Sulfate reduction in a lake and the groundwater of a former lignite mining area studied by stable sulfur and carbon isotopes

The isotopic and chemical composition of water and dissolved sulfur and carbon compounds from a flooded lignite-mining lake and a dump site in Central Germany (today partly covered by a landfill) is used to investigate the sulfate reduction and degradation process of organic matter in the lake and the groundwater. The isotopic composition of both the sulfate/sulfide and the dissolved inorganic carbon (DIC) confirm similarities between the processes in the subhydrical landfill site and in the anaerobic zone of the lake (monimolimnion). The strong reducing conditions are expressed by an enrichment of delta(34)S of sulfate (landfill: delta(34)S-SO4 = +37 parts per thousand, bottom of the lake: delta(34)S-SO4 = +27 parts per thousand), despite varying sulfate supply. More than 50% of the sulfate in the landfill and up to 95% of the sulfate in the monimolimnion were calculated to have been reduced according to the Rayleigh relation. The carbon isotopic composition of DIC ranges from delta(13)C = -10 parts per thousand to delta(13)C = -19 parts per thousand in the landfill and from delta(13)C = -8 parts per thousand to delta(13)C = -18 parts per thousand in the lake. It was shown that these values represent the complex interactions between the production of methane and carbon dioxide and the formation of bicarbonate from intermediate products.