In-Lake Neutralization: Quantification and Prognoses of the Acid Load into a Conditioned Pit Lake (Lake Bockwitz, Central Germany)

Abstract

The formerly highly acidic pit Lake Bockwitz south of Leipzig (Germany) has been repeatedly treated since 2004 with soda ash to meet water quality criteria for the lake effluent. Intense monitoring of water quality parameters showed that previous predictions underestimated the acid load into the lake. Field research and lab experiments were designed to identify and quantify the processes responsible for re-acidification. Monitoring data and key parameters from intermittent-flow column experiments were integrated in hydrogeochemical and physical transport models. The combined lake budget model indicated that re-acidification was dominated by leaching of acidic sulfide mineral weathering products from the Tertiary bank substrates. High inputs of iron, aluminum, and sulfate were generated by infiltrating rain water, interflow, and groundwater recharge. In contrast, acid loads from surface runoff and soil erosion were minor at this particular site. Based on this work, a methodology is proposed to obtain critical parameters from field and lab investigations and integrate those into hydrogeochemical and physical transport models. These process-based models offer tools to reliably predict the water quality of mining pit lakes, develop appropriate treatment measures for the rehabilitation period, and plan the requirements for cost-effective lake water conditioning.