Assessment of hydrogeochemical characteristics and saltwater intrusion in selected coastal aquifers of southwestern India

Spatial assessment of submarine groundwater discharge influence on aquifer water quality in the coastal region of Chettikulam to Kolachel, southern India: using SMI and HFE-D techniques

Submarine Groundwater Discharge (SGD) has a global impact, affecting coastal aquifers, the freshwater environment, and contributing to coastal development. The present study investigates the impact of Submarine Groundwater Discharge (SGD) on groundwater geochemistry along the coast from Chettikulam to Kolachel in Southern India, with an emphasis on regional changes pre and post monsoons in the years 2023-2024. A total of 80 groundwater samples (40 from both monsoons) were analyzed using hydrochemical plots such as Piper, Wilcox, Gibbs, and Hydrochemical Facies Evolution Diagrams (HFE-D), along with AquaChem software and spatial mapping techniques. Hydrogeochemical analysis reveals significant seawater intrusion during the pre-monsoon period, with 48% of groundwater samples categorized under the Mixed Ca-Mg-Cl facies and 30% under the Ca-Cl facies, leading to elevated salinity, total dissolved solids (TDS), chloride, and sulfate concentrations. In contrast, post-monsoon conditions demonstrate improved water quality due to monsoonal recharge, with 55% of samples in the mixed Ca-Mg-Cl facies but exhibiting reduced salinity. Water Quality Index (WQI) analysis shows an increase in 'Excellent' water samples from 22.5% pre-monsoon to 37.5% post-monsoon. The Wilcox diagram highlights a reduction in salinity hazards, improving groundwater suitability for irrigation. The Seawater Mixing Index (SMI) indicates the seawater intrusion in coastal zones, although monsoonal dilution mitigates its effects. Seasonal variation in hydrochemical facies reveals a decrease in the freshening phase from 60% pre-monsoon to 45% post-monsoon, alongside an expansion of the intrusion phase from 40 to 55%. The study underscores the need for ongoing groundwater monitoring and effective water management strategies to sustainable coastal aquifers impacted by Submarine Groundwater Discharge (SGD).

Quality and health impact of groundwater in a coastal region: a case study from west coast of southern India

Seawater intrusion seriously threatens the quality of coastal groundwater, affecting nearly 40% of the world's population in coastal areas. A study was conducted in the Kamini watershed situated in the Udupi district of Karnataka to assess the groundwater quality and extent of seawater intrusion. During the pre-monsoon period, 57 groundwater and 3 surface water samples were analyzed to understand the impact of seawater on the groundwater and surface water. The analysis revealed that the groundwater in the study area is slightly alkaline. The weighted overlay analysis map indicated that 11% of the study area is unsuitable for drinking water due to the influence of seawater. The Piper plot analysis revealed that the groundwater is predominantly CaMgCl facies. The hydrogeochemical facies evolution diagram (HFED) showed that 62% of the groundwater is affected by seawater. The HFED and Piper plots also indicate that the surface water is also affected by seawater. These results are also supported by various molar ratios such as Cl- vs. Cl⁻/HCO3⁻, Cl⁻ vs. Na⁺/Cl⁻, Cl- vs. SO42-/Cl-, and Cl⁻/HCO3- vs. Mg2+/Ca2+, suggesting that the majority of the water sample has been affected by seawater. The saturation indices indicated that mineral dissolution has significantly contributed to groundwater salinization. The correlation between sulfate concentration and calcite and dolomite dissolution suggested the influence of seawater intrusion in the coastal aquifer. The process of reverse ion exchange mainly influences the groundwater chemistry according to chloroalkali indices. The total hazard index (THI) values of nitrate and fluoride exceeded limits, posing health risks to adults and children. Studies suggest that with time and space, seawater intrusion is increasing in some pockets of the study area, especially along the west coast.

MLR index-based principal component analysis to investigate and monitor probable sources of groundwater pollution and quality in coastal areas: a case study in East India

