Groundwater Solute Research Articles

Using Stable Isotopes to Assess Groundwater Recharge and Solute Transport in a Density-Driven Flow-Dominated Lake–Aquifer System

Saline lakes are mostly located in endorheic basins in arid and semi-arid regions, where the excess of evaporation over precipitation promotes the accumulation of salts on the surface. As the salinity of these lakes increases, their mass balance changes, and biogeochemical processes may be intensified. In that sense, Pétrola Lake (SE Spain) is a terminal lake located in an endorheic basin with elevated anthropic pressure, mainly derived from agricultural inputs and wastewater discharge. The goal of this study was to evaluate the interaction between groundwater and saline water from Pétrola Lake to improve our knowledge of groundwater recharge processes by density-driven flow (DDF) in terminal lakes. A combination of hydrochemical (chloride concentration) and stable isotope (δ18OH2O and δ2HH2O) data were used. In order to test the conceptual model, a simple numerical experiment was performed using a one-dimensional column that represents the relationship between the lake and the aquifer incorporating the variable density coupling control in solute migration. The isotopic composition of 190 groundwater and surface water samples collected between September 2008 and July 2015 provides a regression line (δ2HH2O = 5.0·δ18OH2O − 14.3‰, R2 = 0.95) consistent with dominant evaporation processes in the lake. The DDF towards the underlying aquifer showed a strong influence on the mixing processes between the groundwater and surface water. Nevertheless, groundwater chemistry at different depths beneath the lake remains almost constant over time, suggesting an equilibrium between DDF and regional groundwater flow (RGF). Modelling isotope changes allowed inferring the temporal pattern of saline water recharge, coinciding with the summer season when water loss through evaporation is most significant. Consequently, the transport of solutes suitable for chemical reactions is then feasible to deeper zones of the aquifer.

Groundwater Pollution Model and Diffusion Law in Ordovician Limestone Aquifer Owe to Abandoned Red Mud Tailing Pit

Red mud is a strong alkaline solid waste pollutant produced in the process of aluminum smelting, which causes great pollution to the regional groundwater environment due to its high content of fluorine and aluminum and high concentration of strong alkali. In this study, fluoride ion was selected as the model contaminant, and a numerical model of the groundwater flow field and solute transport was developed using GMS software to simulate and analyze the migration patterns of fluoride contaminants caused by the red mud pit for the fractured karst geohydrological conditions. The results demonstrated that the groundwater model and flow pattern were mainly controlled by atmospheric precipitation recharge, given flow boundary conditions and leakage of rivers and drains. When the concentration of fluorine pollutants in the red mud yard was 60.0 mg/L, the maximum migration distance of F− in the groundwater of the ordovician limestone aquifer was 473, 1160, 1595 and 1750 m after 1, 5, 10 and 15 years of bottom leakage, and the additional transport distances were 687, 435 and 155 m every 5 years, respectively. The range of F− pollution plume was 0.37 km2, 1.15 km2, 1.95 km2 and 2.14 km2, respectively and the range of newly added pollution plume was 0.78 km2, 0.80 km2 and 0.19 km2, respectively, every five years. Both indicated that with the extension of time, the migration and diffusion rate of pollutants slow down, and the diffusion volume increased first and then decreased. The F− pollution plume spread from the red mud pit to the northeast, which was consistent with the flow of groundwater. The high-concentration pollution plume was mainly distributed in the Ordovician limestone fractured aquifer in the northeast. This study revealed the migration law of red mud pollutants, and provided a scientific decision-making basis for the prevention and control of red mud groundwater pollution in the future.

