STREAM CHEMISTRY IN THE MIDDLE HILLS AND MOUNTAINS OF THE HIMALAYAS, NEPAL

The major ion chemistry of samples from 216 headwater streams in the Everest, Annapurna, Langtang and Nuwakot regions of the middle and high mountains of the Nepal Himalayas is described. Samples were collected at low flow during February-March 1992, the dry season between the winter and summer monsoon periods. The resulting database provides a baseline against which to assess natural and anthropogenic influences on water chemistry in these environments. Differences in bedrock geology generally determine differences in chemical characteristics between the four regions. Ion concentrations tend to decrease with altitude, reflecting differences in land use, land management, natural vegetation and atmospheric deposition, all of which are correlated with altitude. The well buffered nature of the pristine, high altitude streams indicates that increased atmospheric deposition of S and N compounds is unlikely to cause acidification of streamwater, although these systems currently leak low concentrations of NO3. Terraced agriculture contributes significantly to differences in chemistry both between and within regions. Water draining agricultural catchments has higher concentrations of nutrients (NO3, PO4) and acid anions (Cl, SO4), probably as a result of mineral fertiliser inputs and of trace metals (Fe, Al, Ba, Sr, Mn) Si and F, potentially due to increased weathering.