Patterns and drivers of groundwater quality change in the Ganga River Basin: Linkages to hydro-meteorological and land-surface conditions.
Abstract
Safeguarding groundwater quality at basin scale requires identifying the areas where the conditions are improving, remaining stable, or deteriorating, and understanding the factors driving these changes. Using paired observations from 559 wells across the Ganga River Basin for the years 2010 and 2022, we quantified observed changes in major indicators, including electrical conductivity (EC), concentrations of chloride (Cl-), nitrate (NO3-), and total hardness (TH). A percentile-based framework was adopted to classify the wells as improved, stable/moderate change, or highly deteriorated, while an Explainable Boosting Machine (EBM) related these classes to hydroclimatic and land-surface variables derived from remote sensing and reanalysis datasets. Across the basin, 22.9% of wells improved, 58.5% remained stable/moderate, and 18.6% deteriorated. Highly deteriorated wells exhibited substantial median increases in nitrate (+211.2%), EC (+158.1%), chloride (+487.6%), and TH (+164.3%), indicating localised contamination and salinity-hardness hotspots. Groundwater-quality change class was significantly associated with aquifer/lithological category (χ2 = 63.88, Monte Carlo p < 0.0001), with alluvial aquifers showing a comparatively high improvement share among well-represented categories. Under an untouched test-set evaluation after applying SMOTE only to the training subset, the EBM achieved class-wise F1-scores of 0.35, 0.62, and 0.27 for improved, stable/moderate, and highly deteriorated wells, respectively. EBM feature responses identified rainfall, evapotranspiration, soil moisture, NDVI, and groundwater levels as key factors associated with groundwater quality change. The proposed framework offers an interpretable basis for identifying groundwater quality vulnerability hotspots and prioritising targeted management interventions, including nitrate mitigation, salinity control, recharge-zone protection, and improved agricultural water and nutrient management.