Open-cast lignite mining has exposed formerly anaerobic layers to the air. Pyrite and marcasite are oxidized to sulfuric acid and iron sulfate, which later hydrolyses to iron hydroxide. When the pit is refilled with groundwater, its pH is 2–3. These geogenically acidified mining lakes are brown, with high iron content. Biological conditions differ greatly from those in most natural lakes and resemble those in sulfur-acidic lakes impaired by acid rain. The usability of these fish-free lakes is also restricted for recreational purposes.

Neutralization with lime is expensive. The iron-buffered water has a high base buffering capacity stabilizing at pH 2–4. The complex of potential measures for neutralization comprises: the revegetation of the waste heaps, the hydrological regime and coupled transport processes, the selection of suitable filling water and in-lake technologies. Desulfurication takes place with sulfidic binding of iron and natural neutralization under locally anaerobic conditions in the reclaimed overburden heaps, in the deep water of mining lakes, in macrophyte stands and in the sediments.

Anaerobic conditions should be established within the lakes during stratification periods: in the hypolimnion of eutrophic or the monimolimnion of meromictic lakes. Acidic lakes often are not sufficiently productive to achieve deep-water oxygen depletion. Measures to increase the stability of stratification may be necessary, for example by installation of barriers to establish an environment suitable for biological neutralization. Carbon sources for microbial desulfurication may be added artificially or autochthonically produced by primary producers. Therefore, a controlled eutrophication may be useful in the initial filling stages of acidic mining lakes.