Integrated Hydrogeochemical Analysis of Stream Water in Parts of Proterozoic Shillong Basin Meghalaya, India

Hydrogeochemical surveys were conducted in parts of the Proterozoic Shillong basin in Meghalaya to locate the unconformity-related uranium deposits, related alteration patterns, and the influence of the rock formations on the groundwater solute chemistry. Shillong Basin comprises Proterozoic metasediments and felsic volcanics of Tyrsad Formation, undeformed arenaceous Barapani Formation with intrusive granites, and metabasic sills and dykes. The groundwater quality is determined for drinking, and domestic-agro-industrial utilities using hydrogeochemical tools and physicochemical parameters. The water is acidic to slightly alkaline and has an oxidizing redox environment, Electrical Conductivity (EC), Total Dissolve Solids (TDS), and major ions fell below the World Health Organization (WHO) and Indian Standards Institute (ISI) acceptable limits except for Zn, Fe, and SiO2 showing higher concentrations.The sequences of abundance of major cations and trace elements are Fe>Zn>SiO2>Ca2+>Na+ >Mg2+>K+ >U and anions as HCO3>Cl- >SO4 2- .Groundwater character was assessed by ion exchange, simple dissolution, and unusual dissolution mechanisms. The area has dominant CaHCO3, NaHCO3, and restricted CaMgHCO3 type water. Plagioclase dissolution and high SiO2 and cation exchange of Ca for Na were identified. The groundwater indices of Sodium Percent, Kelley’s Index, Sodium Adsorption Ratio, Magnesium Ratio, Electrical Conductivity, TDS, USSL, and Wilcox index were found suitable for agro-industrial uses. Permeability Index is found to be suitable in most areas and the Corrosivity Ratio shows the areas of galvanized pipes and PVC pipes to be used for water supply. AHC analysis shows three distinct groups of water types, as well as the factor analysis, also shows the three prominent factors of water types defining the dimensionality of water types. Magnesium metasomatic alteration zones and Zn anomalous zones are delineated.

An integrated inversion framework for heterogeneous aquifer structure identification with single-sample generative adversarial network

Generating reasonable heterogeneous aquifer structures is essential for understanding the physicochemical processes controlling groundwater flow and solute transport better. The inversion process of aquifer structure identification is usually time-consuming. This study develops an integrated inversion framework, which combines the geological single-sample generative adversarial network (GeoSinGAN), the deep octave convolution dense residual network (DOCRN), and the iterative local updating ensemble smoother (ILUES), named GeoSinGAN-DOCRN-ILUES, for more efficiently generating heterogeneous aquifer structures. The performance of the integrated framework is illustrated by two synthetic contaminant experiments. We show that GeoSinGAN can generate heterogeneous aquifer structures with geostatistical characteristics similar to those of the training sample, while its training time is at least 10 times faster than that of typical approaches (e.g., multi-sample-based GAN). The octave convolution layer and multi-residual connection enable the DOCRN to map the heterogeneity structures to the state variable fields (e.g., hydraulic head, concentration distributions) while reducing the computational cost. The results show that the integrated inversion framework of GeoSinGAN and DOCRN can effectively and reasonably generate the heterogeneous aquifer structures.

The effective pore volume of multiscale heterogenous fracture-porous media systems derived from the residence time of an inert tracer

Pores in the subsurface form during the Earth forming process, e.g., sedimentation and diagenesis which evolve over the geologic time scale. Pores serve as the needed ‘room’ for the storage of natural resources and for transporting various fluids. Therefore, quantifying the volume of pores, i.e., the pore volume, is central to many geophysical and engineering problems. However, an accurate determination of subsurface pore volume has remained a challenge, primarily related to the spatial heterogeneity found at multiple length scales. Here, we develop a theoretical framework to quantify the effective pore volume of a multiscale heterogeneous porous matrix and preferential fracture flow system by evaluating groundwater age or residence time of an inert tracer. The robustness of the proposed theoretical model is tested against numerical solutions from a typical fracture (network)-matrix flow system with heterogeneous pore space distribution. We find that, although there are some uncertainties for the application of theory by measuring groundwater age or solute breakthrough curves, our proposed theoretical model is overall robust and insensitive to the length scales with relative errors of less than 9% in estimated effective pore volume. In contrast, traditional methods remain either unrepresentative or lack resolution for determining subsurface pore volume in heterogeneous formations.