Identifying groundwater contamination sources and supervising groundwater quality conditions are urgently needed to protect the groundwater resources of coastal areas like Contai of India, as communities here are heavily relying on groundwater which deteriorates progressively. So current research aims to address in detail about origins and influencing factors of groundwater contamination, status, and monitoring water quality by employing extremely useful leading technologies like principal component and factor analyses (PCA/FA), groundwater quality index (GWQI), and multiple linear regression (MLR) that helps to simplify complicated works instead of the conventional methods. Eight groundwater quality parameters were evaluated here, such as pH, TH (total hardness), Tur (turbidity), EC (electrical conductivity), TDS (total dissolved solids), Mn (manganese), Fe (iron), and Cl (chloride) for 38 sites. Three principal components with ~ 81% of the total variance were extracted from the PCA/FA analysis. The origin of maximum loadings of each factor is identified as a result of saline water, disintegration and leaching process, organic or else biogenic activities, and lithogenic or otherwise non-lithogenic links through percolating water. GWQI results show that ~ 87% of the samples fall into the good category and ~ 13% of the samples fall into the poor to very poor category. A model consisting of Tur, Fe, EC, Mn, TH, and Cl as independent parameters is more feasible and is proposed to predict GWQI obtained from MLR analysis. This MLR model also suggests that turbidity with the highest beta coefficient (0.820) is a key contributor relative to the entire groundwater class in this affected area. The findings relating to this research may support the designer and officials in monitoring and protecting coastal groundwater resources like selected areas.

Groundwater and pore water inputs to the Bay of Bengal supported by geochemical tracers: Example from Tamil Nadu and Pondicherry, East Coast of India

Submarine groundwater discharge (SGD) and Saline water intrusion (SWI) are the two major processes that influence coastal aquifers resulting in severe water stress. It is essential to characterize groundwater dynamics and discriminate geochemical characterization to understand both approaches. The present study investigates the zones influenced by SGD and SWI along the coastal aquifers of Tamil Nadu and Pondicherry. Pore water (PW) and groundwater (GW) samples were collected during the monsoon season at low tide and analyzed for major ions adopting standard procedures. Hydrogeochemical characterization of water samples revealed by the piper, ionic ratio, ionic delta (Δm_i), and seawater fraction (f_Sea) plots suggest that the PW samples, irrespective of location and GW samples in specific areas, were attributed to SWI, while the remaining samples suggested SGD. The average chloride-attributed SGD flux calculated for PW was 23.45 × 10^-7 L cm^-2 S^-1 and for GW, 0.58 × 10^-7 L cm^-S^-1. Higher fluxes observed in PW suggest seawater recirculation as the primary mechanism, and GW samples were found to be influenced by fresh, recirculated, and saline intrusions. Overall, the northern parts of the study region were influenced by freshwater discharge. However, the central and southern parts of the study regions were influenced by mixed RSGD and SWI water types. The present work suggests locations influenced by fresh, recirculated, and saline water zones that can benefit the stakeholders in planning strategies to identify proper aquifer recharge and suggest ideal pumping scenarios to sustain groundwater resources.

Impact of limestone caves and seawater intrusion on coastal aquifer of middle Andaman

Seawater intrusion has become a common problem in coastal and island aquifers with the rise in climate change that greatly affects the majority of developing countries. The island hydrology is very complex and associated with a unique set of environmental characteristics with the dynamic interaction of groundwater, surface water, and seawater. Further, Sea level rise, erratic rainfall, and over-extraction of groundwater triggered salt-water intrusion. A study on seawater intrusion and the effect of limestone caves on groundwater was carried out in middle Andaman using a combination of ionic ratios of major ions. A total of 24 samples and a reference sample from the sea were collected and analysed using ICP, spectrophotometer, and flame photometer. A combination of 10 ionic ratios Cl/HCO₃, Ca/(HCO₃ + SO₄), (Ca + Mg)/Cl, Ca/Mg, Ca/Na, Cl/(SO₄ + HCO₃), Ca/SO₄, K/Cl, Mg/Cl, and SO₄/Cl was used to assess the dissolution of limestone minerals and the level of saltwater intrusion into groundwater. The geospatial method was used to extract and combine all the hydrogeochemical parameters and ionic ratios in the GIS platform. Durov plot was used for the interpretation of groundwater chemistry and the identification of natural processes controlling the hydrogeochemistry of the area. The dominance of Ca-HCO₃ and Na-HCO₃ was confirmed in 48% and 24% of the sample respectively. The equiline graph of chloride with other major ions showed the enrichment of alkali and alkaline earth metal salt in groundwater. Schoeller's diagram depicted the dominance of Cl, Ca, and the sum of CO₃ and HCO₃ in seawater near Mayabunder. The lower concentration of Na with respect to Cl (64%) and Ca (100%) showed the presence of a reverse ion exchange process. Further, the correlation matrix showed a strong relationship between Cl, K, Ca, and Na. The analysis of X-ray diffraction of the rock samples confirmed the presence of limestones such as Aragonite, Calcite, Chlorite, Chromite, Dolomite, Magnetite, and Pyrite in the study area. The integration of ionic ratios showed moderately affected and slightly affected saline regions in 44% and 54% of the region respectively. Finally, the role of tectonic activities and active lineaments connected to the sea was found to play a major role in the intrusion of seawater where interconnected faults created an opening for surface water to recharge groundwater leading to the deep aquifer.

