Regional and Local Controls on Dissolved Mercury and Dissolved Organic Carbon in Streams

In streams, dissolved mercury (HgD) is often strongly associated with dissolved organic carbon (DOC), but due to spatial and temporal variability the ratio of HgD:DOC can vary between watersheds and even within an individual watershed. This thesis examines soil organic carbon (SOC) and stream pH as regional and local controls, respectively on HgD:DOC. A meta-analysis of published studies and geospatial datasets was conducted to test the regional importance of SOC, while stream pH was evaluated during a field-based study in a small, first-order stream. At the regional scale, HgD:DOC from 19 watersheds across the U.S. was related to mean watershed SOC derived from a soil geospatial database. DOC quality measured by specific ultraviolet absorbance at 254nm and Hg wet deposition from the Mercury Deposition Network were also considered as possible controls. SOC was a strong primary control on HgD:DOC, while DOC quality and Hg wet deposition were secondary in comparison. Results from this study show that SOC data may be utilized to predict stream HgD:DOC ratios on a more geographically widespread basis. For streams with DOC data, HgD could also be predicted without expensive, time consuming field work. At the local scale, pH was lowered in a small, first-order stream to analyze the impacts on HgD and DOC. Two stream acidification experiments revealed that at acidic pH levels HgD and DOC concentrations are reduced in the water column. High-quality (i.e. aromatic) DOC was preferentially adsorbed, which caused the HgD:DOC ratio to decrease with decreasing pH. The likely mechanism is adsorption to stream sediments and suspended particulates which is enhanced by hyporheic mixing and exchange. Results from this study suggest that changes in stream and soil solution pH may influence the export of HgD and DOC. This work shows that regional and local watershed characteristics can help explain variance in the ratio of HgD:DOC. Identifying and understanding controls on the association of HgD and DOC in streams can help better predict HgD export to streams under changing environmental conditions. Future regulations on Hg emissions need to consider watershed processes that influence Hg and DOC concentrations to accurately predict water quality impacts.