Groundwater In Coastal Aquifers Research Articles

General Hydrogeochemical Characterization of Groundwater in Coastal Aquifers of Kilwa Kisiwani Oceanic Island, South East Tanzania

General Hydrogeochemical Characterization of Groundwater in Coastal Aquifers of Kilwa Kisiwani Oceanic Island, South East Tanzania

Understanding the impact of climate-induced sea level rise on groundwater inundation in a low-lying coastal area: A numerical simulation in Southeast India

Global warming has led to rising sea levels, severely threatening coastal areas, particularly in low-elevation coastal zones. The groundwater in coastal aquifers of lower elevation is susceptible to groundwater inundation (GWI), an emerging hidden long-term hazard earlier to marine inundation. The inundation is mainly due to the hydraulic connection between sea level and shallow unconfined aquifers, resulting in groundwater upsurging towards the ground surface. To understand the potential impact of sea level rise (SLR) induced GWI on low-lying coasts, a transient state model was simulated and predicted in the Cuddalore regions of South India under 0.3 m, 0.6 m, and 1 m SLR scenarios. The simulation reveals that the aerial extent of groundwater rise (GWR) was 331.9 km2, 566.2 km2, and 650.8 km2 for 0.3 m, 0.6 m, and 1 m SLR scenarios, respectively. About 50% of the response was observed in the northern and central parts of the study area within a 5 km buffer, which is attributed to the deeper groundwater table. In contrast, the southern region exhibited a 10% decline in response due to shallow groundwater discharge to drainage, which dampened the shoaling effect. The present-day shallow groundwater tables in the southern region are more vulnerable to GWI from rising sea levels. Specifically, a 0.3 m SLR could inundate 14.9 km2. In comparison, a 1 m SLR could inundate 40.5 km2 of the area, including the Pichavaram mangrove forest, agricultural land, municipal structures, and the other regions that flooding could threaten. These inundations could have potential impacts, such as wetland loss, agricultural flooding, deteriorating water quality, and infrastructural damage. The present study provides valuable insights into the subsurface hydrodynamic and exposure impacts of sea rise-induced inundations. This can aid coastal engineers, policymakers, and decision-makers to plan appropriate mitigation measures.

Understanding the vulnerability of coastal groundwater aquifers in Odisha, India

Understanding the vulnerability of coastal groundwater aquifers in Odisha, India

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

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

Multi-isotopic and hydrochemical evidence of water resources evolution and recharge estimation in the tropical coastal aquifer

The study integrates hydrochemical and isotopic (δ18O, δ2H, 3H, and δ13C) techniques to investigate water evolution in the North Kelantan River basin. Groundwater facies were mainly classified as Ca–Mg–Cl, Ca–Mg–Cl–HCO3, and Na–Cl during the rainy season and shift toward Ca–Mg–HCO3 and Na–Ca–HCO3–Cl during the dry season in surface water and shallow aquifer, while facies in intermediate and deep aquifers were classified as Na–HCO3 and Na–Cl. The δ18O and δ2H compositions of most groundwater samples have not deviated significantly from NKMWL (δ2H = 8.4 δ18O + 11.5) with slightly depleted isotopic values due to the humid climate. Isotopically, evaporation does not impact recharged water significantly. However, shallow aquifers are slightly enriched than deep aquifers of the study area. The estimated recharge was 20.17% and 22.52% of annual rainfall based on the CMB and δ18O values, respectively. The recharge mainly occurs during the wet season and is influenced mostly by the amount of rain. Aquifers clustered in distinct groups based on their isotopic signatures and hydrochemical results. The decomposition of organic matter is the primary carbon source in the study area. Building on this, by utilizing isotope hydrology to study water resources, we can gain a deeper insight into the susceptibility of groundwater in coastal aquifers of monsoonal tropical humid regions.

