Assessment of geochemistry and irrigation suitability of the River Ganga, Varanasi, India: PCA reduction for water quality index and health risk evaluation.

Surface water chemistry of the River Ganga at Varanasi was analyzed at 10 locations over 3 years (2019-2021) across pre-monsoon, monsoon, and post-monsoon seasons. The study aimed to assess water parameters using principal component analysis (PCA), calculate the water quality index (WQI), determine processes governing water chemistry, evaluate irrigation suitability, and estimate non-carcinogenic health risks. The physical parameters measured included pH (8.12 ± 0.37, 8.17 ± 0.30, 7.80 ± 0.30), electrical conductivity (EC) (857.58 ± 163.17, 667.17 ± 162.15, 754.46 ± 179.32 µS/cm), total dissolved solid (TDS) (428.98 ± 81.74, 334.99 ± 80.75, 376.14 ± 88.91 mg/L), and total coliform (6201.96 ± 6914.17, 4363.76 ± 3379.98, 6176.53 ± 7080.62 MPN/100 mL) for pre-monsoon, monsoon, and post-monsoon seasons, respectively. WQI, based on 9 major parameters via PCA reduction, indicated poor water quality (53-74) across sites and seasons, except for S1, which was rated as good during the monsoon. The major ion concentrations in the samples followed the trend: (bicarbonate) HCO⁻ (360.20 mg/L) > (chloride) Cl⁻ (59.80 mg/L) > (sodium) Na⁺ (53.10 mg/L) > (calcium) Ca⁺ (44.61 mg/L) > (sulphate) SO₄⁻ (43.81 mg/L) > (magnesium) Mg⁺ (24.50 mg/L) > (potassium) K⁺ (5.20 mg/L) > (phosphate) PO₄⁻ (2.41 mg/L) > (fluoride) F⁻ (0.52 mg/L), indicating alkaline water. The Piper diagram is dominated by Ca-Mg-HCO and mixed SO-Cl hydrochemical facies, while the Gibbs plot suggested rock-water interaction as the key driver of ion chemistry. Irrigation indices [sodium percentage (Na%); sodium absorption ratio (SAR); Kelly index (Ki); permeability index (PI); magnesium hazard (MH)] indicated suitability for irrigation. Non-carcinogenic risk assessment showed children were more sensitive to fluoride and chloride ingestion, while teenagers were more affected through dermal exposure.