Association of Dissolved Mercury with Dissolved Organic Carbon in Rivers and Streams: The Role of Watershed Soil Organic Carbon

Abstract:

Surface waters are an important pathway for the transport of atmospherically deposited mercury (Hg) from terrestrial watersheds. Dissolved Hg (Hg_D) is thought to be more bioavailable than particulate Hg and has been found to be strongly correlated with dissolved organic carbon (DOC) in numerous watersheds. The ratio of Hg_D to DOC is highly variable from site to site, which we hypothesize is strongly dependent on local environmental factors such as atmospheric deposition and soil organic carbon (SOC). Sixteen watersheds throughout the United States were used in this study to determine the relationship between the ratio of Hg_D:DOC, Hg wet deposition, and SOC. The Soil Survey Geographic database (SSURGO) and Northern Circumpolar Soil Carbon Database (NCSCD) were used to determine SOC values while Hg_D:DOC values were obtained from previous studies. Hg wet deposition was reported by the Mercury Deposition Network. There was no correlation found between atmospheric mercury wet deposition and Hg_D:DOC (r² = 0.04; p = 0.44) but SOC was able to explain about 71% of the variation in the Hg_D:DOC ratio (r² = 0.71; p < 0.01). A mathematical framework was developed to explain the power-law relationship between SOC and Hg_D:DOC based on soil carbon pools. The framework infers that the amount of Hg adsorbed to SOC does not increase in proportion to SOC at high SOC levels and points towards a Hg supply limitation for adsorption to soils with relatively deep carbon pools. Overall, this study identifies SOC as a first-order control on the association of Hg_D and DOC and indicates that globally available SOC datasets can be utilized to predict Hg transport in stream systems.