Arsenic and fluoride exposure in drinking water caused human health risk in coastal groundwater aquifers

Groundwater (GW) is a precious resource for human beings as we depend on it as a source of fresh drinking water, agricultural practices, industrial and domestic uses, etc. Extreme exposure of arsenic (As) and fluoride (F-) concentrations along the coastal GW aquifers of “South 24 Parganas and East Medinipur” diluted the quality of GW and created serious health issues. Various chronic health disorders such as – black foot disease, fluorosis skin cancer, cardiac problems, and other water borne diseases have been noticed in these two coastal districts. The comprehensive entropy-weighted water quality index (EWQI) and health risk assessment (HRA) were applied to evaluate the quality of GW and probable health risks in the coastal districts. Monte Carlo simulation and sensitivity analysis methods were simultaneously adopted to identify the non-carcinogenic health risk assessment due to regular ingestion of contaminated GW. As the study region is densely populated and part of the Sundarbans Ramsar site, it has greater importance at the international level along with regional importance to address the GWQ of this region. The major findings of the present study highlight that almost 55% of the study area is confronting serious GW quality issues and associated probable health risk (HR) due to the intense accumulation of As and F- in the GW aquifers of the study area. Children's health is more vulnerable due to the consumption of As containing GW, and adults are highly affected due to the intake of F- bearing GW in the coastal districts. The findings of the current study will draw the attention of hydrologists, groundwater management authorities, government bodies, and NGOs to regulate and monitor the GW aquifers routinely, enhance GW quality, minimizing the health hazards and sustainable water management in a more scientific and sustainable way which must be advantageous for coastal people.

Wind-modulated groundwater discharge along a microtidal Arctic coastline

Groundwater discharge transports dissolved constituents to the ocean, affecting coastal carbon budgets and water quality. However, the magnitude and mechanisms of groundwater exchange along rapidly transitioning Arctic coastlines are largely unknown due to limited observations. Here, using first-of-its-kind coastal Arctic groundwater timeseries data, we evaluate the magnitude and drivers of groundwater discharge to Alaska’s Beaufort Sea coast. Darcy flux calculations reveal temporally variable groundwater fluxes, ranging from −6.5 cm d−1 (recharge) to 14.1 cm d−1 (discharge), with fluctuations in groundwater discharge or aquifer recharge over diurnal and multiday timescales during the open-water season. The average flux during the monitoring period of 4.9 cm d−1 is in line with previous estimates, but the maximum discharge exceeds previous estimates by over an order-of-magnitude. While the diurnal fluctuations are small due to the microtidal conditions, multiday variability is large and drives sustained periods of aquifer recharge and groundwater discharge. Results show that wind-driven lagoon water level changes are the dominant mechanism of fluctuations in land–sea hydraulic head gradients and, in turn, groundwater discharge. Given the microtidal conditions, low topographic relief, and limited rainfall along the Beaufort Sea coast, we identify wind as an important forcing mechanism of coastal groundwater discharge and aquifer recharge with implications for nearshore biogeochemistry. This study provides insights into groundwater flux dynamics along this coastline over time and highlights an oft overlooked discharge and circulation mechanism with implications towards refining solute export estimates to coastal Arctic waters.

The Spatio-Temporal Dynamic Patterns of Shallow Groundwater Level and Salinity: The Yellow River Delta, China

