Sources of stream water sulfate during the spring snowmelt in boreal streams: Evidence from delta S-34 isotope measurements

Episodic hydrological events, such as snowmelt during spring, have a marked effect on stream water chemistry. Here we investigated how spring snowmelt affected delta S-34 values of sulfate in six streams situated in northern Sweden. Four streams had high delta S-34(SO4) values during base flow with values ranging from +11.9 to +8.6%. During snowmelt the delta S-34(SO4) decreased to around +6%. In one of the streams and in the forested upper reaches of a second stream, delta S-34(SO4) values were close to +5% during base flow and decreased to about +3.8% during the spring snowmelt. One stream, which drained cultivated postglacial sediments dominated by acid sulfuric soils, was differentiated from the other streams by low delta S-34(SO4) values (-5.0% to -0.5%). We could identify two stream water SO4 sources: sedimentary sulfides and anthropogenic S. Bacterial dissimilatory sulfate reduction was identified as an important process affecting stream water delta S-34(SO4) values and suggests that in this boreal landscape, peatlands and possibly riparian zones have a large influence on the biogeochemistry of SO42- during base flow conditions. Our results suggest that during the spring snowmelt, snow S and desorbing SO4 of mainly anthropogenic origin are the two major S sources in four of the investigated streams. Two streams in forested areas also indicate that reoxidation of reduced S may be released during the spring flood. The stream in the cultivated area was found to be strongly influenced by the acid sulfuric soils independent of stream flow conditions.