Assessment of the Extent of Sulfate Reduction in Lignite Mining Dumps Using Thermodynamic Equilibrium Models

Abstract

Following the description of immobilizable reactions in dump aquifer - residual lake - systems, this article deals with the assessment of the extent of sulfate reduction processes in lignite mining dump aquifers in Germany. Laboratory experiments with and without addition of easily degradable organic substances were carried out with sediments and groundwater of the Rhineland mining area. The natural organic carbon (matter) of the sediments did not lead to extensive sulfate reduction during the laboratory experiments. In contrast, the addition of degradable organic substances led to the formation of higher amounts of acid volatile sulfide (AVS) as product of sulfate reduction during the laboratory experiments. Stability diagrams were constructed from hydrogeochemical calculations of measured groundwater samples for the Rhineland and Lusatian mining area. The pH-pe-diagrams were calculated using all complexation reactions of the computer code PHREEQM, corrected for ionic strength effects and for real groundwater temperatures. Fe(III)-hydroxide and Fe(II)-carbonate (siderite) turned out to be the relevant solid phases which control the solubility of iron. The stability field of pyrite is located at lower redox values as measured in the groundwater. Altogether the laboratory experiments and the hydrochemical calculations point out that extensive natural sulfate reduction in the investigated dump aquifers is not likely without addition of degradable organic material.