Shallow groundwater in coastal aquifers is a highly dynamic and complex system with a high risk of seawater intrusion. Analyzing the spatio-temporal dynamic patterns of groundwater can help to manage the groundwater resource and prevent it from degradation. Based on the groundwater level (GWL) and electrical conductivity (EC) monitoring data of 18 observation wells in the Yellow River Delta (YRD) from 2004 to 2010, this research analyses the groundwater dynamics using a robust seasonal trend decomposition technique (STL) and spatial interpolation method to detect the groundwater spatio-temporal dynamic patterns of groundwater level and salinity. Combined with hydro-climatic data, the Pearson correlation method and the Mann-Kendall (MK) trend analysis were used to further reveal the impacts that induce their trends and seasonal variations. Our analyses show that the risk of seawater intrusion into local shallow aquifers in this region is high, with the mean groundwater level over 42% of the region lower than the local sea level, and the mean groundwater EC over 96% of the region met the standards for seawater intrusion. In addition, the trends of groundwater level generally declined by 0.01~0.45 m/a and salinity increased by 1.153~25.608 μs/cm.a, which are consistent with the trend of precipitation decline. The seasonal dynamics of groundwater level and salinity are highly correlated with the seasonal components of rainfall and evaporation. It can be concluded that the extent of seawater intrusion will increase in the future with sea level rise. The approaches used in this study proved to be effective and can certainly serve as an example for the analysis of the spatio-temporal dynamics of groundwater in other coastal regions.

Hydrogeochemical evaluation for human health risk assessment from contamination of coastal groundwater aquifers of Indo-Bangladesh Ramsar site

The suitability of groundwater (GW) and the corresponding health risk caused by heavy metals become prime concerns in the recent era; so, the determination of GW quality most important for drinking, agricultural, and domestic purpose is the foremost responsibility of the concurrent researchers. Our study intends to develop an innovative approach to delineate the health hazard risk zone (HHRZ) association to the hydro chemical condition of available GW of the most prominent Indo-Bangladesh Ramsar region by coupling the groundwater quality measurement (GWQM), degree of concentration (Cd), and health risk index (HRI) with the help of hazard quotient (HQ) method. A total of 199 GW samples were collected from the Ramsar sites of both countries and twelve (12) significant physiochemical properties, namely Sodium (Na+), Potassium (K+), Magnesium (Mg2+), Nitrate (NO3−), Bicarbonate (HCO3-), Phosphate (PO42−), Fluoride (F−), Chloride (Cl−), Sulphate (SO42−), pH, Arsenic (As) and Electoral conductivity (EC) considered as water quality and health hazard causative factors for this research work. This study also assesses the individual health of each parameter and anthropogenic pressure on this Ramsar region. The produced GWQM reveals that the GW of extensive areas in the eastern, north-east, central, and south-central zone is characterized by unfit for consumption (11.47%), very poor (14.27%), and poor (20.76%) categories; whereas, health hazard map also identified quite a similar zone for significant health risk and wide geographical area of Indo-Bangladesh Ramsar site fall under very high (17.49%), high (13.26%), moderate (7.91%) health risk zone. Therefore, our study will help the policymakers, and governments of both countries in ensuring the monitoring plans, strict water management policies, and adoption of health hazard mitigating measures to ensure the well beings of the exposed inhabitants of this coastal Ramsar site region.

Groundwater quality assessment along the West of New Damietta Coastal City of Egypt using an integrated geophysical and hydrochemical approaches

Recently, the groundwater became very important source for the Egyptian water balance. Therefore, assessing its quality and quantity is necessary before initiating any developmental plans using this resource. In this research, a new operated power plant, which is located in the coastal area of West New Damietta City, north-east of Egypt was designed to work using the coastal groundwater aquifer. Therefore, an integrated approaches of both geophysical methods and hydrochemical analysis were applied to identify the subsurface lithology, the water-bearing layers and determine the potential use of such coastal groundwater. Therefore, a total of seventeen vertical electrical soundings (VES) and five time-domain electromagnetic soundings (TEM) were performed. Moreover, groundwater samples were collected from seven existing wells to be analyzed for water quality assessment. The geophysical results identified five geoelectric layers. The first geoelectric layer has a resistivity of 1–7.9 Ω m and a thickness range of 7–9 m. The second layer has a resistivity of 0.9–4.4 Ω m and a thickness range of 7–8 m. The third layer is 10–21 m thick with a resistivity value of 0.5–2 Ω m. The fourth layer, which is thicker (29 to 42 m), has a resistivity of 0.7–3 Ω m, while the last layer has a resistivity of 1.7–9 Ω m. According to the hydrochemical analysis, the aquifer is dominated by Na–Cl water type. The brackish nature of the water is revealed by the TDS range of 7035 mg/l to 7735 mg/l. The results collected demonstrate the groundwater's quantity and quality availability for the power plant’s sustainable use.