Numerical Benchmark Studies on Flow and Solute Transport in Geological Reservoirs

Predicting and characterising groundwater flow and solute transport in engineering and hydrogeological applications, such as dimensioning tracer experiments, rely primarily on numerical modelling techniques. During software selection for numerical modelling, the accuracy of the results, financial costs of the simulation software, and computational resources should be considered. This study evaluates numerical modelling approaches and outlines the advantages and disadvantages of several simulators in terms of predictability, temporal control, and computational efficiency conducted in a single user and single computational resource set-up. A set of well-established flow and transport modelling simulators, such as MODFLOW/MT3DMS, FEFLOW, COMSOL Multiphysics, and DuMuX were tested and compared. These numerical simulators are based on three numerical discretisation schemes, i.e., finite difference (FD), finite element (FE), and finite volume (FV). The influence of dispersivity, potentially an artefact of numerical modelling (numerical dispersion), was investigated in parametric studies, and results are compared with analytical solutions. At the same time, relative errors were assessed for a complex field scale example. This comparative study reveals that the FE-based simulators COMSOL and FEFLOW show higher accuracy for a specific range of dispersivities under forced gradient conditions than DuMuX and MODFLOW/MT3DMS. FEFLOW performs better than COMSOL in regard to computational time both in single-core and multi-core computing. Overall computational time is lowest for the FD-based simulator MODFLOW/MT3DMS while the number of mesh elements is low (here < 12,800 elements). However, for finer discretisation, FE software FEFLOW performs faster.

Provenance of Groundwater Solute and Its Controlling Factors in Yancheng Area

Affected by human activities and climate change, the deterioration of groundwater quality could continue to intensify, and it is difficult to repair after being polluted. In order to reduce the vulnerability of groundwater safety, it is urgent to strengthen research on groundwater quality. To analyze the characteristics of hydrochemical composition and control factors and to explore the impact of human activities, groundwater sampling was conducted in the alluvial and marine plains of Yancheng area, where industrial and agricultural activities were intense. The results showed that:① the TDS value of shallow groundwater was between 211 and 3790 mg·L-1, of which, brackish water samples accounted for 30.28% in Yancheng. The order of the concentration of anions and cations was:HCO3->Cl->SO42->NO3- and Na+>Ca2+>Mg2+>K+. However, from Type Ⅰ/Ⅱ water to Type Ⅴ water, with the increase in TDS value, the dominant anions and cations evolved from HCO3 type to Cl/SO4 type and from Ca type to Na type, respectively. ② In the initial Type Ⅰ/Ⅱ water bodies, Na+ should have mainly originated from the weathering erosion of albite, whereas Ca2+, Mg2+, and HCO3- should have had a common rock weathering source. In the subsequent water quality evolution process, the factors that led the TDS value to increase, or even to exceed the standard, were not limited to rock weathering, and the effects of evaporation-concentration and ion exchange were also very worthy of attention. ③ The enrichment factor analysis showed that the Cl- and SO42- were mainly from the input of sea salt sources, and their proportions were gradually increasing from the freshwater areas of Type Ⅰ/Ⅱ in the west to the brackish water areas in the east. However, Ca2+, K+, and HCO3- were mainly derived from crustal sources, and Ca2+ was precipitated due to saturation. ④ The NO3- derived from agricultural fertilizers was limited to water of category Ⅰ/Ⅱ and Ⅲ-1 (500 mg·L-1 < TDS ≤ 750 mg·L-1). Discharge of urban sewage mainly affected Type Ⅲ water bodies. The nitrates formed by agricultural organic fertilizers were mainly distributed in the water of Type Ⅳ and Ⅴ, which was the main rice-producing area of Sheyang, Jiangsu, and the SO42- formed by human activities in this area could be ignored. These should be one of the achievements in the development of regional ecological agriculture.

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p e , montmorillonite void ratio e m and critical sand content α s are introduced to determine the e m - p e relationship and finally the k - e m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α s , the e m - p e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α s , the e m - p e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p esα as the intersection. In addition, the e m - k relationship also shows the linear trend when α ≤ α s , and the slope of the line increases with the increase of the salt solution concentration. When α > α s , the k - e m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.

