Hydrogeochemical investigations were carried out with an objective to identify the processes affecting the chemistry of groundwater in the Coimbatore district of Tamil Nadu, India. Thirty-three groundwater samples were collected from representative wells for chemical analysis. Groundwater types identified from piper plots were Ca-Mg-Cl and Na-Cl. The dominance of ions was in the order of Na>Ca>Mg>K and Cl>HCO₃>SO₄>CO₃. Spatial variation diagrams of ions were generated using the geostatistical analyst tool ArcGIS 9.3. According to these diagrams, most of the ions were higher in the northeast and southeast regions. This is attributed to the flow direction of the groundwater and high residence times. Gibbs diagrams identified rock–water interaction as an important geochemical process in the district. Evaporation, ion exchange, silicate weathering and dissolution of carbonate minerals were identified as other important hydrogeochemical processes which influence the groundwater chemistry of the study area.

Groundwater depletion is one of the most critical concerns for users and policymakers. Identifying groundwater potential (low to high) helps properly plan the available groundwater resource. This study has used the possibilities of a geographical information system (GIS), remote sensing and, of course, field data to delineate the groundwater potential zones in the Chennai River Basin (CRB). Thematic layers generated for eleven controlling factors, such as geology, water level, drainage, soil, lineament, rainfall, land use, slope, aspect, geomorphology, and depth to bedrock, were brought into the GIS environment. Then, appropriate weightage was given to each layer using a multi-criteria decision-making technique, namely, the analytic hierarchical process (AHP). A groundwater potential map is generated using weighted overlay analysis, with the following five classes: very poor, poor, moderate, good, and very good. The results were comparable to the actual specific yield data from the field and accuracy was 78.43%. Thus, AHP-aided GIS–RS mapping is a useful tool in groundwater prospecting in this region of the world.

Anthropogenic factors are contaminating crystalline aquifers more rapidly than natural sources and affecting human health in many states in India. Since a large population depends on untreated groundwater, identifying the sources of this contamination and assessing the related human health risk are essential to ensure a good-quality water supply. Nitrate is one of the most widespread means of groundwater contamination in many parts of India. Coimbatore and Tirupur districts are the most rapidly growing industrial urban areas in southern India. This paper deals with nitrate contamination and possible health risks for children and adults in the Coimbatore and Tirupur districts based on 93 groundwater samples. To achieve this goal, classical hydrochemical and deterministic hazard identification methods coupled with spatial mapping technologies were applied. A wide variation in nitrate concentration, between 1 and 415 mg/L, was observed, with 37% of the samples exceeding the WHO permissible limit of 50 mg/L. The distinct concentrations of nitrate and other ions observed spatially can be attributed to the diverse geochemical and land use settings in the study area. The bivariate plots of NO₃ with other ions suggested that the principal origin of nitrate in this study is related to the excess application of fertilizers and sewages. The spatial variation of NO₃, in comparison with the land use map, confirmed these results. The values of hazard quotient (HQ) via ingestion exceeded the critical value, one in 40% in males, 42% in females, and 45% in children. However, HQ values via oral pathways are within one and pose no exposure risk. Thus, the hazard index corresponds to HQ_ingestion only. The health risk was in the increasing order of male>female>children, and shows that body weight is the most critical factor that is influencing the health impact to children as compared to adults. The spatial variation of hazard index values showed that groundwater quality is highly polluted with NO₃ in the north and northeastern parts of the study area, mainly due to intensive agricultural practices, and poses critical health concerns. Considering the increasing population and higher dependencies on groundwater, immediate and sufficient measures are proposed.