Groundwater Solute Transport Model Research Articles

Adaptive machine learning surrogate based multiobjective optimization for scavenging residual saltwater behind subsurface dams

High-fidelity physically based groundwater flow and solute transport models have been limited for seawater intrusion remediation design because of computationally intensive evolutionary algorithms. Data-driven machine learning approaches are promising to substitute expensive-cost groundwater numerical models within optimization due to computing efficiency. However, machine learning surrogates may accumulate error of forecasting and thereby result in infeasible optimal solutions. To achieve desired fidelity level, this study proposes a novel adaptive machine learning surrogate based multiobjective optimization method for coastal aquifer desalination. An adaptive modeling algorithm is newly introduced to iteratively retrain poorly-performed machine learning models and enhance predicting accuracy. The method is demonstrated to seek optimal extraction and injection strategies for scavenging residual saltwater trapped in an upstream aquifer behind a subsurface dam. Two conflicting objectives of minimizing the total extraction-injection rate and maximizing saltwater removal effectiveness are considered. Three machine learning models including artificial neural network (ANN), Gaussian process (GP) and response surface regression model (RSR) are developed to replace a high-fidelity seawater intrusion model for predicting chloride concentration and salinity mass. Non-dominated Sorting Genetic Algorithm II (NSGA-II) is employed to derive Pareto fronts. Pareto optimal solutions obtained from machine learning models are compared against those from the seawater intrusion model. Results indicate that the developed machine learning models do not only have strong predicting capability, but also maintain good quality of Pareto optimal solutions, while achieving substantial computational saving up to 95%. Especially, the adaptively retrained RSR model inhibits error accumulation of forecasting and accelerates correct convergence to the true Pareto front. The proposed method is found superior performance in accurate convergence, widely-spread diversity as well as high computing efficiency than conventional methods.

Unravelling the influence of heterogeneities and abstraction on groundwater flow and solute transport in a fractured carbonate aquifer, Sicily, Italy

The migration of a groundwater contaminant plume consisting of light nonaqueous phase liquids (LNAPLs) along the Ionian coastline of Sicily, Italy, has been reported to follow a trajectory that is inconsistent with the regional hydraulic gradient. The influence of several faults affecting the fractured carbonate bedrock aquifer, and groundwater abstraction from a well, were hypothesized to be responsible for the anomalous trajectory of the contaminant plume. A conceptual hydrogeological model was developed for the study area that incorporated structural information derived from geophysical surveys and the mapping of fractures in bedrock outcrops. This conceptual model was incorporated into numerical groundwater flow and solute transport models to simulate the groundwater transport of the light nonaqueous phase liquids. Four model scenarios representing different levels of complexity were tested to assess the relative influence of geological heterogeneity and groundwater abstraction on the migration of the contaminant plume. Results show that underground major discontinuity systems, invoking the presence of the faults in the model domain accounted for the observed migration of the contaminant plume, act as conduits for groundwater flow. Conversely, groundwater abstraction from a well was found to result in relatively minor, localized impacts to the migration of the contamination plume. This study demonstrates the importance of incorporating geological heterogeneity into groundwater modelling and environmental risk assessments associated with the storage of LNAPLs.

Impacts of groundwater dynamics around a macro-tidal river on agricultural soil salinity

