Pilot Study of Mercury in Litterfall at National Atmospheric Deposition Program Mercury Deposition Network Sites

Martin R. Risch
U.S. Geological Survery,
Department of Natural Resources and Environmental Science,
Indianapolis, Indiana USA

Atmospheric mercury can be transported to aquatic or terrestrial ecosystems through wet and dry deposition. Mercury wet deposition has been monitored for more than 10 years at National Atmospheric Deposition Program Mercury Deposition Network (MDN) sites in North Anerica. Mercury dry deposition generally is not measured directly and a national monitoring program to estimate dry deposition is in development. Data on mercury wet and dry deposition are needed to evaluate regional- and continental-scale deposition models and to assess changes in deposition resulting from reductions in mercury emissions required by regulation.

Forests can enhance mercury dry deposition by scavenging mercury from the air. Rates of mercury dry deposition to forests are known to be several times the rates of mercury wet deposition to open areas at the same location. Information about mercury in litterfall is a measure of mercury dry deposition to forests. Litterfall is forest canopy material, mostly leaves and needles, that drop from the trees to the forest floor. Mercury in litterfall consists primarily of mercury attached to the surface of leaves and needles and mercury incorporated into the foliage tissue. Because previous studies of mercury in litterfall have been for small watersheds, litterfall data from MDN sites could provide a larger scale for comparing wet and dry deposition to forest landscapes in North America.

In fall 2007, the U.S. Geological Survey (USGS) began investigating mercury in litterfall at MDN sites through a pilot study in which litterfall samples were collected at 12 MDN sites in 6 states. The purpose of the pilot study was to test methods that could be implemented at a greater number of MDN sites in the future. In the pilot study, the USGS provided sampling kits and instructions to MDN site operators who placed sampling boxes at randomly selected points in a forest study plot near each of the 12 MDN sites. Litterfall was allowed to accumulate in the sampling boxes for one month; two sets of samples were collected at each site. The litterfall samples were shipped to the USGS where they were frozen, dried, weighed, ground, homogenized, and analyzed for total mercury and methylmercury. Mercury dry deposition in litterfall was computed as the product of sample concentration and sample dry weight per sampling-box area.

This presentation provides results from the pilot study, compares litterfall mercury dry deposition with mercury wet deposition, and examines factors contributing to differences in litterfall dry deposition among the MDN sites in the pilot study.