Where Brimstone Meets Quicksilver: Interactions between Agricultural Sulfur Applications and Methylmercury Production along a Vineyard to Wetland Gradient

Mercury (Hg), specifically as methyl Hg (MeHg), is a potent neurotoxin with significant consequences for humans and wildlife. This study examines the interactions between sulfur used as a fungicide in California, U.S., vineyards and Hg methylation. We collected surface soils and soil porewater across a vineyard to wetland gradient situated at the terminus of the Napa Valley winegrowing region for analyses of sulfate, total Hg, and MeHg. We found that sulfate runoff from vineyards is transported into adjacent wetlands via drainage ditches and a grassland transition zone. In addition, concentrations of MeHg and percent of Hg present as MeHg in the wetland and drainage ditch were approximately three to eight times higher than values measured in vineyard and grassland transition soils, while potential net Hg methylation rates (<i>k</i>_meth) over a 48 h time period were approximately ten times higher in the wetland compared to the vineyard. Stimulation of Hg methylation in the wetlands and drainage ditch is likely driven by wet, reduced soil conditions. These results show that while Hg methylation is possible in vineyards and adjacent land covers, downstream drainage ditches and wetlands-which intercept sulfate added in vineyards-may be the most notable zones for MeHg production.