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

Abstract

In many forested headwater catchments, peak SO_4^2- 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 SO_4^2- 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 SO_4^2- 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 SO_4^2- 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 SO_4^2- concentrations continued to increase up to 568 µeq/L, indicating that a major source of SO_4^2- in addition to bedrock weathering contributed to peak SO_4^2- concentrations. The further increase in SO_4^2- concentrations coincided with an abrupt decrease of δ³⁴S values in stream water SO_4^2- 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 SO_4^2- indicates that secondary sulfides in C-horizons were oxidized to SO_4^2- during the particularly dry summer of 2001. The newly formed SO_4^2- was transported to the streams during re-wetting of the watershed contributing ~60% of the SO_4^2- during peak concentrations in the stream water. Thereafter, the contribution of SO_4^2- from oxidation of secondary sulfides in C-horizons decreased rapidly and pedogenic SO_4^2- reemerged as a dominant SO_4^2- source in concert with decreasing SO_4^2- concentrations in spring of 2002. The study provides evidence that a quantitative assessment of the sources of stream water SO_4^2- in forested watersheds is possible by combining hydrological, chemical and isotopic techniques, provided that the isotopic compositions of all potential SO_4^2- sources are distinctly different.

Citation

Mayer, B.; Shanley, J.B.; Bailey, S.W.; Mitchell, M.J. 2010. Identifying sources of stream water sulfate after a summer drought in the Sleepers River watershed (Vermont, USA) using hydrological, chemical, and isotopic techniques. Applied Geochemistry. 25(5): 747-754.