Estuaries are vulnerable to oceanic and atmospheric climate change. Much of the research investigating climate change impacts on estuaries is focused on saltwater intrusion within surface water due to drought and rising sea levels, with implications for ecosystems and humans. Groundwater and soil near estuaries may also be influenced, as estuary salinity and hydraulic head changes can impact soils and aquifers not previously at risk of salinization. This study was conducted to address knowledge gaps related to present and future groundwater salinity distribution in a groundwater system connected to a macro-tidal estuary. The studied estuary experiences a tidal bore due to its hydraulic connection to the Bay of Fundy in Nova Scotia, Canada. A parcel of agricultural land adjacent to the estuary was selected to assess the groundwater response to episodic fluctuations in estuary water levels and salinity. Groundwater monitoring and electromagnetic surveys were conducted to map soil and groundwater salinity patterns. A numerical model of groundwater flow and solute transport informed by field data was used to investigate how varying estuary salinity due to droughts and sea-level rise could impact groundwater salinity. Results showed that, in contrast to salt wedges observed along marine coasts, the saline groundwater existed as a plume immediately around the estuary. Model simulations showed that short-term droughts had an insignificant impact on the adjacent groundwater salinity. However, permanent increases in salinity caused by sea-level rise increased the plume volume by 86 %, or an additional ∼11 m horizontally and ∼ 4.5 m vertically. Our results suggest that increased river salinity in this setting would not result in widespread salinization of porewater and agricultural soils, but more extensive salinization may be experienced in permeable aquifers or along more saline estuarine zones. Findings may inform land management decisions in regions exposed to increased salinity in the future.

Island hydrogeology in the tropics: Constraining a 3D variable-density groundwater flow and solute transport model with geophysics

Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A combination of traditional hydrogeological datasets and an airborne electromagnetic survey were used to develop a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. This investigation represents one of very few studies that have taken such an approach. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d−1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d−1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.

A numerical modelling approach to investigate the fate of brine reject of farm scale desalination plants on groundwater aquifers in arid environments

Farm-scale desalination units are gaining popularity for agricultural irrigation in arid countries, such as Kuwait to meet freshwater demands. However, less attention has been given to the management of environmentally hazardous brine reject water they produce. In this study we investigated the fate of brine water produced by the inland desalination units on the underlying aquifers using numerical modelling and field investigations. The methodology involved developing groundwater flow and solute transport models using Flex VMF-SEAWAT to simulate the movement of reject brine. The field investigations included collecting 150 water samples and conducting pumping tests on newly drilled wells. This numerical simulation considered advection, dispersion, and adsorption processes with variable groundwater density following rigorous validation and calibration of the developed numerical models. The results show that the RO reject brine will significantly increase groundwater salinity, exceeding 10,000 mg/L when accounting for advection, dispersion, and adsorption processes. The sustainable yield of the aquifer, with a salinity of <10,000 mg/L, averages 500 Mm3 but is expected to be depleted within 16 years with the current extraction rate. The resulting hydraulic properties are favourable with K about 100 m/d, T > 1000 m2/day, and Sy just >0.1. The adopted values for dispersivity and adsorption coefficients for chloride and sulphate salts in the aquifer were 10 m and 1 × 10−7 [mg/L]−1 respectively. Chemical and numerical analyses indicate a mixing ratio between the reject brine and groundwater in the study area of approximately 10 %. Uncontrolled groundwater extraction, combined with the surface disposal of RO reject brine, has led to a significant decline in groundwater levels and an increase in the salinity. The adsorption ratio of simulated brine plume was 13 %. The authors recommend to dispose the RO reject water in a safe location or transfer it to the nearest wastewater treatment plant for proper treatment and reuse.

Pumping-induced groundwater aging and rejuvenation in aquifer-aquitard systems: A perspective from regional groundwater flow

As an indicator of the renewability of a groundwater basin, the spatial distribution of groundwater age has attracted intensive research. As reported in the literature, 3H measurements in 74 US aquifers showed pumping-induced groundwater rejuvenation, whereas 14C ages in two heavily developed aquifers in France and China indicated pumping-induced groundwater aging. Unfortunately, the mechanisms leading to the seemingly opposite behavior of pumping-induced aging and rejuvenation remain unclear. In this study, we conducted numerical simulations of groundwater age in layered aquifer-aquitard systems under different pumping scenarios and aquitard permeability to show that pumping-induced groundwater rejuvenation and aging always occur simultaneously. Groundwater rejuvenation widely occurs throughout the basin wherever the well is located, whereas groundwater aging mainly occurs to the downgradient parts of the basin. Although groundwater rejuvenation is larger in area, groundwater aging is greater in magnitude, and the magnitude of aging increases with pumping intensity and duration. Moreover, when pumping occurs in the discharge area, aging could occur in almost the entire aquifer to the downgradient of the pumping well, and the magnitude of aging is larger. The large area and large magnitude of aging in the discharge area is a direct result of upward leakage of old groundwater from underlying aquitards/aquifers, which is controlled by the topography-driven groundwater flow. By demonstrating the importance of topography-driven groundwater flow and underlying formations in groundwater age redistribution, this study has implications for determining the bottom of a groundwater flow and solute transport model and interpretation of various tracers.

