HYDROCHEMISTRY AND ROCK WEATHERING IN A SUBTROPICAL LESSER HIMALAYAN RIVER BASIN IN KUMAUN, INDIA

Major ion chemistry of the water of the Gaula catchment - a medium-sized Lesser Himalayan basin in Kumaun, was measured in 1983 and 1984. The results show the influence of rock weathering on the concentration of major constituents in the ground water and surface water. The catchment lithology contributes a large part of the major constituents in the waters; Ca and Mg account for 40-73% of the cations and HCO3 accounts for 48-98% of anions, and the equivalent ratio of Ca + Mg to Na + K is about nine. Cl + SO4 accounts for 12-16% in the anion balance and does not represent a contribution from soil salt. The low Mg: Ca ratio (0.1-0.4) and high bicarbonate suggest the possibility of carbonate precipitation between the upper and the lower reaches. The excess Na over Cl, low Mg:Ca ratio and relatively high abundance of silica in the upper reaches indicate weathering of aluminosilicate minerals of crystalline rocks (granite, mica schist and quartz porphyry), and particularly of Na- and K-feldspar and quartz. Intense weathering of pyrite and or gypsum associated with the carbonate rocks is reflected by the substantially higher abundance of SO4, low silica and the low Na:Cl ratio. In comparison with the Indian peninsular rivers, the low concentrations of the major constituents in the water of this Lesser Himalayan catchment indicate rapid infiltration and quick outflow of the rainwater and thus a short residence time for water, during which interaction with the rocks could occur, because of the steep slopes.