Decoupled geogenic and faecal hazards in shallow coastal groundwater: evidence and a screening framework from the lower Indus Delta, Pakistan

Shallow hand pumps supply drinking water to millions of people in low-lying deltas worldwide, where groundwater is exposed to geogenic salinisation and faecal contamination. Because these hazards are usually assessed together, it remains unclear whether they share a pathway and, therefore, which interventions are needed. In the lower Indus Delta (Pakistan), we analysed 93 hand pumps for 16 physicochemical variables, total coliforms and Escherichia coli, and combined hydrochemical diagnostics, a seawater mixing index (SMI), multivariate statistics, entropy-weighted and weighted-arithmetic water quality indices (EWQI, WA-WQI) and deterministic and Monte Carlo health-risk models. TDS exceeded 1000 mg/L in 58.1% of wells. A single salinity factor (65.9% of variance) integrated evaporation, cation exchange (slope −0.95, R² = 0.97) and limited marine mixing (SMI > 1 in 23.7%). E. coli occurred in 23.7% of wells, was independent of salinity (OR = 0.83, p = 0.81), was predicted only by turbidity (OR = 4.18 per log₁₀ NTU; AUC = 0.82) and, unlike salinity, was not spatially clustered, indicating wellhead ingress. WA-WQI assigned 66% of its weight to F⁻ and turbidity and agreed poorly with EWQI (weighted κ = 0.20). Child hazard index exceeded 1 with 46.1% probability, driven by F⁻, and drinking water supplied up to 122% of the WHO daily sodium limit. A well-by-well typology showed that only 26.9% of wells were safe, 8.6% needed only wellhead repair and 64.6% required treatment or a new source. The framework uses only routine monitoring data and is transferable to other salinising coastal deltas.