The legacy of hard coal mining in Germany: hydrochemistry and stable isotopes of mine waters

           Underground hard coal mining occurred in Germany over several centuries in the Ruhrgebiet in Northrhine-Westphalia and ceased in 2018. Nineteen mine waters draining historic and (sub)recent mines were studied in respect to their hydrochemistry and selected stable isotopes (H, O, C, S) in order to identify prevailing processes that determine their chemical composition.

All mine waters show near neutral pH values. Electrical conductivity displays a wide range with values between 370 and 1690 μS/cm (average value of 700 μS/cm). Major ions are in decreasing importance: dissolved inorganic carbon, calcium, sulfate, magnesium, sodium and chloride.

Sulfate concentrations range from 27 to 363 mg/L, showing a positive correlation with electrical conductivity. Sulfate sulfur and oxygen isotopes display some variability between -4.9 and +20.4 ‰ (average value of +4.5 ‰) and between +0.7 and +12.7 ‰ (average value of +4.5 ‰), respectively. Most isotope values suggest that sulfate derived from pyrite oxidation and/or the oxidation of organo-sulfur compounds in the coal. In contrast, the very positive sulfur and oxygen isotopes could indicate dissolution of sulfate minerals.

Apart from sulfate, dissolved inorganic carbon (DIC) is a major ion with concentrations ranging from 60 to 600 mg/L (average value of 249 mg/L). The carbon isotopic composition of DIC is somewhat variable with values between -19.3 and -4.5 ‰. Samples center around an average value of -11,40 ‰, suggesting the dissolution of carbonate minerals through carbonic acid.

In summary hydrochemical and stable isotope data from these mine waters suggest water-rock interaction as the main compositional driver, specifically carbonate dissolution, pyrite oxidation and sulfate dissolution.