Hydrogeochemistry of sulfur isotopes in the Kalix River catchment, northern Sweden

The 34S-to- 32S ratio in dissolved SO 4 has been studied in the Kalix River, Northern Sweden, and its catchment. Weekly sampling over 17 months revealed temporal variations from +5.3‰ up to +7.4‰ in the δ 34S values in the river. Snow and rain samples showed lower δ 34S values (average +5.6‰ and +5.0‰, respectively). The atmosphere is the major source for S in surface waters in the catchment, and the heavier δ 34S values in the river are a result of SO 4 reduction within the catchment. Most of the temporal variations in the δ 34S value in the river are caused by a mixing of water from the mountain areas (relatively light δ 34S) and the woodland. The δ 34S value is relatively heavy in the woodland tributaries because of bacterial SO 4 reduction in peatland areas influenced by groundwater. The highest δ 34S values were measured during the spring flood, in June and in November. These heavy δ 34S values are related to different types of water with diverse origins. The heavy δ 34S values coinciding with the early spring flood originate from peatland areas in the woodland. Relatively heavy δ 34S values (up to +14.4‰) were registered in mire water. Smaller variations of the δ 34S value during summer and early autumn most likely were caused by the input of ground-mire water during heavy rains. A correlation between increased TOC concentrations and increased δ 34S values was observed. The heavy δ 34S values in June and November probably originate from SO 4 reduction in bottom water and sediments in lakes within the catchment. Bottom water, enriched in 34S SO 4, was transported in the river during the spring and autumn overturn.