SULFUR ISOTOPE RATIOS IN SULFATE AND OXYGEN ISOTOPES IN WATER FROM A SMALL WATERSHED IN CENTRAL SWEDEN

During 1988-89 water samples for sulphur and oxygen isotope measurements were collected in the Lake Mjosjon watershed (7.3 km2), central Sweden. Samples included: precipitation, throughfall, lakewater, shallow groundwater and inlet and outlet streams. The delta-S-34 of sulphate in precipitation ranged from + 6.41 parts per thousand. in winter to + 3.88 parts per thousand in summer, the higher winter values attributed to seasonal differences in the kinetic and equilibrium isotope fractionation during oxidation of atmospheric sulphur dioxide to sulphate. The delta-S-34 in rain samples and in pine and spruce throughfall were similar, indicating no gain of sulphur from the trees. In the inflowing stream, the delta-S-34 value increased as discharge decreased, from + 5.57 parts per thousand in spring to + 26.21 parts per thousand in summer, indicating bacterial sulphate reduction. The fluctuations in the inlet water were damped by the lake and in the outlet water, only a small decrease in the delta-S-34 value during spring discharge was observed. During winter 1988-89, the near surface waters in the lake showed the same delta-S-34 as snow indicating that meltwater governs the isotopic composition. During the winter, the delta-S-34 in the near bottom waters increased while oxygen decreased due to bacterial sulphate reduction in the sediments. This also caused an increase in the alkalinity in the near bottom waters. Based on the delta-O-18 data the water within the watershed is derived largely from meteoric water. During spring discharge, meltwater governs the inflow and outflow stream while additional groundwater influences occurred during the drier period. Most sulphur is derived from atmospheric deposition and the delta-S-34 in sulphate increased during passage through the watershed due to bacterial sulphate reduction.