Human Impact on the Mobilization of a Naturally-Occurring Source of Selenium in an Urbanizing Catchment

The central part of the San Diego Creek Watershed was historically occupied by an extensive swamp that was drained in the late 19^th century to accommodate the expansion of ranching and agriculture. Stable isotopes (δ³⁴S—SO₄, δ¹⁵N—NO₃, δ¹⁸O—NO₃, and δ^82/76Se) are used to examine the link between denitrification of fertilizer-derived nitrate and sulfide/selenide oxidation along three discrete groundwater flowpaths that begin at the inferred outer boundary of the historic marsh and terminate at or near a surface water boundary. Isotopes along one flowpath strongly suggest a link between denitrification and sulfide oxidation: both δ¹⁵N—NO₃ and δ¹⁸O—NO₃ increase from upgradient values of 6.8߮ and 6.8߮ to downgradient values of 19߮ and 14.3߮ respectively while δ³⁴S—SO₄ becomes more negative from an upgradient value of 0.1߮ to a downgradient value of –12.7߮. The other two flowpaths show little to no isotopic evidence of denitrification and δ³⁴S—SO₄ signatures do not change dramatically, suggesting that, in the vicinity of these flowpaths, oxidation may not be the primary mechanism mobilizing sulfate and selenium from more reduced forms. Selenium isotopes do not directly demonstrate a link between denitrification and oxidation of reduced forms of selenium, but rather preserve information about the redox gradient in the historic marsh. The data do not rule out the possibility of nitrate acting as an electron acceptor in the oxidation of reduced forms of selenium. This hypothesis is further supported by results from soil slurry experiments where nitrate-rich water was more effective than oxygenated artificial rainwater in mobilizing selenium in select soil samples.