UC Santa Cruz

Agricultural sulfur loading to vineyard soils has been reported to drive in situ mercury (Hg) methylation via microbes (with the hgcAB gene cluster) using sulfate in their metabolic process as an electron acceptor whereby they interconvert Hg to toxic methylmercury (MeHg). The Napa Valley, famous for world-renowned wines, is an ideal landscape to study the impact of sulfur loads potentially increasing MeHg concentrations in soils and fluvial waters. Fluvial system MeHg production is largely understudied. The goal is to characterize Hg abundance in vineyard irrigation and effluent waters, as well as the Napa River, that discharge into the San Pablo Bay critical wildlife habitat where bioaccumulation and biomagnification occur. Surface waters were collected and analyzed for: total mercury (THg), MeHg, dissolved organic carbon (DOC), sulfate (SO₄²⁻), and total suspended solids (TSS) during the rainy season of 2022/2023. Hg species were found in low concentrations - THg (0.53 - 20.09 ng/L, x̄ = 4.90 ng/L), MeHg (0.01 - 0.29 ng/L, x̄ = 0.10 ng/L) despite high [SO₄ ²⁻] (7.58 - 192.08 mg/L, x̄ = 40.80) clearly indicating the agricultural signal. Positive THg and DOC correlations were noted in vineyard and background sites (R² = 0.53 and R² = 0.96, respectively) in the dissolved fraction. Agrarian SO₄ ²⁻ loading was not associated with elevated [MeHg] in fluvial waters. MeHg production in fluvial systems is complex with many geochemical and environmental controls governing MeHg fate and requires further investigation to resolve unknowns.