RESUMO
The Pachin river is an upland tributary of the Brahmaputra river, originating in the foot-hills of the lesser Himalayas. A systematic study of major ions and heavy metals in the surface water of the river was carried out at peak, intermediate and low flow conditions during an one-year interval to assess the relative contributions from weathering and pollution. The major ion chemistry indicates that silicate weathering and precipitation are the major contributing factors to the river's chemical composition. The Pachin river is characterized by a low overall conductivity, even during times of evaporative concentrations during low flow. This corresponds to the fact that the catchment area is still relatively pristine, i.e., well forested and without significant industry. However, heavy metal patterns clearly reflect source input from agricultural activity and urban development. The levels of Fe and Co were amongst the highest of any rivers of the Indian sub-continent, while these and Cr, Mn, Cu and Se each exceeded the world average value. These results clearly indicate that rivers draining pristine areas can be significantly modified by human activities, and consequently, these must be monitored to ensure that useable water supplies meet the prescribed safety standards.