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Groundwater contamination is a global problem that has a significant impact on human health and ecological services. Studies reported in this special issue focus on contaminants in groundwater of geogenic and anthropogenic origin distributed over a wide geographic range, with contributions from researchers studying groundwater contamination in India, China, Pakistan, Turkey, Ethiopia, and Nigeria. Thus, this special issue reports on the latest research conducted in the eastern hemisphere on the sources and scale of groundwater contamination and the consequences for human health and the environment, as well as technologies for removing selected contaminants from groundwater. In this article, the state of the science on groundwater contamination is reviewed, and the papers published in this special issue are summarized in terms of their contributions to the literature. Finally, some key issues for advancing research on groundwater contamination are proposed.

Nowadays, GIS and remote sensing techniques are effectively used to find groundwater prospect zones in various troublesome landscapes throughout the world. In the present work, groundwater potential zonation mapping was carried out for the Vattamalaikarai River basin in South India by overlaying ten thematic maps such as soil, drainage density, lineament density, geology, slope, land use/land cover, geomorphology, topographic position index, rainfall and groundwater level by giving appropriate weightages to each significant parameter with respect to its influence on groundwater. As the basin mainly depends on the groundwater resources, it is necessary to assess the groundwater prospect for the better management of aquifer system. Groundwater potential zonation map illustrates that more than 50% of the basin region falls under moderate to low groundwater potential category. Highly influential thematic layers were integrated to generate groundwater recharge zonation map. Based on this output, artificial recharge sites were selected to replenish the groundwater resources in the basin. Three check dam sites were suggested across the third- and fourth-order streams. Four suitable sites for the construction of percolation ponds and ten locations for the construction of recharge pits were also identified. Four injection well sites were recommended to augment groundwater in the aquifer present under the black cotton soil regions in the western part of the basin.

The main objective of the present research was to examine the risk of fluoride-rich groundwater in the Shanmuganadhi River basin, south India on human health. The non-carcinogenic risks were estimated into two classes: (1) risks associated with oral intake of water and (2) risks associated with dermal contact. Hazard Quotient for oral intake and dermal contact was separately calculated for adult men, adult women and children from the geochemical results of 61 representative samples collected from the wells constructed in hard rock aquifers during the post- (January-2018) and pre-monsoon (May-2018) seasons. The collected samples were analyzed immediately after the field work for all the major ions and fluoride. Finally, total hazard index was calculated for adults (men and women) and children to evaluate the risk. It directed that 41%, 49% and 74% of post-monsoon samples and 30%, 43% and 62% of pre-monsoon samples possessed a non-carcinogenic risk for men, women and children, respectively. Because the basin falls in the drought-prone region, the water supply for drinking and cultivation are commonly based on groundwater resources. The study revealed that the minerals such as apatite, fluorite, biotite and pyroxene in the hornblende–biotite gneiss formation contribute fluoride ions to the groundwater system due to water–rock interaction mechanism. The Durov diagram depicted that dissolution of silicate minerals and cation exchange are the foremost hydrogeochemical activities, which decide the overall chemical composition of groundwater in this region. The ionic concentrations including fluoride increased with respect to depth of occurrence of groundwater. Escalation of the water table due to monsoon recharge and artificial recharge through a check dam decreased the total dissolved solids and fluoride ion concentration. The investigation conducted around the existing check dam at Kaldurai village highlighted that the fluoride concentration is below the allowable limit of 1.5 mg/l (WHO in World health statistics 2017: monitoring health for the SDGs, Sustainable Development Goals. World Health Organization, Geneva, 2017) in the wells closer to the check dam toward the downstream side. The concentration increased with distance, which lead the groundwater unsuitable for consumption. Therefore, it is recommended to implement the managed aquifer recharge using check dams in the other parts of the basin to enrich the quantity and applicability of groundwater.

This study integrates hydrological simulation and crop yield modeling using the SWAT model to assess paddy production in the Palakkad and Alathur blocks of the Bharathapuzha River basin, South India. A majority of Indians rely on paddy as a staple food, particularly in Kerala; hence, the consequences of a warming planet on paddy productivity are causing concern. Palakkad, known as the granary of Kerala, produces about 34% of the total paddy in the state. The Bharathapuzha River basin’s monthly streamflow was simulated using the robust SWAT (Soil and Water Assessment Tool) model. Irrigation of paddy cultivation in the Palakkad district depends on the water from Malampuzha Irrigation Project, which lies in the Bharathapuzha River basin. Two blocks, Palakkad and Alathur, which completely lie within the Bharathapuzha watershed were considered for checking the model’s paddy yield prediction capacity. The watershed’s streamflow and paddy output were jointly simulated using the SWAT model, integrating hydrological processes with crop yield prediction. The monthly observed discharge at the Kuttipuram River gauge point was used to calibrate the model. The streamflow simulation for calibration gave a Nash Sutcliffe Efficiency (NSE) value of 0.67 during tuning period (1998–2012) and 0.64 for testing duration (2013–2017). The RMSE-observations standard deviation ratio (RSR) value for both training and testing spans was 0.58. Paddy yield prediction was evaluated by checking the performance indices NSE and RSR. The NSE value of 0.80 and RSR value of 0.45 for Palakkad block demonstrate the model’s ability to predict paddy yield reasonably. NSE of 0.68 and RSR value of 0.57 for Alathur suggest that SWAT can predict the paddy yield with good agreement to observed values. The findings provide useful insights for water resource planning and sustainable paddy cultivation in Kerala’s rice-growing regions. This dual integration of hydrological and yield modeling provides a methodological framework for climate-resilient agricultural planning in humid tropical river basins.