Groundwater fluoride and arsenic mobilization in a typical deep aquifer system within a semi-arid basin

Contamination of groundwater with fluoride and arsenic in deep aquifers poses new health challenges worldwide. Integrated hydrogeochemical parameters with multiple isotopes (δ18O, δ2H, δ13C, δ34S, 87Sr/86Sr and 14C) is applied to demonstrate the origins and evolutions of deep groundwater and mobilization of groundwater solutes, fluoride and arsenic at Yuncheng Basin, northern China. Over 80% of deep samples have elevated fluoride (up to 3.73 mg/L) and/or arsenic values (up to 24.3 µg/L) above the WHO guideline of 1.5 mg/L and 10 µg/L, respectively. Deep saline groundwater normally belongs to Na-SO4-(Cl) type and is characterized by distinctly depleted δ18O and δ2H values (as low as −10.4‰ and −78.5‰, respectively) than deep fresh water and shallow groundwater, indicating palaeo-climatic effect. Results of ionic ratios and isotopes reveal that three types of solute sources are delineated for deep aquifers. Deep saline groundwater with old residence times of ∼ 11000 yrs showed the most depleted δ18O and δ2H values, high B/Cl and low Br/Cl, pointing towards the palaeo-saline water for the locally distributed groundwater solutes in the Qiji and Wenxi County. Saline sample near the faults with residence time of 7403 yrs showed strong evidence of a geothermal origin. In the central basin, some deep groundwater yield modern ages (3696 yrs and 4348 yrs), higher salinities (up to 1200 mg/L) and lower δ34S values (8.74‰-9.96‰), indicating the vertical leakage of shallow groundwater suffering from pyrite oxidation. High F-As fresh groundwater, characterized by Na-HCO3(-SO4) type with diverse hydrogeochemical and isotopic compositions, are evolved in closed reducing environment and semi-closed facultative environment and controlled by palaeo-recharge and shallow leakage, respectively. Organic degradation mainly controls the enrichment of arsenic and fluoride in groundwater.

Radon and Salinity Mass Balance Constraints on Groundwater Recharge on a Semi-Arid Island (Catalina, California)

Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget and groundwater recharge of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to watershed-specific groundwater recharge rates obtained from soil water balance (SWB) modeling. Calculated FSGD rates vary only slightly with season and are remarkably similar to the recharge estimates from the SWB model. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study support the SWB-model-based recharge rates. The findings of this study highlight the utility of the radon-and-salinity-mass-balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in establishing more refined sustainable groundwater yields.

Development of mini column experiments (MCE) by coupling microliter flow HPLC with ICP-MS for the analysis of metal retention under conditions close to nature

The development and implementation of mini column experiments (MCE) can be of great importance to improve existing analytical methods such as highly standardised but unrealistic batch laboratory experiments or lengthy long-term diffusion experiments to study metal sorption/desorption properties. One envisaged application would be to test the retention of repository-relevant metals in claystone, which is a promising candidate for the host rock of a final repository site for high-level radioactive waste (HLW). The used MCE setup is derived from classical high performance liquid chromatography (HPLC). On this basis, coupling of MCE to ICP-MS (inductively coupled plasma mass spectrometry) detection was realised. This online separation enables the dynamic monitoring of metal sorption and desorption experiments on clay minerals (kaolinite), natural clay (Opalinus Clay) and mixtures with quartz sand in background solutions close to nature, and without the common use of metal complexing ligands, other reaction partners or buffers for the analysed metal ions. The optimised method allows the analysis of the retention of various contaminants in real groundwater solutions and compact clay samples in a reasonable amount of time and a small amount of sample. In addition, the coupling to the unspecific mass detector of ICP-MS enables the study of different radionuclides and homologues rather than the limited UV detection of a common HPLC.

Migration mechanism of pollutants in karst groundwater system of tailings impoundment and management control effect analysis: Gold mine tailing impoundment case

