Annual and seasonal trends in mercury deposition in Maryland

Abstract

Wet deposition collections for total mercury (Hg) and methylmercury (MMHg) have been made at four sites in Maryland. The Chesapeake Biological Laboratory (CBL) site has been in operation since May 1995; the other sites (the Science Center (SC) in urban Baltimore; Stillpond (STP) on the eastern shore of the Chesapeake Bay; and Frostburg (FRB) in western Maryland) have been operation 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 accounted for by variability in annual and seasonal rainfall. Fluxes were higher for the regional Maryland sites (CBL, STP and FRB) than those in the mid-west USA. The results for CBL do not show a measurable annual trend in Hg deposition with time over this period. Seasonal variation is apparent with higher fluxes in summer, and the lowest fluxes in winter at all sites. The seasonal signal apparent in the total Hg data is not reflected in the MMHg record. Wet depositional fluxes at the urban SC site were 2–3 times higher than the rural sites. This difference was comparable to differences in particulate 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 urban influence for Hg deposition.