Water quality of open pit Lake Cospuden status and modeling approaches

Abstract

Lake Cospuden is an open pit lake in the mining region of Middle Germany south of the city of Leipzig. The lake which will be used for recreation is filling with recovering groundwater, surface water, and groundwater pumped form the neighboring Zwenkau mine, and is projected to reach its final water level by 2000. The physical limnology of Lake Cospuden is changing during the filling period from an unstratified to a stratified lake with a hypolimnion that is approximately twice as large as the epilimnion. The water quality is typical for a mining lake with low pH and high metal concentration. Due to the filling with neutral surface water, recovery of the initially low pH lake water can be observed. The microbiological environment is attenuated by relatively high phosphorus and nitrogen concentrations, but low chlorophyll-a concentrations, which might be attributed to carbon limitation of photosynthesis due to the low pH. After a rev1ew of general lake models, two models COLQUAL and AQUAMODl, were chosen to simulated the eutrophication process. COLQUAL indicated phosphorus and light as the limiting factors. AQUAMODl, which was modified to incorporate carbon limited photosynthesis, indicated that the eutrophication process is governed by carbon limitation and a mechanism that keeps the soluble reactive phosphorus concentration low, despite of high total phosphorus concentrations. This mechanism was assumed to be related to binding of phosphorus by suspended matter (e.g. clays) which creates turbidity observed in the lake. This study has a preliminary character and identifies future research. Because of its mining history, Lake Cospuden is a unique system and will require alternative approaches in order to evaluate and predict the water quality. Some first steps were attempted in this study.