Tailing impoundments are sources of high-risk pollution due to the accumulation of mining tailings and wastewater. As a result of thin karst overburden as well as actions of precipitation leaching and scouring, pollutants in the tailing impoundment can easily enter the karst aquifer through sinkholes, corrosion fissures and other channels, causing pollution to groundwater and is not conducive to the utilization of groundwater resources. Through the analysis of hydrogeological conditions, the boundary condition, aquifer structure, source-sink term, and hydraulic characteristics of groundwater are generalized, the groundwater flow model and solute migration model were established, and the migration mechanism of the characteristic pollutant (arsenic ions) was analyzed. The research results show that with the passage of time, the area of high-concentration polluted areas showed a decreasing trend, and the area of low-concentration polluted areas showed a decreasing trend; Under the influence of groundwater flow and the pollutant concentration difference, the pollution plume presents an elliptical shape with the long axis along the direction of water flow; The migration law of pollutants is controlled by the direction of groundwater flow, the concentration difference of pollutants and the permeability of the aqueous medium; The direction of groundwater flow determines the migration direction of pollutants, the concentration difference of pollutants determines the diffusion range of pollutants, and the permeability of aqueous media determines the migration speed of pollutants; Through the prediction of the pollutant control effect, it can be found that a reasonable number of pumping wells that do not interfere with each other are arranged around the groundwater flow channel, which can effectively control the migration of pollutants to the target area. The above research reveals the migration mechanism of pollutants in karst groundwater and main control factors, and also provides a scientific basis for the protection of groundwater resources in karst areas.

Bentonite–Concrete Interactions in Engineered Barrier Systems during the Isolation of Radioactive Waste Based on the Results of Short-Term Laboratory Experiments

Bentonite clays have unique properties that determine their use as the main component of engineered barrier systems (EBS) for the isolation of radioactive waste. At present, the Russian Federation is elaborating the concept of deep geological disposal of radioactive waste in crystalline rocks of the Yeniseisky site, where bentonite clays play an important role in ensuring the safety of the repository for a long period of time. This work demonstrates the first results of short-term laboratory experiments (1 and 3 months) on the thermochemical interaction of bentonite and concrete in the presence of synthetic water at an elevated temperature. These experiments will help predict the effect of EBS materials on montmorillonite. Bentonite from the 10th Khutor deposit (Russia) and Portland cement were used in the experiments. At the first stage of the experiments, solutions were obtained after leaching the concrete with a synthetic groundwater solution for 1 month at 90 °C. At the second stage, the interactions of the obtained solutions with bentonite at 90 °C were studied. As a result of the experiments, the processes of concrete leaching were revealed, which changed the composition and acidity (an increase in the pH from 6.1 to 12.1) of the synthetic water and led to an increase in the porosity of the material in contact with the solution. However, no dissolution of montmorillonite was observed, and the changes were quite small. The research results show the high stability of bentonite from the 10th Khutor deposit under model conditions, which was confirmed by modeling. Thus, we can say that at pH ≈ 12 and at elevated temperatures, montmorillonite retains a stable structure for a long time, which is important for ensuring the safety of disposal in general.

Coupling microscale zero-valent iron and autotrophic hydrogen-bacteria provides a sustainable remediation solution for trichloroethylene-contaminated groundwater: Mechanisms, regulation, and engineering implications

Coupling microscale zero-valent iron (mZVI) and autotrophic hydrogen bacteria (AHB) has gained increasing attention owing to its potential to improve dechlorination performance by bridging H2 donors and acceptors. However, few studies have attempted to test its sustainable remediation performance and to comprehensively unveil the governing mechanisms. This study systematically compared the performances of different systems (mZVI, H2-AHB, and mZVI-AHB) for trichloroethylene (TCE) removal, and further optimized dechlorination and H2 evolution of mZVI-AHB synchronously by regulating the mZVI particle size and dosage to achieve a win-win remediation solution. The final removal efficiency and removal rate of TCE by mZVI-AHB were 1.67-fold and 5.30-fold of those by mZVI alone respectively, and mZVI-AHB resulted in more complete dechlorination than H2-AHB alone. Combining H2 evolution kinetics, material characterization data, and bacterial community analysis results, the improved dechlorination performance of mZVI-AHB was mainly due to the following mechanisms: H2 generated by mZVI corrosion was efficiently utilized by AHB, lasting corrosion of mZVI was facilitated by AHB, and dechlorination functional bacteria were highly enriched by mZVI. Finally, the remediation performance of mZVI-AHB with different mZVI particle sizes and dosages was evaluated comprehensively in terms of dechlorination reactivity, H2 utilization efficiency and chemical cost, and suggestions for possible engineering applications are provided.

