Atmospheric deposition to the Chesapeake Bay watershed—regional and local sources

Atmospheric deposition is an important source of mercury to the Chesapeake Bay and its watershed. Estimates from our collections at four sites around the Chesapeake Bay and in western Maryland show that the regional wet flux is between 10 and 20 μg m−2 yr−1 with higher concentrations being found close to local sources. The Hart-Miller Island site, in close proximity to Baltimore, had a wet flux of 25 μg m−2 yr−1 Amount of rainfall is also important. At Frostburg, in western Maryland, the flux estimate of 23 μg m−2 yr−1 is largely a result of the higher than average rainfall during the study period. Our measurements of total throughfall deposition in western Maryland and our measurements of particulate mercury at the Chesapeake Biological Laboratory indicate that dry deposition is not a dominant source of mercury at either site. Direct deposition to the Chesapeake Bay contributes more than half the mercury entering the Bay and our preliminary estimates for watershed retention suggest that most of the mercury deposited to the watershed (90% or greater) is retained in the terrestrial domain. For methylmercury, atmospheric deposition is not the most important flux to the Bay. Additionally, our results suggest that methylmercury is being generated within the watershed and transported to the Bay in river runoff. Where comparisons can be made, our results confirm that deposition to the Chesapeake Bay is elevated compared to that of other regions of the U.S.A. except perhaps Florida.