Identifying sources of stream water sulfate after a summer drought in the Sleepers River watershed (Vermont, USA) using hydrological, chemical, and isotopic techniques

In many forested headwater catchments, peak SO42- concentrations in stream water occur in the late summer or fall following drought potentially resulting in episodic stream acidification. The sources of highly elevated stream water SO42- concentrations were investigated in a first order stream at the Sleepers River watershed (Vermont, USA) after the particularly dry summer of 2001 using a combination of hydrological, chemical and isotopic approaches. Throughout the summer of 2001 SO42- concentrations in stream water doubled from ∼130 to 270 μeq/L while flows decreased. Simultaneously increasing Na+ and Ca2+ concentrations and δ34S values increasing from +7‰ towards those of bedrock S (∼+10.5‰) indicated that chemical weathering involving hydrolysis of silicates and oxidation of sulfide minerals in schists and phyllites was the cause for the initial increase in SO42- concentrations. During re-wetting of the watershed in late September and early October of 2001, increasing stream flows were accompanied by decreasing Na+ and Ca2+ concentrations, but SO42- concentrations continued to increase up to 568 μeq/L, indicating that a major source of SO42- in addition to bedrock weathering contributed to peak SO42- concentrations. The further increase in SO42- concentrations coincided with an abrupt decrease of δ34S values in stream water SO42- from maximum values near +10‰ to minimum values near −3‰. Soil investigations revealed that some C-horizons in the Spodsols of the watershed contained secondary sulfide minerals with δ34S values near −22‰. The shift to negative δ34S values of stream water SO42- indicates that secondary sulfides in C-horizons were oxidized to SO42- during the particularly dry summer of 2001. The newly formed SO42- was transported to the streams during re-wetting of the watershed contributing ∼60% of the SO42- during peak concentrations in the stream water. Thereafter, the contribution of SO42- from oxidation of secondary sulfides in C-horizons decreased rapidly and pedogenic SO42- reemerged as a dominant SO42- source in concert with decreasing SO42- concentrations in spring of 2002. The study provides evidence that a quantitative assessment of the sources of stream water SO42- in forested watersheds is possible by combining hydrological, chemical and isotopic techniques, provided that the isotopic compositions of all potential SO42- sources are distinctly different.