Chemical composition of rainwater in a tropical urban area of northern India

Rainwater samples during the monsoon of 2001 and 2002 were collected from Roorkee urban area, a medium sized “town group” situated on the right bank of Solani River, a tributary of the Ganga River, near the Himalayan foothills and analyzed for EC, pH, TSS, TDS and major ions. The median value of pH was 7.05, well above 5.6, which is the reference pH. The ratios of SO42−+NO3− and Ca2++Mg2+ (TA/TC) have been considered for acidity. In this study, ratio of TA/TC is quite below 1.0, indicating alkaline nature of rainwater. The concentration of ions in rain water have been observed to follow the pattern Ca2+>HCO3−>Cl−>NO3−>Na+>Mg2+>SO42−>K+. In order to estimate the marine and non-marine contribution, sea salt fraction has been calculated taking Na+ as reference. All ionic ratios have been found to be higher than the recommended sea water ratios in all three types of land use, viz. residential, commercial and industrial, suggesting a significant contribution of non-marine origin for these components. A comparison with the data of the other Indian sites validates the inverse relation of Cl− and Na+ with distance from the sea and highlights higher Ca2+ concentration and lower SO42− concentration.