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

The isotopic and chemical composition of water and dissolved sulfur and car bon 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 su lfate reduction and degradation process of organic matter in the lake and t he groundwater. The isotopic composition of both the sulfate/sulfide and th e dissolved inorganic carbon (DIC) confirm similarities between the process es in the subhydrical landfill site and in the anaerobic zone of the lake ( monimolimnion). The strong reducing conditions are expressed by an enrichme nt of delta(34)S of sulfate (landfill: delta(34)S-SO4 = +37 parts per thous and, 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 re duced according to the Rayleigh relation. The carbon isotopic composition o f DIC ranges from delta(13)C = -10 parts per thousand to delta(13)C = -19 p arts per thousand in the landfill and from delta(13)C = -8 parts per thousa nd to delta(13)C = -18 parts per thousand in the lake. It was shown that th ese values represent the complex interactions between the production of met hane and carbon dioxide and the formation of bicarbonate from intermediate products.