Mercury and methylmercury transport through an urban watershed

Samples for mercury (Hg) and methylmercury (MMHg) were collected during both base flow and storm flow over the period of a year in the two branches of the Anacostia River, an urban, impacted river within greater Washington, D.C. The concentrations in each branch were correlated for comparable samples suggesting that similar processes are contributing Hg and MMHg to each branch. Concentrations of total Hg during base flow were less than 10 ng/l but were 3–5 times higher during storm flow, mainly as a result of the high particulate loading (up to 800 mg/l). Storm flows are therefore the major vector for Hg transport in this river. Total Hg concentrations generally increased as SPM and POC increased. However, the Kd decreased with increasing SPM for both Hg and MMHg, and the magnitude of the Kd was a function of the %POC, suggesting that the strength of particulate binding was a function of the organic content of the particles. Reactive Hg was a small fraction of the total and there was little relationship between dissolved Hg and DOC. Equilibrium calculations suggest that all the dissolved Hg is bound to DOC even at the lowest DOC concentrations encountered. Watershed yield calculations suggest that the flux from these rivers is a relatively large fraction of the atmospheric input, in contrast to what has been found in other systems.