Delineation of Salinization and Recharge Sources Affecting Groundwater Quality Using Chemical and Isotopic Indices in the Northwest Coast, Egypt

Salinization of coastal aquifers is a serious issue affected by climate change and enhanced by overexploitation of groundwater resources. This research aims to explore the hydrogeochemical processes that cause salinization of groundwater in coastal aquifers, such as the area located between Barrani and Baqbaq, on the northwestern coast of Egypt. Various techniques were applied, including Gibbs plots and hydrochemical facies diagrams (HFE-D), ion ratios and stable isotope bivariate plots, statistical analyses, a groundwater quality index for seawater intrusion (GQISWI), and a seawater mixing index (SMI). Based on the total dissolved solids (TDS), groundwater can be classified into four groups: slightly saline (9%), moderately saline (45%), highly saline (43%), and salty water (3%). The geochemical properties were further catergorized on the basis of other parameters and ion ratios, such as Caexcess, Nadeficit, Na/Cl, Cl/HCO3, and Br/Cl, which suggest the influence of cation exchange, seawater, and marine sediment dissolution. Additionally, stable isotopes indicated two groups. One of these has relatively high salinity and low isotopic content and is affected by the leaching and dissolution of marine deposits. The other group is enriched in δ18O and δD content, with much higher salinity due to mixing with seawater and evaporation. The GQISWI categorizes groundwater as saline and mixed (55 and 41%, respectively), followed by saltwater (4%), whereas the SMI calculations indicate that about 10% of the groundwater samples are impacted by seawater. Finally, the areal distribution of GQISWI and SMI identified some patches along the coastline as well as other inland places located about 12.5 km away from the sea that have undergone saltwater intrusion. In conclusion, overexploitation of groundwater should be avoided because the amount of annual rainfall is very limited.

Hydro-chemical assessment of coastal groundwater aquifers for human health risk from elevated arsenic and fluoride in West Bengal, India

The vulnerability of groundwater in the coastal regions in terms of As, F−, and NO3− exposure is growing rapidly. Hence, the present study focused on assessing groundwater quality, ecological richness, and HR in the coastal districts of West Bengal by applying field-based CD, GWQI, ERI, and HRI techniques. After assessing the GW vulnerability, it is stated that approximately 40–50 % area of the two selected coastal district's GW is poor to very poor in quality, the ecology of GW is threatened, and human health is faced serious risk for both dry and wet season. The Wilcox and USSL diagram verified that nearly 50 % GW aquifers of coastal district of West Bengal are not fit for irrigation and drinking. The findings of this study will be beneficial to manage and control groundwater vulnerability in the coastal regions for water scientists, policy makers, and researchers as well in sustainable way.

Ecohydrology of coastal aquifers in humid environments and implications of a drying climate

AbstractCoastal groundwater‐dependent ecosystems (GDEs), such as wetlands, estuaries and mangrove forests, are globally important habitats that promote biodiversity, provide climate regulation and serve as refugia for plant and animal communities. However, global warming, coastal development and over‐abstraction threaten the availability and quality of groundwater in coastal aquifers and, by extension, the ecohydrological function of dependent ecosystems. Because ecohydrological knowledge of coastal groundwater is disparate across disciplines and habitat types, we begin by summarising the physiochemical, biological and hydrological processes supported by groundwater across coastal watersheds. Groundwater makes a significant but poorly recognised contribution to the function and resilience of coastal ecosystems and will play an essential role in climate change mitigation and adaptation. This review then explores how critical ecosystem processes supported by groundwater will be affected in areas of the humid subtropics that are expected to be impacted by climatic drying. Where rainfall is predicted to decrease, reduced groundwater recharge will interrupt the hydrology of coastal GDEs, while anthropogenic pressures, such as land‐use intensification and pollution, will diminish the quality of remaining groundwater. The challenges of managing groundwater for multiple purposes under climate change predictions are highlighted. To improve the management of coastal GDEs, research should be aimed at developing robust conceptual models of coastal groundwater systems that quantify biophysical linkages with ecological communities across relevant spatiotemporal scales.