Transport of pollutants in groundwater of domestic waste landfills in karst regions and its engineering control technologies

Domestic waste produces leachate with a high concentration of pollutants in the landfill process due to biochemical degradation stages like compaction and fermentation. A large number of cases show that anti-seepage membranes widely used in refuse landfills tend to rupture under long-term tension and corrosion, causing leachate to enter the groundwater system and pollute the environment. To reveal the phenomenon of groundwater contamination in refuse landfills, typical domestic waste landfills in karst regions were examined, on the basis of a summary of hydrogeological conditions and hydrochemical characteristics, a three-dimensional groundwater flow model and solute transport model were constructed to analyze the pattern of pollutant diffusion, and its controlling factors, under the current conditions and massive rupture of anti-seepage membrane. The results show that with a minor rupture of the anti-seepage membrane, the area of the low pollution region increases first and then decreases while that of the slight pollution region continuously increases; When a massive rupture of the anti-seepage membrane appears, the ranges of heavy pollution region and total pollution regions continue to grow; Pollutant migrates along the same direction as the groundwater flow and diffuse from high concentration region to low concentration regions under the differential concentration effect. Based on the temporal-spatial distribution characteristics of groundwater pollutants, two engineering control schemes, namely, curtain grouting blocking and group well pumping, were established. A comparison of the two control schemes shows that group well pumping stably maintains water quality safety over the long term, pollutants overflow from both sides of the curtain after they have accumulated to a certain point of concentration, causing damage to the groundwater environment in the conservation area.

A Novel Simulation-Optimization Model Built by FloPy: Pollutant Traceability in a Chemical Park in China

Heavy metal pollution of groundwater will not only destroy the ecological environment but also negatively affect the functioning of the human liver. Tracing the source of groundwater pollution is an important way to protect groundwater resources. FloPy is promoting the use of big data in the groundwater field, especially in groundwater resource planning and management and contaminant traceability. This paper takes Mn as an example and codes a simulation-optimization model for solving the groundwater pollutant traceability problem using FloPy. The Bayesian optimization and strengthen elitist genetic algorithm (SEGA) algorithms are then used to optimize the hydraulic conductivity and pollutant sources in the study area. The results show that the model runs in 411 s, which is an acceptable amount of time spent, the slope of the fitted curve between the model-calculated water level and the actual observed water level is 0.914, and the contaminant traceability results can successfully locate the contaminant sources in real engineering problems. The numerical groundwater flow model and solute transport model can be quickly built, modified, and run by writing code, and can be easily and efficiently coupled with various optimization algorithms with FloPy.

Use of stacked Gaussian processes regression method to improve prediction of groundwater solute transport model

The complex groundwater solute transport model can generally be divided into several coupled submodels, the systematic error is inevitable in each submodel due to the unreasonable simplification or biased representation of the true groundwater environment system. The submodels’ systematic errors and their propagation lead to great challenge for reliable model predictions. To improve the predictive performance of groundwater solute transport model, the Stacked Gaussian processes regression (GPR) method was proposed to quantify and reduce the error propagation among the submodels in this study. Stacked GPR corrects the structure errors of groundwater model submodel by submodel gradually. Two case studies are performed to demonstrate the effectiveness of Stacked GPR method, including a synthetical groundwater transport case and a laboratory NaCl solute transport experiment. The results demonstrated that Stacked GPR can reduce the error propagation among submodels and improve model’s predictive performance apparently. Moreover, it is more feasible to correct the structure error of each submodel in sequence by Stacked GPR than to correct the lumped structure error by traditional GPR for a component-based groundwater solute transport model. The errors in prior submodels could aggravate the complexity of the error of the posterior submodel though error propagation, which increases the difficulty of constructing an effective error-correcting model.

