depletion at the system level. The concepts and techniques developed in this study can be used to resolve classic externality problems in the context of common-pool groundwater management.

Assessing the impact of groundwater abstractions on aquifer depletion in the Cauvery Delta, India

Many of the world’s deltas are highly productive areas for agriculture as well as important places of socio-economic development but are currently under stress. This study assesses the impacts of stresses on groundwater pumping, changing cropping patterns, and saltwater intrusion on groundwater resources in the Cauvery Delta in Tamil Nadu, India. A transient groundwater flow model of the delta was constructed for this assessment. The historical changes in groundwater resources in response to decreasing irrigation canal flows and increasing groundwater abstractions were assessed for the past 30 years. Furthermore, the model was used to formulate and analyze future sustainable groundwater development scenarios. Farmers’ narratives about a drying delta, as they experience water scarcity and quality issues most closely, were ascertained in the research. Farmers have abandoned many shallow wells in their fields. The model simulation shows groundwater levels are decreasing and aquifer storage depleting. Furthermore, salinization has increased, with continuous declining groundwater levels in the deep aquifer from 1990 to 2019. A more robust hydro-chemical assessment and further modeling of seawater intrusion are needed to better assess the sources and distribution of groundwater salinity. The pathway for future sustainability requires enhancing groundwater recharge in the shallow aquifer and controlling groundwater abstraction in the deep aquifers.