Hydrochemical Evolution and Nitrogen Behaviors in Coastal Groundwater Suffered From Seawater Intrusion and Anthropogenic Inputs

Coastal aquifers play key roles in providing freshwater resources to maintain the social and economic development in coastal areas. However, climate change and human activities have dramatically affected the quantities and qualities of groundwater in coastal aquifers. In this study, stoichiometric analysis of hydrogeochemistry, multivariate analysis, and isotopic trancing techniques were used to reveal the local hydrochemistry characteristics, the natural and anthropogenic origins, and the major hydrochemical evolution in a typical coastal aquifer located in the Pearl River estuary. According to hydrogeological conditions and groundwater burial conditions, the aquifer was divided into three zones, namely, semiconfined fissure groundwater (SFGW), recharged fissure groundwater (RFGW), and porous medium groundwater (PGW). Seawater intrusion, ion exchange, water–rock reaction, and human activities were the main controlling factors affecting the characteristics of groundwater, but there were significant differences in the main controlling effects of different zones. Among them, the samples from the SFGW was severely affected by seawater intrusion, and the contributions of seawater ranged from 6% to 97%. Obvious cation exchange process occurred during the seawater intrusion. The hydrochemical characteristics of the PGW and the RFGW were mainly controlled by water–rock interaction. In addition, human activities had further influence on the hydrochemical characteristics of groundwater, which resulted in elevated nitrate–nitrogen (NO3−–N). The mean NO3–N concentrations in the PGW and the SFGW were 6.58 and 3.07 mg/L, respectively. Furthermore, the δ15N–NO3− and δ18O–NO3− values in these two regions ranged from +2.35‰ to +27.54‰ and from +0.39‰ to +18.95‰, respectively, indicating that the anthropogenic input contributed to the increased nitrate. Redox analysis and dual nitrogen isotopic evidence indicated that denitrification was the predominant biogeochemical process in the PGW and the RFGW. This study highlights the impacts of seawater intrusion and anthropogenic inputs on hydrochemical evolution and nitrogen behaviors in coastal groundwater, which provides a scientific basis for the management of groundwater resources in coastal aquifers.

Computational assessment of groundwater salinity distribution within coastal multi-aquifers of Bangladesh

The rising salinity trend in the country’s coastal groundwater has reached an alarming rate due to unplanned use of groundwater in agriculture and seawater seeping into the underground due to sea-level rise caused by global warming. Therefore, assessing salinity is crucial for the status of safe groundwater in coastal aquifers. In this research, a rigorous hybrid neurocomputing approach comprised of an Adaptive Neuro-Fuzzy Inference System (ANFIS) hybridized with a new meta-heuristic optimization algorithm, namely Aquila optimization (AO) and the Boruta-Random forest feature selection (FS) was developed for estimating the salinity of multi-aquifers in coastal regions of Bangladesh. In this regard, 539 data samples, including ten water quality indices, were collected to provide the predictive model. Moreover, the individual ANFIS, Slime Mould Algorithm (SMA), and Ant Colony Optimization for Continuous Domains (ACOR) coupled with ANFIS (i.e., ANFIS-SMA and ANFIS-ACOR) and LASSO regression (Lasso-Reg) schemes were examined to compare with the primary model. Several goodness-of-fit indices, such as correlation coefficient (R), the root mean squared error (RMSE), and Kling-Gupta efficiency (KGE) were used to validate the robustness of the predictive models. Here, the Boruta-Random Forest (B-RF), as a new robust tree-based FS, was adopted to identify the most significant candidate inputs and effective input combinations to reduce the computational cost and time of the modeling. The outcomes of four selected input combinations ascertained that the ANFIS-OA regarding the best accuracy in terms of (R = 0.9450, RMSE = 1.1253 ppm, and KGE = 0.9146) outperformed the ANFIS-SMA (R = 0.9406, RMSE = 1.1534 ppm, and KGE = 0.8793), ANFIS-ACOR (R = 0.9402, RMSE = 1.1388 ppm, and KGE = 0.8653), Lasso-Reg (R = 0.9358), and ANFIS (R = 0.9306) models. Besides, the first candidate input combination (C1) by three inputs, including Cl− (mg/l), Mg2+ (mg/l), Na+ (mg/l), yielded the best accuracy among all alternatives, implying the role importance of (B-RF) feature selection. Finally, the spatial salinity distribution assessment in the study area ascertained the high predictability potential of the ANFIS-OA hybrid with B-RF feature selection compared to other paradigms. The most important novelty of this research is using a robust framework comprised of the non-linear data filtering technique and a new hybrid neuro-computing approach, which can be considered as a reliable tool to assess water salinity in coastal aquifers.

