COMPOSITION OF SEDIMENTS IN DRINKING-WATER RESERVOIRS AS A BASIS FOR THE ASSESSMENT OF POTENTIAL CHANGES IN WATER-QUALITY

The sediments of two drinking water reservoirs in the Erzgebirge Hills , Saxony, Germany, were compared. One had a high external phosphorus l oad (Saidenbach Reservoir), the other (Neunzehnhain Reservoir) a low e xternal phosphorus load. However, both had high iron, aluminium and ni trate loads. The goal was to determine whether the sediment compositio n can yield information on the potential changes in water quality subs equent to changes in the external phosphorus load. The catchment of th e meso- to eutrophic Saidenbach Reservoir is mainly under agricultural use, whereas the catchment of the oligotrophic Neunzehnhain Reservoir is largely forested and not populated. The bottom sediment in both ca ses was relatively high in total phosphorus, total number of bacteria and esterase activity (fluoresceine diacetate hydrolysis). In the Neun zehnhain Reservoir sediment, however, aluminium was substantially high er, and sulphide was absent. Obviously, the phosphorus-binding capacit y of the Saidenbach Reservoir sediment is already nearly exhausted. In the Neunzehnhain Reservoir sediment, the phosphate mobility was extre mely low, and its capacity to immobilise phosphate remains high. The s ubstantial high aluminium input resulting from the acidification of th e catchment and the consequent precipitation in the reservoir may have been effective in maintaining the oligotrophic character of the Neunz ehnhain Reservoir.