Assessment of Hydrochemical Controls and Water-Use Suitability of Surface Waters in The Gadilam River Basin, Tamil Nadu, India

Abstract

Coastal river basins are influenced by multiple interacting factors while most index-based assessments focus on groundwater and evaluate a single water use based suitability analysis. This study involved thirty-eight surface water samples collected from estuaries backwaters tanks and the Gadilam River within the Gadilam River Basin. Eighteen water quality parameters were measured and separate indices computed for drinking, domestic and irrigation use, with entropy-derived weights verifying the drinking classification. Composition is dominated by a longitudinal salinity gradient. Electrical conductivity ranges from 284 to 8,788 µS/cm, the sodium to chloride ratio converges on the seawater value of 0.858, and chloride-based mixing fractions reach 15% downstream. Two additional controls act independently with nutrient enrichment indicated by the strong correlation between nitrate and phosphate (r = 0.942) and higher concentrations upstream of the tidal limit while redox mobilization of iron and manganese is strongly correlated (r = 0.828) and unrelated to salinity. The drinking water quality index spans 28.8 to 466.1, averaging 42.4, 73.8 and 241.9 in the upper, middle and lower reaches, and classifies 17 samples Excellent, eight Good, seven Poor, one Very poor and five Unsuitable. Twenty samples comply with the Indian drinking water specification while 18 do not with iron being the largest contributor to poor drinking water quality in 20 samples. Fifteen of the 21 lakes rated Excellent for drinking are classified as Good for domestic use while six sources classified as non-potable due to iron remain suitable for irrigation.