Integrated stable S isotope, microbial and hydrochemical analysis of hydrogen sulphide origin in groundwater from the Legnica-Głogow Copper District, Poland

Hydrogen sulphide (H2S) was first reported in 2010 in an underground mining area of the Legnica-G & lstrok;og<acute accent>ow Copper District (LGCD). However, during the deposit exploration and exploitation stage in 1970, H2S was not detected. Previous studies have not indicated the origin of H2S in LGCD mine waters. Indirect evidence has suggested that, most likely, H2S migrated with the groundwater from the Main Dolomite aquifer to the mine galleries through cracks in the separating anhydrite and halite layers resulting from long-term mining operations. Interdisciplinary studies have been conducted to confirm this hypothesis. Investigations of the delta 34S in H2S of 12 groundwater samples taken from leakages from cracks in galleries and drilled boreholes showed high isotopic similarity to H2S in natural gas and S-organic compounds of oil from petroleum reservoirs in the Main Dolomite unit, where this gas was generated in high volumes mainly as a result of thermochemical sulphate reduction (TSR). Microbial investigations of mine water indicated the presence of Desulfitobacterales, Desulfobacterales, Desulfotomaculales, Desulfovibrionales, and Desulfuromonadia as possible indicators of contact between the analysed water and rocks inhabited by sulphur-reducing bacteria and Halothiobacillales, as an indicator of contact between water and salt. Hydrochemical analysis revealed that water flowing out of mine galleries and boreholes has a higher salinity than groundwater that is naturally associated with the horizon where mines operate; this phenomenon is associated with H2S occurrence. This investigation confirmed the hypothesis that the H2S in the LGCD mines originates mainly from the Main Dolomite aquifer. Furthermore, this study confirms that a multifaceted approach is necessary to answer complex geochemical questions in mining.