How Findings from a Multi-Annual International Modeling Initiative Are Implemented in a Nuclear Waste Management Organization

In the present paper, we discuss various aspects of the SKB Task Force on Modeling of Groundwater Flow and Transport of Solutes (TFGWFTS). The TFGWTS is a multi-lateral forum for modeling of groundwater flow and solute transport, focusing on issues of relevance for disposal of nuclear waste. We discuss the objectives and set-up of the different tasks performed during the last 30 years, and specifically how the results of the modeling have informed performance and safety assessment applications within SKB (Swedish Nuclear Fuel and Waste Management Company, Solna, Sweden). We conclude that the TFGWFTS has been instrumental in developing modeling methodologies and tools, and in training and fostering modelers. While the early tasks were related to the construction of the Äspö Hard Rock Laboratory in Sweden and developed general modeling competence, the later tasks have served performance and safety assessment purposes in a more substantial manner.

A Computationally Efficient Method for Estimating Multi‐Model Process Sensitivity Index

AbstractIdentification of important processes of a hydrologic system is critical for improving process‐based hydrologic modeling. To identify important processes while jointly considering parametric and model uncertainty, Dai et al. (2017), https://doi.org/10.1002/2016WR019715, developed a multi‐model process sensitivity index. Numerical evaluation of the index using a brute force Monte Carlo (MC) simulation is computationally expensive, because it requires a nested structure of parameter sampling and the number of model simulations is on the order of (N being the number of parameter samples). To reduce computational cost, we develop a new method (here denoted as quasi‐MC for brevity) that uses triple sets of parameter samples (generated using quasi‐MC sequence) to remove the nested structure of parameter sampling in a theoretically rigorous way. The quasi‐MC method reduces the number of model simulations from the order of to 2N. The performance of the method is assessed against the brute force MC approach and the recent binning method developed by Dai et al. (2017), https://doi.org/10.1002/2016WR019715, through two synthetic cases of groundwater flow and solute transport modeling. Due to its rigorous theoretical foundation, the quasi‐MC method overcomes the limitations imposed by the inherently empirical nature of the binning method. We find that the quasi‐MC method outperforms both the brute force MC and the binning method in terms of computational requirements and theoretical aspects, thus strengthening its potential for the assessment of process sensitivity indices subject to various sources of uncertainty.

Heterogeneity-Driven Hydrodynamics Conditions the Hydrochemistry of Spring Water in Volcanic Islands.

Perched aquifers represent significant unexploited groundwater reserves in volcanic islands and contain valuable freshwater resources. These water reserves provide critical resources to indigenous populations suffering water scarcity. Groundwater discharged from a perched aquifer into two adjacent (14 m) springs in the volcanic summits constituted by basaltic and pyroclastic deposits of Gran Canaria Island (Spain) was examined. Based on springs discharge data, a three-dimensional groundwater flow and solute transport model of the investigated perched aquifer was calibrated to reproduce its hydraulic regime, as well as to explain the hydrochemical and isotopic composition of its main discharge systems, the studied springs. Groundwater flow simulations effectively replicated flow paths of the two springs affected by the existing geological heterogeneities, with differential travel times of 246 and 130 years, respectively, and with a convergent flow toward them partially explaining the averaged differences in electrical conductivity, δ18 O, and tritium observed between the springs. It can be concluded that, although water quality in both springs is similar and homogenous, as they come from the same aquifer system, geological heterogeneities in the upper elevation volcanic areas is likely the cause for the differences in the residence times of the two springs, which suggests that the flow regimes for the two springs are independent. The chemistry of the two springs, however, is essentially the same, with the exception of tritium, which is used to ascertain residence time.

