Isotopic evidence for mercury photoreduction and retention on particles in surface waters of Central California, USA

Cache Creek (Coast Range, California) and the Yuba River (Sierra Nevada Foothills, California) are two river systems affected by extensive mercury (Hg) contamination due to legacy sources of Hg related to mining. Stable Hg isotope techniques have proven useful for elucidating the complex cycling of Hg within aquatic ecosystems, and we applied these techniques to improve understanding of Hg and methylmercury (MeHg) transformations in these watersheds. Total mercury (THg) concentrations and Hg stable isotope ratios were measured in filtered surface waters and suspended particulate matter collected from 14 sites within the Cache Cr. and Yuba R. watersheds. Filtered surface waters from both watersheds exhibited values of Delta Hg-199 (0.37 parts per thousand to 0.71 parts per thousand), consistently elevated above those observed in sediments (Delta Hg-199 average = 0.07%). Associated suspended particulates from these surface water samples displayed a much greater range of values for Delta Hg-199 (-0.61 parts per thousand to 0.70 parts per thousand), although suspended particulates from the Yuba R. exhibited mostly negative Delta Hg-199 values (-0.61 parts per thousand to 0.10 parts per thousand). The relationship between Delta Hg-199 and Delta Hg-201 in the filtered surface waters and associated suspended particulates was calculated using a bivariate York regression, yielding a slope of 1.57 +/- 0.49 (+/- 2SE) for the Yuba R. and 1.40 +/- 0.27 (+/- 2SE) for Cache Cr., both within error of the previously reported experimentally-derived slopes for MeHg- and inorganic Hg(II)-photoreduction. This provides isotopic evidence that Hg photoreduction is occurring within these surface waters to a significant degree, and suspended particulate phases are retaining the reduced product of Hg photoreduction, particularly within the Yuba R. The isotopic compositions of filtered surface waters are consistent with the isotopic signatures recorded in biota at low trophic positions within these watersheds, suggesting that the reservoir of Hg incorporated within the biota of these systems is similar to the filter-passing Hg fraction in surface waters. (C) 2019 Elsevier B.V. All rights reserved.