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

The major ion chemistry of samples from 216 headwater streams in the E verest, 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 wi nter and summer monsoon periods. The resulting database provides a bas eline 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 th e four regions. Ion concentrations tend to decrease with altitude, ref lecting differences in land use, land management, natural vegetation a nd atmospheric deposition, all of which are correlated with altitude. The well buffered nature of the pristine, high altitude streams indica tes that increased atmospheric deposition of S and N compounds is unli kely to cause acidification of streamwater, although these systems cur rently leak low concentrations of NO3. Terraced agriculture contribute s significantly to differences in chemistry both between and within re gions. Water draining agricultural catchments has higher concentration s of nutrients (NO3, PO4) and acid anions (Cl, SO4), probably as a res ult of mineral fertiliser inputs and of trace metals (Fe, Al, Ba, Sr, Mn) Si and F, potentially due to increased weathering.