Seasonal variability, Hydrochemical processes, and drinking water suitability in groundwater of Madhepura District, Bihar, India

Seasonal variation in drinking water quality poses significant challenges to consumption safety in monsoon-dominated alluvial regions. This study evaluates the seasonal and spatial variability of groundwater quality in Madhepura district, Bihar, based on 250 drinking water samples collected from 13 administrative blocks during pre-monsoon, monsoon, post-monsoon, and winter seasons. Physicochemical parameters including pH, electrical conductivity (EC), total dissolved solids (TDS), major ions, nitrate, and fluoride were analyzed and integrated using Water Quality Index (WQI), hydrochemical facies, and geochemical process evaluation. Mean EC and TDS values were highest during the pre-monsoon season (892 µS/cm and 546 mg/L, respectively), reflecting evaporative concentration and enhanced water-rock interaction. Monsoon recharge led to a significant reduction in EC (612 µS/cm) and TDS (382 mg/L), indicating dilution; however, nitrate concentrations increased markedly during this period, with a mean value of 52 mg/L, exceeding drinking water guidelines in 47% of samples. Fluoride concentrations showed seasonal decline from 1.21 mg/L during pre-monsoon to 0.82 mg/L during monsoon. WQI values indicated poor water quality during pre-monsoon (mean WQI=91) and very poor quality during monsoon (mean WQI=112), while winter showed relative improvement (mean WQI=74). Piper diagram analysis revealed dominance of Ca-HCO₃ facies across all seasons, confirming carbonate weathering as the primary geochemical process. Gibbs diagrams further demonstrated rock-water interaction as the dominant control, with minor evaporation influence during dry periods. Spatial WQI mapping identified higher contamination risk in agriculturally intensive and flood-prone blocks. The study highlights pronounced seasonal and spatial variability in groundwater quality and emphasizes the need for seasonally adaptive and location-specific drinking water management strategies in monsoon-affected alluvial aquifers.