Methylmercury degradation and exposure pathways in streams and wetlands impacted by historical mining

Monomethyl mercury (MMHg) and total mercury (THg) concentrations and Hg stable isotope ratios (delta Hg-202 and Delta Hg-199) were measured in sediment and aquatic organisms from Cache Creek (California Coast Range) and Yolo Bypass (Sacramento Valley). Cache Creek sediment had a large range in THg (87 to 3870 ng/g) and delta Hg-202 (-1.69 to -0.20 parts per thousand) reflecting the heterogeneity of Hg mining sources in sediment. The delta Hg-202 of Yolo Bypass wetland sediment suggests a mixture of high and low THg sediment sources. Relationships between %MMHg (the percent ratio of MMHg to THg) and Hg isotope values (delta Hg-202 and Delta Hg-199) in fish and macroinvertebrates were used to identify and estimate the isotopic composition of MMHg. Deviation from linear relationships was found between %MMHg and Hg isotope values, which is indicative of the bioaccumulation of isotopically distinct pools of MMHg. The isotopic composition of pre-photodegraded MMHg (i.e., subtracting fractionation from photochemical reactions) was estimated and contrasting relationships were observed between the estimated delta Hg-202 of pre-photodegraded MMHg and sediment IHg. Cache Creek had mass dependent fractionation (MDF; delta Hg-202) of at least-0.4 parts per thousand whereas Yolo Bypass had MDF of +0.2 to +0.5 parts per thousand. This result supports the hypothesis that Hg isotope fractionation between IHg and MMHg observed in rivers (-MDF) is unique compared to +MDF observed in non-flowing water environments such as wetlands, lakes, and the coastal ocean. (C) 2016 Elsevier B.V. All rights reserved.