Evaluation of groundwater quality in parts of the Southern Gangetic Plain using water quality indices

In India, persistent dependence on groundwater for intensively irrigated agriculture, industrial use and domestic activities imposes substantial threat to the quality and quantity of groundwater. This study assesses the suitability of groundwater for drinking purposes in parts of the Southern Gangetic Plain based on samples collected from 33 dug wells. In this scope, two robust indices are utilized, namely Groundwater Quality Index (GQI) and the Canadian Council of Ministers of the Environment (CCME) Water Quality Index (CCME_WQI), to assess the overall quality of groundwater by integrating observed water quality determinants like pH, TDS and major ion concentration. The concentration of major anions and cations is observed to be in the order of HCO₃ ⁻ > Cl⁻ > SO₄ ²⁻ > NO₃ ⁻ and Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, respectively. The pH values of the analyzed samples vary between 6.2 and 7.37 during the post-monsoon (POM) period and between 6.6 and 7.9 during the pre-monsoon (PRM) period. Similarly, TDS concentrations are observed to vary between 6.7 and 1535 mg/l (POM) and 1.43 and 1327 mg/l (PRM). The values of GQI in the study area range between 10.41–108.56 and 16.03–130.06 for the PRM and POM periods, respectively. Among the groundwater samples collected during the PRM period, about 21, 73 and 6% are found to be excellent, good and poor quality, respectively. While the percentage of poor quality water for POM period is exactly the same as that of PRM, about 42 (52%) of the samples are found to be of excellent (good) quality for drinking purposes. The analysis of CCME_WQI for groundwater samples reveals that 3 (21%), 42 (36%), 33 (18%), 21 (24%) of the samples collected during PRM (POM) are classified to be of excellent, good, fair and marginal category, respectively. The spatial distribution of GQI and CCME_WQI illustrates that excellent quality water occurs primarily in the northeast part of the study area during the POM period. However, some localized patches with excellent quality are also observed in both the seasons. Results from this study will be crucial for proper utilization and management of already depleted groundwater resources of the study area.