The suitability of surface waters from small west-flowing rivers for drinking, irrigation, and aquatic life from a global biodiversity hotspot (Western Ghats, India)

The present study provides surface water types and water quality indices (WQI) for 70 large coastal rivers of the Western Ghats (WG). Irrespective of seasons and lithology, concentration of cations (Ca²⁺ > Na⁺ > Mg²⁺ > K⁺) and anions (HCO₃⁻ > Cl⁻ >SO₄²⁻ > NO₃⁻ > PO₄³⁻) follow a typical trend all along the coast. The WG rivers can broadly be classified as calcium-bicarbonate-chloride (Ca²⁺-HCO₃⁻-Cl⁻) type. Pearson correlation analysis of major ions demonstrates natural sources influence on the riverine water composition across the WG region. Gibbs plot suggests water composition of these rivers is the result of the interaction of rock and precipitation. It means that ionic contributions from precipitation and chemical weathering of rock-forming minerals largely determine surface water quality. This biodiversity hotspot is facing high population pressure and anthropogenic activities. Despite it, quantitatively, all the physical parameters and chemical constituents are within the permissible limits of the World Health Organization (WHO) and Bureau of Indian Standards (BIS), thus making it suitable for drinking and domestic purposes. About 86% of the surface water samples are found to be suitable for irrigation (KR < 1) in non-monsoon seasons. Rivers near to Goa coast are only found unsuitable (KR > 1) for irrigation exclusively during non-monsoon seasons. From the majority of the calculated indices, it may be inferred that the river waters draining from the WG region are suitable for irrigation. Overall, the calculated WQI for studied rivers showed excellent to good water quality for drinking, agriculture, and aquatic life in monsoon seasons, which are then ranked from good to marginal in non-monsoon seasons.