Machine learning-based optimal design of groundwater pollution monitoring network

It is an important task of environmental management to design groundwater pollution monitoring network (GPMN) to find out the occurrence of pollution events and carry out remediation in time. However, there are many uncertain factors in the process of designing GPMN, which affect the GPMN design result. In the process of applying the Monte Carlo method for uncertainty analysis, groundwater numerical simulation model may be utilized thousands of times, which results in a huge computational load. In order to overcome this disadvantage, a machine learning (ML)-based surrogate model is constructed with Kriging method, to replace the computational simulation model under uncertainty of pollution sources and parameters. The 0–1 integer programming optimization model is constructed to maximally cover serious polluted area to detect the occurrence of groundwater pollution in time. The optimal design framework of GPMN based on proposed ML algorithm was applied in a domestic landfill in Baicheng City, China. The results showed that the ML-based surrogate model has a great fitness with the groundwater solute transport simulation model. The optimal results of GPMN indicated that monitoring wells should be mainly placed at the downstream of the leachate equalization basin. If more wells are allowed to be placed, part of wells could be placed at the downstream of the landfill. Moreover, the area where the pollution plumes of landfill site meet that of leachate equalization basin should be set as the key monitoring objective. Verification and comparison showed that the pollutant detection rate of the optimal layout scheme is far higher than random layout schemes, which proves the reliability of the ML-based optimal design scheme of GPMN.

Cyanotoxin transport from surface water to groundwater: Simulation scenarios for Lake Erie

Harmful algal bloom (HAB) and cyanotoxin studies in the Great Lakes region have been typically focused on surface-water issues, with few investigating or reporting on groundwater. This study aims to theoretically explore whether groundwater can be contaminated by microcystins from HABs in surface water due to surface-water and groundwater interaction. Specifically, a 3-D MODFLOW/MT3DMS model was developed to simulate pumping-induced reverse groundwater flow and solute transport from Lake Erie to the aquifer underneath the South Bass Island in Ohio. Our simulation results based on typical, base case settings showed that after microcystins were detected and released from the lake, it would take about two, three, and 13 months for the water in a well on the island to reach the EPA advisory levels of microcystin for detection (0.1 µg/l), infants and children (0.3 μg/l), and school-age children to adults (1.6 μg/l), respectively. Furthermore, our scenario analyses showed that, as expected, higher pumping rate and higher lakebed leakance would accelerate the microcystin transport to the well. However, higher hydraulic conductivity would increase the time to reach the EPA levels due to mixing and dilution effects. The 3-D modeling scheme developed in this study was suitable to simulate the complex surface-water and groundwater interaction and transport processes occurring in the Great Lakes. This theoretical study provides useful insight for managing coastal groundwater aquifers and resources under threat from HABs in the Great Lakes. Future improvements to the model would include incorporating reactions and fractures and obtaining water-quality data for model calibration.

Solving hindered groundwater dynamics in restored tidal marshes by creek excavation and soil amendments: A model study

Groundwater fluxes in tidal marshes largely control key ecosystem functions and services, such as vegetation growth, soil carbon sequestration, and nutrient cycling. In tidal marshes restored on formerly embanked agricultural land, groundwater fluxes are often limited as compared to nearby natural marshes, as a result of historical agricultural soil compaction. To improve the functioning of restored tidal marshes, knowledge is needed on how much certain design options can optimize soil-groundwater interactions in future restoration projects. Based on measured data on soil properties and tidally induced groundwater dynamics, we calibrated and evaluated a 2D vertical model of a creek-marsh cross-section, accounting for both saturated and unsaturated groundwater flow and solute transport in a variably saturated groundwater flow model. We found that model simulations of common restoration practices such as soil amendments (increasing the depth of porous soil on top of the compact layer) and creek excavation (increasing the creek density) increase the soil aeration depth and time, the drainage depth and the solute flux, and decrease the residence time of solutes in the porewater. Our simulations indicate that increasing the depth to the compact layer from 20 cm to 40 cm, or increasing the creek density from 1 creek to 2 creeks along a 50 m marsh transect (while maintaining the total creek cross-sectional area), in both cases more than doubles the volume of water processed by the marsh soil. We discuss that this may stimulate nutrient cycling. As such, our study demonstrates that groundwater modelling can support the design of marsh restoration measures aiming to optimize groundwater fluxes and related ecosystem services.