Geochemistry of groundwater and metal(loid) behavior in the costal aquifers of the Maharlu Lake, Iran.

Groundwater in coastal aquifers of the Maharlu Lake, southern Iran, has suffered from quality degradation and salinity increases in recent decades. These aquifers are important sources for drinking and agricultural uses in area around the lake. Hydrogeochemical analyses were carried out to identify the geochemical processes controlling groundwater chemistry and heavy metal concentrations in the coastal aquifer of the Maharlu Salt Lake. A total number of 54 water samples (45 samples from three coastal aquifers and 9 water samples from lake water) were collected for laboratory analysis of major ions and heavy metals three times from November-2014 to July-2015. In addition, physiochemical parameters of temperature, electrical conductivity, pH, and redox potential were measured in the field. The findings indicate three major controls of groundwater chemistry in the coastal aquifers of the Maharlu Lake including (1) lake water intrusions, (2) evaporites and carbonate dissolution-precipitation, and (3) input of organic matter probably linked to the seepage of domestic sewage with resulting sulfate reduction. In contrast, dissolution of minerals in surface sediments and direct evaporation of groundwater in the coastal aquifers seem to be insignificant based on geochemical ratios and water table depth. Concentrations of B, Li, Co, Cr, Cu, Ni, and U increase, but Zn concentrations decrease in groundwater due to the lake water intrusions into the costal aquifers. Gypsum dissolution and consequent carbonate reactions cause B, Co, Cr, Cu, Ni, Zn, and U enrichment in the groundwater samples. These findings can provide water managers and local authorities with a comprehensive framework of the coastal groundwater geochemistry, allowing a better understanding of the effects of current management practices and the implementation of mitigation approaches such as reduction of groundwater extraction to limit further lake saltwater intrusion and water resources deterioration.

New approach in estimation of seawater intrusion footprint (SWIF) for irrigated crops using coastal groundwater

The ecological and environmental issues caused by seawater intrusion, economic losses should not be neglected, especially from a long-term perspective. This research presents a new approach to calculate the water footprint of the crops that irrigated by coastal groundwater aquifers, considering seawater intrusion is one of the effecting pollutants. Seawater intrusion footprint (SWIF) is a new term of water footprint that defines aquifer pollution due to the excessive withdrawal from the aquifer. The seawater intrusion footprint that defined as the amount of water needed to mitigate the impact of seawater intrusion is different from the grey water footprint, which is defined as the amount of water needed to mitigate the impact of pollutants generated during the life-cycle of a product. Using the new approach, the water footprint of 12 irrigated crops in the Nile Delta in Egypt that use the coastal groundwater was recalculated and compared with the currently published figures that do not consider the pollution caused by seawater intrusion. The main results are; the irrigation using only coastal groundwater, the total water footprint of some crops such as wheat, bananas, sugar cane, and apples increase by more than 80% of published figures for each crop to reach 3068,1079,345 and 1761 m3/ton, respectively. While using coastal groundwater with green water in rainy seasons, the total water footprint of the same crops increases to 2282,1038, 300, and 1617 m3/ton, respectively. Since seawater intrusion causes groundwater contamination and salinization, leading to decreases in agricultural production and ecological and environmental issues. So, the economic losses arising from seawater intrusion impacts should not be neglected, especially from a long-term perspective. Also, it is recommended to incorporate seawater intrusion footprint (SWIF) in the total water footprint for the crops that irrigated by coastal groundwater aquifer. Also, the study suggested applying the new approach whenever the coastal groundwater has been used near the coast or far away if there is a seawater intrusion.