A model of isotope transport in the unsaturated zone, case study

This work deals with a groundwater flow and solute transport model in the near-surface (predominantly unsaturated) zone. The model is implemented so that it allows simulations of contamination transport from a source located in a geological environment of a rock massif. The groundwater flow model is based on Richards’ equation. Evaporation is computed using the Hamon model. The transport model is able to simulate advection, diffusion, sorption and radioactive decay. Besides the basic model concept, the article also discusses potential cases that could lead to non-physical solutions. On three selected examples, which include, for example, rapidly changing boundary conditions, the article shows the solvability of such cases with the proposed model without unwanted effects, such as negative concentrations or oscillations of solution, that do not correspond to inputs.

Numerical modeling of changes in groundwater storage and nitrate load in the unconfined aquifer near a river receiving reclaimed water.

Reclaimed water (RW) has been widely used as an alternative water resource to recharge rivers in mega-city Beijing. At the same time, the RW also recharges the ambient aquifers through riverbank filtration and modifies the subsurface hydrodynamic system and hydrochemical characteristics. To assess the impact of RW recharge on the unconfined groundwater system, we conducted a 3D groundwater flow and solute transport model based on 10years of sequenced groundwater monitoring data to analyze the changes of the groundwater table, Cl- loads, and NO3-N loads in the shallow aquifer after RW recharge to the river channel. The results show that the groundwater table around the river channel elevated by about 3-4m quickly after RW recharge from Dec. 2007 to Dec. 2009, and then remained stable due to the continuous RW infiltration. However, the unconfined groundwater storage still declined overall from 2007 to 2014 due to groundwater exploitation. The storage began to recover after groundwater extraction reduction, rising from 3.76 × 108 m3 at the end of 2014 to 3.85 × 108 m3 at the end of 2017. Cl- concentrations varied from 5-75mg/L before RW recharge to 50-130mg/L in 2years (2007-2009), and then remained stable. The zones of the unconfined groundwater quality affected by RW infiltration increased from 11.7 km2 in 2008 to 26.7 km2 in 2017. Cl- loads in the zone increased from 1.8×103t in 2008 to 3.8 × 103 t in 2017, while NO3-N loads decreased from 29.8 t in 2008 to 11.9 t in 2017 annually. We determined the maximum area of the unconfined groundwater quality affected by RW, and groundwater outside this area not affected by RW recharge keeps its original state. The RW recharge to the river channel in the study area is beneficial to increase the groundwater table and unconfined groundwater storage with lesser environmental impacts.

Predictive Modeling of a Simple Field Matrix Diffusion Experiment Addressing Radionuclide Transport in Fractured Rock. Is It So Straightforward?

The SKB GroundWater Flow and Transport of Solutes Task Force is an international forum in the area of conceptual and numerical modeling of groundwater flow and solute transport in fractured rocks relevant for the deep geological disposal of radioactive waste. Two in situ matrix diffusion experiments in crystalline rock (gneiss) were performed at POSIVA’s ONKALO underground facility in Finland. Synthetic groundwater containing several conservative and sorbing radiotracers was injected at one end of a borehole interval and flowed along a thin annulus toward the opposite end. Several teams performed predictive modeling of the tracer breakthrough curves using “conventional” modeling approaches (constant diffusion and sorption in the rock, no or minimum rock heterogeneity). Supporting information, derived from small-scale laboratory experiments, was provided. The teams were free to implement different concepts, use different codes, and apply the transport and retention parameters that they considered to be most suited (i.e., not a benchmark exercise). The main goal was the comparison of the different sets of results and the analysis of the possible differences for this relatively simple experimental setup with a well-defined geometry. Even though the experiment was designed to study matrix diffusion, the calculated peaks of the breakthrough curves were very sensitive to the assumed magnitude of dispersion in the borehole annulus. However, given the very different timescales for advection and matrix diffusion, the tails of the curves provided information concerning diffusion and retention in the rock matrix regardless of the magnitude of dispersion. In addition, although the task was designed to be a blind modeling exercise, the model results have also been compared to the measured experimental breakthroughs. Experimental results tend to show relatively small activities, wide breakthroughs, and early first arrivals, which are somewhat similar to model results using large dispersivity values.