Assessment of groundwater vulnerability by applying the improved DRASTIC model: a case in Guyuan City, Ningxia, China

Groundwater is the main source of production and living in most arid and semi-arid areas, and it plays an increasingly critical role in achieving local urban development. There is a serious issue regarding the contradiction between urban development and groundwater protection. In this study, we used three different models to assess the groundwater vulnerability of Guyuan City, including DRASTIC model, analytical hierarchy process-DRASTIC model (AHP-DRASTIC) and variable weight theory-DRASTIC model (VW-DRASTIC). The groundwater vulnerability index (GVI) of the study area was calculated in ArcGIS. Based on the magnitude of GVI, the groundwater vulnerability was classified into five classes: very high, high, medium, low, and very low using the natural breakpoint method, and the groundwater vulnerability map (GVM) of the study area was drawn. In order to validate the accuracy of groundwater vulnerability, the Spearman correlation coefficient was used, and the results showed that the VW-DRASTIC model performed best among the three models (ρ=0.83). The improved VW-DRASTIC model shows that the variable weight model effectively improves the accuracy of the DRASTIC model, which is more suitable for the study area. Finally, based on the results of GVM combined with the distribution of F^- and urban development planning, suggestions were proposed for further sustainable groundwater management. This study provides a scientific basis for groundwater management in Guyuan City, which can be an example for similar areas, particularly in arid and semi-arid areas.

Impact of land use/land cover on groundwater resources in tropical unconfined aquifers of south-western India

Groundwater quality assessment is essential to understand land use impacts and implement water management plans. The present study aims to assess the impact of land use/land cover (LULC) on the groundwater table, and its quality in the tropical unconfined aquifers. Two hundred groundwater samples were collected from 100 sampling wells during monsoon and post-monsoon seasons. The drinking water quality index and irrigation quality indices were estimated based on the various parameters obtained from the laboratory analysis. Human health risk concerning nitrate contamination was evaluated based on the USEPA method. The land-use/land-cover map prepared using ArcGIS showed that the study area consists dominantly of croplands. Drinking water quality index results suggested that the groundwater samples were excellent to moderately suitable for drinking purposes. Only one sample was unsuitable for drinking. The different irrigation quality indices revealed various degrees of groundwater suitability for irrigation purposes. The spatial distribution of the corrosivity ratio suggests avoiding the metal pipe, for transportation of groundwater supply in the northern part of the study area. Fertilizers used in agriculture and soak pit leakages have contributed to high nitrate concentration in a few parts of the study area. Human health risk assessment showed that infants are vulnerable to non-carcinogenic health risks. The impact of the LULC assessment revealed that groundwater quality was moderately suitable for drinking in urban land. The study suggests implementing proper sewage treatment measures to avoid groundwater contamination. Overall, the findings are important in adopting site-specific, groundwater management strategies in the study area. Polluted and unpolluted areas demarcated in the study are beneficial for decision-makers to develop suitable groundwater management plans. The study recommends informed LULC development in the study area to improve groundwater quality and reduce human health risks.

Interpreting the salinization and hydrogeochemical characteristics of groundwater in Dongshan Island, China

The groundwater salinization is a global problem that degrades water quality and endangers sustainable use of water resources, particularly in coastal areas. In this paper, 24 water samples were collected from 12 monitoring wells during the dry (January) and wet (June) seasons for analyzing the salinization and hydrogeochemical characteristics of groundwater in Dongshan Island of China through combined hydrogeochemical and multivariate statistical approaches. Results showed that groundwater in the study area is primarily Cl-Na and followed by Cl-Ca·Mg type in the dry season, Cl-Na and followed by Cl-Ca·Mg and HOC₃·Cl-Na type in the wet season. The groundwater chemistry is predominantly controlled by carbonate, gypsum, and silicate dissolution. However, some areas are strongly influenced by seawater intrusion, sewage infiltration and reverse ion exchange process. Around 40% of water samples from the dry season and 50% from the wet season are at injuriously, highly and severely saline levels while other samples at slightly and moderately saline levels, suggesting that groundwater in the area is partially recharged by seawater. Furthermore, the NO₃^-/Cl^- versus Cl^- diagram and principal component analysis (PCA) indicated nitrate pollution in groundwater that is subjected to anthropogenic activities such as domestic sewage, agricultural and industrial practices, which lead to degradation of groundwater quality in the area. The findings of this study provide helpful insights for understanding the genesis and hydrogeochemical evolution of groundwater in those coastal areas.