Isotopic Composition of Inorganic Mercury and Methylmercury Downstream of a Historical Gold Mining Region

We measured total mercury (THg) and monomethyl mercury (MMHg) concentrations and mercury (Hg) isotopic compositions in sediment and aquatic organisms from the Yuba River (California, USA) to identify Hg sources and biogeochemical transformations downstream of a historical gold mining region. Sediment THg concentrations and delta Hg-202 decreased from the upper Yuba Fan to the lower Yuba Fan and the Feather River. These results are consistent with the release of Hg during gold mining followed by downstream mixing and dilution. The Hg isotopic composition of Yuba Fan sediment (delta Hg-202 = -0.38 +/- 0.17 parts per thousand and Delta Hg-199 = 0.04 +/- 0.03 parts per thousand; mean +/- 1 SD, n = 7) provides a fingerprint of inorganic Hg (IHg) that could be methylated locally or after transport downstream. The isotopic composition of MMHg in the Yuba River food web was estimated using biota with a range of %MMHg (the percent of THg present as MMHg) and compared to IHg in sediment, algae, and the food web. The estimated delta Hg-202 of MMHg prior to photodegradation (-1.29 to -1.07 parts per thousand) was lower than that of IHg and we suggest this is due to mass-dependent fractionation (MDF) of up to -0.9 parts per thousand between IHg and MMHg. This result is in contrast to net positive MDF (+0.4 to +0.8 parts per thousand) previously observed in lakes, estuaries, coastal oceans, and forests. We hypothesize that this unique relationship could be due to differences in the extent or pathway of biotic MMHg degradation in stream environments.