Groundwater pollution risk assessment based on groundwater vulnerability and pollution load on an isolated island

Prolonged leakage of surface pollutants has an impact on groundwater quality, especially in shallow groundwater. This paper proposes a new method combining Modflow and geographic information system data for groundwater pollution risk assessment. First, hydrogeological information of the study area was obtained through a field investigation and data referencing, and the groundwater flow field in the study area was constructed. The actual drilling water level data were then used for verification. A groundwater solute transport model was constructed based on the groundwater flow field, combined with pollution source sampling and detection data. Finally, based on the simulation of the migration value of pollutants, the groundwater in the study area was evaluated by constructing a groundwater pollution risk assessment system that combined groundwater vulnerability and pollution load. The results showed that in the study area, the heavy-risk area accounted for 38%, the high-risk area accounted for 29%, the medium-risk area accounted for 22%, and the low-risk area accounted for 11%. Approximately 70% of the area was rated as high-risk or above, indicating that the groundwater in the study area was more susceptible to surface pollutants. Once groundwater is contaminated, recovery is difficult; therefore, groundwater pollution risk assessments are necessary. The groundwater pollution risk assessment system constructed by the groundwater numerical simulation and multi-index comprehensive evaluation method has significance as a reference for regional groundwater pollution risk identification and scientific management.

Optimization of well field management to mitigate groundwater contamination using a simulation model and evolutionary algorithm

Groundwater represents the most important available freshwater reserves and is of critical importance to global water and food security. Old environmental burdens that have led to the spread of contaminants in groundwater limit its use, thus interventions to mitigate contamination must often be carried out to ensure a safe drinking water supply. This study presents the optimization of well field management designs to reduce the desethylatrazine (DEA) concentration in the deep wells of the Brest Water Works (Central Slovenia). It investigates artificial recharge by injection wells using water from the nearby river and elaborates five well field management scenarios prioritizing different objectives. A multi-objective simulation-optimization framework was developed. A transient groundwater flow and solute transport model was applied to simulate the effects of the proposed recharge and pumping regimes. The shuffled complex evolution method was used to identify optimal values of well field management variables (location of injection well(s), minimum required injection rate, maximum pumping rate from production well) in the proposed scenarios. Model simulations showed that optimized well field management designs can significantly reduce DEA concentration in production wells (below 0.05 μg/L), assure compliance with water quality standards with (26%) reduced injection rate, and, with the implementation of two injection wells, achieve lower DEA concentration and higher pumping rate (up to 27 L/s). The optimization solutions depend on the defined well field management priorities and reveal a trade-off between the objectives (reduction of DEA concentration, increase of pumping rate, and reduction of injection rate). The impact of management variables on mitigation efficiency is not uniform and largely depends on the location of the injection well(s), which increases the complexity of mitigation design. The study has shown that the presented approach can be efficiently used for finding optimal mitigation designs and supporting water managers with information for planning mitigation measures.

Impacts of Feedwater Quality Change on the Oldest Continuously Operated Brackish-Water Reverse Osmosis Desalination Plant in the United States

