Annual and seasonal trends in mercury deposition in Maryland

Wet deposition collections for total mercury (Hg) and methylmercury (MMHg) have been made at four sites in Maryland. The Chesapeake Biological Laborat ory (CBL) site has been in operation since May 1995; the other sites (the S cience Center (SC) in urban Baltimore; Stillpond (STP) on the eastern shore of the Chesapeake Bay; and Frostburg (FRB) in western Maryland have been o peration since 1996 or 1997. Annual depositional fluxes for Hg at CBL over the period were variable but a large fraction of the variability could be a ccounted for by variability in annual and seasonal rainfall. Fluxes were hi gher for the regional Maryland sites (CBL, STP and FRB) than those in the m id-west USA. The results for CBL do not show a measurable annual trend in H g deposition with time over this period. Seasonal variation is apparent wit h higher fluxes in summer, and the lowest fluxes in winter at all sites. Th e seasonal signal apparent in the total Hg data is not reflected in the MMH g record. Wet depositional fluxes at the urban SC site were 2-3 times highe r than the rural sites. This difference was comparable to differences in pa rticulate and reactive gaseous Hg between SC and CBL. The rural sites, two on the Chesapeake Bay shoreline (CBL and STP) and one in western Maryland ( FRB), have similar annual fluxes suggesting that these sites represent the average regional Hg depositional signal. The SC site clearly shows the urba n influence for Hg deposition. (C) 2000 Elsevier Science Ltd. All rights re served.