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Buffering mechanisms in an acidic mine lake in Lusatia, Germany were investigated. The titration curve has four sections with different buffering mechanisms: (1) buffering by free hydrogen ions and hydrogen sulphate (pH = 2.55-2.9), (2) buffering by Fe with bound SO 4 (pH = 2.9-4.3), (3) buffering by Al with bound SO 4 (pH = 4.3-5.5), and (4) buffering by surface exchange of SO 4 and basic cations (pH > 5.5). Three different phase models were applied to simulate the titration curve: (1) an iron and aluminium hydroxide model; (2) an iron and aluminium hydroxysulphate model; and (3) an iron hydroxide model with surface exchange for SO 4 , Ca, and Mg, coupled with an aluminium hydroxysulphate model. The uncertainty of model input parameters was accounted for in a sensitivity analysis. Only the third model, which considers surface exchange, was able to adequately reproduce the measured titration curve.

a b s t r a c tAbout one third of several hundred mining lakes in Eastern Germany are highly acidified, and there is a need to restore them to neutral conditions because they constitute an environmental hazard for water resources and downstream environments. The aim of this study is to evaluate the efficiency of three different acid pit lake water remediation treatments: dilution with alkaline (river) water, limestone treatment and biological neutralization by organic carbon-driven alkalinity generation. The efficiency is evaluated for the acidic mining lake Grünewalder Lauch by adjusting input values into a geochemical model and making future projections. Current approaches, such as flooding with neutral surface water or extensive liming, are not suitable for many lakes because of a limited supply of alkaline water or high lime immobilizing potential of Fe-and Al-rich water in acidic lakes, respectively. Further treatment methods are, therefore, designed to combine water supply and biological measures with the management of water quality by the application of in-lake microbial processes. These processes are focused on the metabolic response of aquatic ecosystems to nutrient enrichment (enhancement of primary production and thereby organic carbon supply) and the microbial decomposition of organic matter and their effects on the gain or loss of alkalinity.The results and comparisons of different neutralization measures will be generalized by the application of hydrogeochemical models for alkalinity production showing a) the long term efficiency of the measures, depending on carbon turnover at the sediment/water interface, b) the development of bicarbonate buffering capacity as a consequence of biological measures, c) the importance of pyrite formation instead of FeS.

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