Mercury in Litterfall at Selected National Atmospheric Deposition Program Mercury Deposition Network Sites in the Eastern United States, 2007-2009

Martin R. Risch*, John F. DeWild1, David P. Krabbenhoft1, Randall K. Kolka2, and Leiming Zhang3

Forest canopies can accumulate more mercury from the atmosphere than other landscapes because of their relatively large leaf areas. In autumn in the eastern United States, the mercury mass in the annual litterfall of deciduous or predominantly deciduous forests represents a large portion of the atmospheric mercury dry deposition that was retained in the forest landscape that year. The Mercury Deposition Network (MDN) of the National Atmospheric Deposition Program (NADP) provides a framework for litterfall mercury monitoring because it has long-term sites, a broad geographic coverage, capacity for supplementary sample collection, and weekly mercury wet-deposition data.

A study of mercury in litterfall at selected MDN sites, which were thought to represent different forest-cover types and geographic regions in the eastern US, was completed during autumn months in 2007, 2008, and 2009. Trace-metal-free methods for collecting, processing, and analyzing litterfall samples for mercury were developed for the study. Data obtained in the study include total mercury concentrations in litterfall samples and litterfall sample-catch amounts from four passive collectors randomly deployed for approximately 2 months each autumn in study plots located near as many as 23 MDN sites in 15 states.

The annual total mercury concentrations in litterfall samples ranged from 21.4 to 67.8 ng/g (nanograms per gram) and had a median of 43.7 ng/g. The median percent relative standard deviation of the total mercury concentrations among the four collectors at a site was 7 percent. The median relative percent difference between the total mercury concentrations in 40 pairs of duplicates was 3.8 percent. The annual litterfall sample-catch amounts ranged from 42.1 to 499 g and had a median of 229 g. Mean mercury concentration in the samples and sample-catch amount were used to estimate annual litterfall-mercury deposition at each MDN site. Litterfall-mercury deposition ranged from 1.6 to 23.4 g/m2/yr (micrograms per square meter per year) and had a median of 10.9 g/m2/yr. On average, litterfall accounted for 50 percent of the annual sum of mercury wet deposition plus litterfall-mercury deposition at the MDN sites in the study. The mean ratio of annual litterfall-mercury deposition to mercury wet deposition was 1.2 to 1.

Litterfall-mercury concentrations, sample catch, and litterfall-mercury deposition differed significantly among MDN sites and were highest in areas with deciduous oak-hickory forest-cover types. Annual litterfall-mercury deposition was highest at three sites near urban areas in Indiana and Maryland and two sites in the Ohio River Valley in Indiana and Ohio. The results of this study provide a reference for potential future litterfall-mercury monitoring at NADP-MDN sites.

1*U.S. Geological Survey, 5957 Lakeside Boulevard, Indianapolis, Indiana 46278, USA, 317-290-3333 ext. 163
1U.S. Geological Survey, 8505 Research Way, Middleton, Wisconsin, 53562, 608-821-3846
2U.S. Forest Service, 1831 Highway 169 East, Grand Rapids, Minnesota, 55744, 218-326-7115
3Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, 416-739-5734