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.

Understanding the effects of periodic freshening and salinization on coastal water quality and aquifer sediments through laboratory-based column experiments

The coastal aquifers experience periodic freshening and salinization events due to frequent changes in hydrological conditions. Such frequent changes have vital effects on the coastal groundwater quality and aquifer properties. This forms the basis of the present study. Laboratory-based column experiments have been conducted to investigate the long-term impact of such periodic events on coastal groundwater quality and aquifer sediment characteristics. Conditions similar to the natural coastal aquifer systems were generated in sediment-filled columns by injecting freshwater and seawater periodically. Changes in pore water chemistry and sediment properties were monitored over a period of one year. It has been observed that periodic injections of freshwater (freshening) and seawater (salinization) caused significant changes in the pore water chemistry, mainly due to sediment-water interactions. In addition to seawater-freshwater mixing, geochemical processes such as ion-exchange, mineral precipitation/dissolution, and adsorption/desorption of elements have been identified to control the pore water chemistry. Trace elements, including barium, boron, bromide, lithium, and strontium, have been identified as effective tracers for monitoring the saltwater intrusion. Further, the changes in pore water chemistry are found to be interlinked with the changes in aquifer properties. It is observed that with the increase in the number of salinization cycles, the effectiveness of the freshening process to bring back the original aquifer pore water chemistry diminishes, and the impact of salinization lasts longer. At the end of the experiment, changes in the porosity and permeability of the sediments have been observed. Such changes in the aquifer properties may affect the specific yield and storativity of the aquifer.

Enhancing the resolving ability of electrical resistivity tomography for imaging saltwater intrusion through improvements in inversion methods: A laboratory and numerical study

Mapping and monitoring saltwater intrusion are critical for the sustainable management of groundwater in coastal aquifers around the world. Increasingly, geophysical methods, such as electrical resistivity tomography (ERT), have been used to address these needs. We identify methods for the inversion of ERT data that would most accurately map the location and geometry of an intrusion wedge. This is accomplished using laboratory and synthetic experiments, with the classic representation of an intrusion wedge perpendicular to the coast. The laboratory experiments allow us to collect ERT data on a saltwater intrusion wedge in an environment in which we have supporting data that provided (1) the distribution of salinity within the tank with which to verify our inversion results, (2) the resistivity, porosity, and permeability of the porous medium, and (3) the transform between resistivity and salinity. The synthetic experiments allow exploration of issues of specific interest related to the presence of lithologic heterogeneity at a field site: the role of lithologic heterogeneity in introducing complexity into the resistivity-salinity relationship and the geometry of the saltwater intrusion wedge. We find that using a reference model with a good approximation of the wedge to inform the inversion greatly improves the ability of the resulting resistivity profile to map the wedge. Where there is no, or limited, lithologic heterogeneity, a parametric approach, which constrains the range of possible solutions by solving for a sharp interface between the saltwater and freshwater regions, is very effective at capturing the wedge location and geometry. Where there is lithologic heterogeneity, a hybrid between the parametric and informed inversion approaches is most effective, resolving the wedge with a high level of accuracy with little a priori information.