Review on Groundwater Flow and Solute Transport Modelling in India: Recent Advances and Future Directions

ABSTRACT The declining groundwater level, deteriorating groundwater quality and decreasing well yield necessitate the need to have an informed understanding of the aquifer system vis-à-vis the natural and anthropogenic stresses. The groundwater modelling research has gained momentum in the last 30 years, and the advent of fast computing platform coupled with Graphical User Interface (GUI) based user-friendly software have provided further stimulus. It is being used as an effective tool for groundwater development and management encompassing a wide range of applications in different areas like agriculture, mining and environment. This article is intended to review the efficacy of applications of groundwater flow and solute transport modelling in India over the last 22 years (1997 to 2019) attempted by researchers, research institutes and groundwater agencies. An attempt is made to examine the strength and weakness of the groundwater modelling efforts, gaps in our knowledge and the future research directions. The literature review attests that the groundwater models developed in India are mostly with the help of commercial software namely Visual MODFLOW, GMS and Groundwater Vista. Different aspects in groundwater modelling like conceptualization of the model, steady state and transient calibration, sensitivity analysis and prediction scenario attempted in the model are reported and the research areas for future are identified. The literature review suggests that the modelling efforts need to be strengthened and capability be developed by attempting modelling the migration of heavy metals, nitrate and pesticides. The sensitivity analysis need to be addressed and future efforts should be on Global Sensitivity Index. The modelling efforts need to incorporate the climate change scenario.

Identification of Artificial Recharge Sites for Nand Samand Catchment

Abstract: Security of groundwater is widely regarded as a serious impediment to India's economic and social progress. According to the Central Ground Water Board's (CGWB) assessment, India's groundwater tables are plummeting at an alarming rate, with reserves in some regions reaching critical levels. Unregulated groundwater use in southern peninsular India has also resulted in excessive extraction, lowering the 'critical' threshold. With over 30 million groundwater structures in use, India is on the verge of a disaster of over-extraction that will leave 60% of all aquifers in critical condition within the next two decades. To resolve the issue, a variety of renewable groundwater solutions must be implemented. Artificial recharge is a procedure that augments groundwater at a pace that is significantly greater than the rate of replenishment under natural conditions, which may give a solution. The current study is for the Nand Samand catchment in the district of Rajasthan. The investigation of artificial groundwater recharge sites is being conducted using an integrated Remote Sensing and Geographic Information System (GIS) approach. Thematic maps such as topographic elevation, post-mosoon groundwater level, recharge, slope, transmissivity and soils map are created, and weighted overlay analysis is used to identify areas suitable for artificial recharge. Keywords: Nand Samand catchment, artificial recharge zone, thematic map, remote sensing, GIS

Variable-Density Flow and Solute Transport in Stratified Salt Marshes

Subsurface hydrodynamics underpin the eco-functions of salt marshes. Many studies have investigated these processes under various conditions. However, the impact of soil stratification (a low-permeability mud layer overlying a high-permeability sand layer) on the variable-density groundwater flow (particularly unstable flow) and solute transport in regularly tide-flooded marshes remains poorly understood. The present study numerically explored this question based on a 2D cross-creek section of salt marshes, by comparing cases with and without stratification. Results show that, the low-permeability mud layer delays the initiation of unstable flow and leads to smaller and denser salt fingers. Consequently, solute plume stays in the marsh soil for a longer time and spreads more widely than that in the homogeneous case. Also, soil stratigraphy extends the duration and shrinks the zone of solute discharge across the tidal creek. Sensitivity analysis was conducted based on three key controlling variables: hydraulic conductivity contrast between mud layer and sand layer (Kmud/Ksand), salinity contrast between surface water and groundwater (Csea/Cpore), and mud layer thickness (Dmud). The results demonstrate that the residence time of solute plume in a two-layered salt marsh is less sensitive to Csea/Cpore than to Kmud/Ksand and Dmud. Moreover, the commencement and duration of solute discharge are more sensitive to Kmud/Ksand and Dmud than to Csea/Cpore. While the location of solute discharge zone is highly sensitive to Dmud and slightly influenced by Kmud/Ksand and Csea/Cpore. Findings from this study would facilitate a deeper understanding of the eco-functions of salt marshes.