Brackish groundwater is abundant in many coastal zones of the world. The water can be economically treated with low-pressure reverse osmosis. A key issue is the stability of the feedwater pumped from groundwater systems. Commonly, groundwater solute-transport models are used to evaluate the long-term changes in salinity with time that impact brackish-water reverse osmosis (BWRO) desalination system process design. These models are run to assess changes over a 20- to 40-year period. The City of Cape Coral, Florida operates two regional BWRO facilities with the South Plant being the oldest continuously operated system in the world. This facility has a capacity of 68,182 m3/d and can treat raw water with a total dissolved solids (TDS) concentration up to 4000 mg/L. Two solute transport models were constructed to evaluate future salinity change in the groundwater source. The first model conducted in 1991 produced a range of probable changes with a high, most probable (mid), and low range. Actual data confirm the low range of the model produced an accurate result (within 15%) and that the 4000 mg/L threshold would not be exceeded until beyond 2031. The second modeling effort conducted in 2014 suggested that the 4000 mg/L TDS threshold would be reached in 2018, which did not happen. The use of real data and regression analyses for all wells suggests that the 4000 mg/L TDS concentration will not be exceeded until after 2060. Once the TDS threshold is reached, the plant would require a process change to allow treatment of higher TDS water. The current analysis shows that plant process design modification would not be required for up to 40 years into the future. The standard conceptual model assuming predominantly upward recharge during pumping was accurate with the addition of an enhanced zone of leakage caused by a fracture zone or a fault. A key issue that contributed to the success of the facility was the use of groundwater solute transport modeling prior to the final design of the membrane process during plant expansions.

Intensified salinity intrusion in coastal aquifers due to groundwater overextraction: a case study in the Mekong Delta, Vietnam.

Groundwater salinization is one of the most severe environmental problems in coastal aquifers worldwide, causing exceeding salinity in groundwater supply systems for many purposes. High salinity concentration in groundwater can be detected several kilometers inland and may result in an increased risk for coastal water supply systems and human health problems. This study investigates the impacts of groundwater pumping practices and regional groundwater flow dynamics on groundwater flow and salinity intrusion in the coastal aquifers of the Vietnamese Mekong Delta using the SEAWAT model-a variable-density groundwater flow and solute transport model. The model was constructed in three dimensions (3D) and accounted for multi-aquifers, variation of groundwater levels in neighboring areas, pumping, and paleo-salinity. Model calibration was carried for 13 years (2000 to 2012), and validation was conducted for 4 years (2013 to 2016). The best-calibrated model was used to develop prediction models for the next 14 years (2017 to 2030). Six future scenarios were introduced based on pumping rates and regional groundwater levels. Modeling results revealed that groundwater pumping activities and variation of regional groundwater flow systems strongly influence groundwater level depletion and saline movement from upper layers to lower layers. High salinity (>2.0 g/L) was expected to expand downward up to 150 m in depth and 2000 m toward surrounding areas in the next 14 years under increasing groundwater pumping capacity. A slight recovery in water level was also observed with decreasing groundwater exploitation. The reduction in the pumping rate from both local and regional scales will be necessary to recover groundwater levels and protect fresh aquifers from expanding paleo-saline in groundwater.

Efficient simulation of groundwater solute transport using the multipoint flux approximation method with arbitrary polygon grids

Groundwater solute transport models are critical for simulating subsurface contaminant processes, guiding policy making for groundwater management, and pollution remediation. In this study, we develop an improved groundwater solute transport model which uses arbitrary-shaped polygons for spatial discretization. This allows for the efficient simulation of solutes in complex boundary aquifer systems. The governing equation is discretized in space using the finite volume method to ensure flux conservation both locally and globally. The dispersion term is discretized by the multipoint flux approximation (MPFA) method which is a finite volume method that is suitable on arbitrary polygon grids. A total variation diminishing (TVD) scheme is used to control numerical oscillation in the advection term. The functionality and performance of the new model are demonstrated by comparing the simulated results between our model and MT3DMS model in five case studies. The comparisons show that the new model can well represent the solute transport processes. In a real-world watershed with complex-shaped boundaries, the new model outperforms the conventional groundwater solute transport model with smaller computed root-mean-square errors. These modeling results suggest that our model can provide robust modeling solutions for simulating groundwater solute transport processes, especially in aquifer systems with complex shaped boundaries. Furthermore, our model can provide a more flexible discretization solution for coupling surface water models and groundwater solute transport models.