The aim of the study is to address the issues and associated health risks due to consumption of high-fluoride water supplied for drinking in a rural part of Shanmuganadhi River basin, Tamil Nadu, India. In this study, 61 groundwater samples were gathered from various tube and open wells and analysed for fluoride and other physicochemical parameters. The abundance of cations is Na+ > Ca2+ > Mg2+ > K+, and that of anions is HCO3− > SO42− > Cl− > F−. The fluoride concentration in drinking groundwater varied from 0.10 to 3.3 mg/l. According to the WHO standards, about 26% of the samples were unfit for drinking requirements (16 out of 61 samples) Water quality index (WQI) method was adopted to categorize the water into different classes to understand its suitability for drinking requirements. WQI signified that nearly 52% of the samples denoted poor, very poor and not suitable categories, whereas 48% of samples denoted good and excellent categories for consumption. Health risks associated with high-fluoride drinking water were assessed for various age groups of inhabitants such as children, teens and adults. The hazard quotient estimated based on the oral intake ranged from 0.00E+00 to 5.50E+00, from 0.00E+00 to 4.22E+00 and from 0.00E+00 to 3.45E+00 for children, teens and adults, respectively. It suggested that the health risks are associated with 75%, 59% and 43% of samples, respectively, among children, teens and adults. Therefore, children are more inclined towards risk than teens and adults in this region based on the intake of fluoride-rich drinking water. To improve the present scenario, groundwater should be either treated before drinking water supply or must be artificially recharged to lower the concentration of ions.

Samples of groundwater were collected during a post-monsoon period (January) and a pre-monsoon period (May) in 2020 from 30 locations in the rapidly developing industrial and residential area of the Coimbatore region in southern India. These sampling periods coincided with times before and during the lockdown in industrial activity and reduced agricultural activity that occurred in the region due to the COVID-19 pandemic. This provided a unique opportunity to evaluate the effects of reduced anthropogenic activity on groundwater quality. Approximately 17% of the wells affected by high fluoride concentrations in the post-monsoon period returned to levels suitable for human consumption in samples collected in the pre-monsoon period. This was probably due to ion exchange processes, infiltration of rainwater during the seasonal monsoon that diluted concentrations of ions including geogenic fluoride, as well as a reduction in anthropogenic inputs during the lockdown. The total hazard index for fluoride in the post-monsoon samples calculated for children, adult women, and adult men indicated that 73%, 60%, and 50% of the groundwater samples, respectively, had fluoride levels higher than the permissible limit. In this study, nitrate pollution declined by 33.4% by the pre-monsoon period relative to the post-monsoon period. The chemical facies of groundwater reverted from the Na-HCO3-Cl and Na–Cl to the Ca-HCO3 type in pre-monsoon samples. Various geogenic indicators like molar ratios, inter-ionic relations along with graphical tools demonstrated that plagioclase mineral weathering, carbonate dissolution, reverse ion exchange, and anthropogenic inputs are influencing the groundwater chemistry of this region. These findings were further supported by the saturation index assessed for the post- and pre-monsoon samples. COVID-19 lockdown considerably reduced groundwater pollution by Na+, K+, Cl–, NO3¯, and F– ions due to shutdown of industries and reduced agricultural activities. Further groundwater quality improvement during lockdown period there is evidence that the COVID-19 lockdown by increased HCO3¯ ion concentration. Overall results illustrate the positive benefits to groundwater quality that could occur as a result of measures to control anthropogenic inputs of pollutants.

