NATURAL AND ANTHROPOGENIC ORIGIN SULPHATE IN AN MOUNTAINOUS GROUNDWATER SYSTEM: S AND O ISOTOPE EVIDENCES

摘要

Concentration of sulphate, δ~(34)S(SO_4~(2-)), δ~(18)O(SO_4~(2-)) and δ~(18)O(H_2O) values, in surface waters and groundwaters from crystalline rocks in mountainous Rudawy Janowickie Massif (RJ) in SW Poland, have been analysed. The RJ is located close to the area of the biggest anthropogenic emission source of sulphur compounds in Europe called the "Black Triangle". The lowest concentration of sulphate ions dissolved in water (13 mg/l) were found in groundwaters at the highest altitude (750m asl). In the course of groundwater downward infiltration (hydraulic gradient) the concentration of sulphate ions increased with decreasing altitude. The highest concentrations of SO_4~(2-) (100 mg/l in springs at metamorphic rocks and 60 mg/l in springs at granites) were observed at the foothills (altitudes 410 and 580 m asl, respectively). Likewise, the sulphur and oxygen isotope composition of sulphate ions showed systematic increasing trends with respect to the altitude decrease (from 3.45 to 11.05 per thousand and from -6.53 to 10.03 per thousand, respectively). Three main sources of sulphate for the surface-water/groundwater systems in RJ have been iso-topically discriminated. The high δ~(34)S-δ~(18)O sulphate showed waters from precipitation and peat bogs. The lower δ~(34)S-δ~(18)O sulphate originated due to decomposition of organic matter. The lowest δ~(34)S-δ~(18)O sulphates formed due to microbial (Thiobacillus ferrooxidans) pyrite oxidation within the pyrite-deposit weathering zone. The highest contribution of anthropogenic sulphate have been observed at the top-hills and during low water levels at lower altitudes. The contribution of biogenic and geogenic sulphate relatively increased with the increasing distance (time?) of the water flow through rocks or/and increasing water level in springs. The isotope, hydrochemical and hydrogeological observations confirmed a negligible role of anthropogenic sulphur in the groundwaters analysed. This suggests significant improvement in the natural environment in the Western parts of Sudety Mountains.