In order to assess the geochemical mechanism liable for fluoride contamination in groundwater and its health effects on the people of the Shanmuganadhi River basin, Tamil Nadu, India, 61 groundwater samples were collected during post- and pre-monsoon seasons from the wells used for drinking purposes. Collected samples were analysed for various physico-chemical parameters. The parameters estimated in the present study are hydrogen ion concentration (pH), total dissolved solids, electrical conductivity, calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), bicarbonate (HCO3−), chloride (Cl−), sulphate (SO42−), nitrate (NO3−), phosphate (PO43−) and fluoride (F−). The fluoride ion concentration in the groundwater samples of this region varied from 0.01 to 2.50 mg/l and 0.01 to 3.30 mg/l during post- and pre-monsoon seasons, respectively. Out of 61 groundwater samples, 14 samples of post-monsoon season and 16 samples of pre-monsoon season represented high, very high and extremely high classes of fluoride, which cause dental fluorosis in this region. The fluoride-bearing minerals in the granitic and gneissic rocks such as apatite, hornblende, muscovite, biotite and amphiboles are the major sources for fluoride contamination in this area. In addition to the geogenic sources, applications of synthetic fertilizers in the agricultural fields also contribute significant amount of fluoride ions to groundwater. The spatial distribution of fluoride in different geological formations clearly indicate that the wells located in charnockite terrain were possessing very low fluoride concentration when compare with the wells located in the hornblende–biotite gneiss formation. Therefore, dental fluorosis risks are mostly associated with rock types in this region. People living over the basement rock comprising of hornblende–biotite gneiss are prone for fluorosis. Fluoride exhibited good positive correlation with bicarbonate in groundwater. As fluoridated endemic regions normally acquire lot of bicarbonate in groundwater samples, Shanmuganadhi basin falls under fluoride endemic category. The present study identified 26 villages in Shanmuganadhi basin as probable fluorosis risk areas where attention should be given to treat the fluoride-rich groundwater before drinking water supply. The groundwater level fluctuation study also designates that rise in water level reduces the concentration of fluoride due to dilution mechanism. Therefore, recharge of groundwater by artificial methods will definitely improve the present scenario.

Water contamination from sulphate is a growing concern in the Neyveli region of South India, where industrial activities have significantly impacted water quality. This study presents a comprehensive assessment of sulphate contamination and associated non-carcinogenic health risks in surface- and groundwater systems within the Neyveli lignite-mining region of Tamil Nadu, India. A stratified pre-monsoon survey of 81 samples in which 30 groundwater samples (bore wells, hand pumps, and open wells), and 51 surface water (ash ponds, drainage canals, ponds) were analysed for pH, EC, TDS, salinity, and sulphate using UV–VIS spectrophotometry, and spatial patterns were visualised via IDW interpolation in ArcGIS 10.8. Sulphate concentrations ranged from ~ 6–2,486.7 mg L⁻¹, exceeding the WHO permissible value of 400 mg L⁻¹; hotspots clustered near mine pits, ash-pond cells, and drainage corridors (SW3, SW36, SW38, SW50, GW10). Correlation analysis indicated negligible to very weak associations with EC/TDS/salinity ( r  ≈ 0.15–0.17), suggesting localised enrichment mechanisms rather than uniform ionic loading. Age-/gender-specific risk appraisal showed Hazard Quotient (HQ) > 1 at 19 sites for children (23.4%), 18 for teenagers (22.2%), 13 for adult males (16.0%), and 11 for adult females (13.6%). Critical contamination hotspots were identified at SW3, SW36, SW38, SW50, and GW10, where HQ values consistently exceeded acceptable risk thresholds across multiple demographic categories. Analytical QA/QC supported data reliability (recoveries 96.4 ± 2.1%, duplicate RPD 1.8–4.2%). In response to these findings, Neyveli Lignite Corporation India Limited (NLCIL) has undertaken several mitigation initiatives, including Water Treatment Plants and groundwater recharge structures. To enhance water quality management, remediation strategies like RO, gypsum ion exchange, fly ash adsorption, and sulphate-reducing bioreactors are recommended.

Hydrogeochemical investigations were carried out in Chithar River basin, Tamil Nadu, India to identify the major geochemical processes that regulate groundwater chemistry. For this study, long-term (1991-1997) and recent water quality data (2001-2002) for 30 groundwater wells spread over the study area were used to understand the groundwater geochemistry and hydrogeochemical process regulating groundwater quality. Groundwater quality data obtained from more than 400 water samples were employed. Results of electrical conductivity and chloride express large variation between minimum and maximum values and high standard deviation, which suggests that the water chemistry in the study region is not homogeneous and influenced by complex contamination sources and geochemical process. Nitrate and depth to water table expose the influences of surface contamination sources, whereas dissolved silica, fluoride and alkalinity strongly suggest the effect of rock-water interaction. In the study region, weathering of carbonate and silicate minerals and ion exchange reactions predominantly regulate major ion chemistry. Besides, the concentrations of sulphate, chloride and nitrate firmly suggest the impact of agricultural activities such as irrigation return flow, fertiliser application, etc on water chemistry in the study region.