Mercury Accumulation in the Forest Floor of the United States

C.H. Perry1*, M.C. Amacher1, P.L.K. Zimmerman1, W. Cannon2, R.K. Kolka1 and L. Woodruff2

Atmospherically-deposited Hg has a strong affinity for soil organic matter. Fluxes of Hg in soil water of upland watersheds are generally small, but forest fires may release stored Hg to the ecosystem. The contribution of Hg from forest fires relative to other anthropogenic sources is an important unknown. The Forest Service, US Department of Agriculture, Forest Inventory and Analysis (FIA) program collects soil samples from forested areas across the United States as part of its sampling program, and annual soils inventories are underway or completed in 45 of the 50 states (Alaska, Hawaii, Mississippi, New Mexico, and Oklahoma have yet to be sampled). Our objective is to inventory the spatial distribution of forest floor Hg for a transect running across the United States, from Arizona in the southwest to Maine in the northeast. The collection of forest floor samples was accomplished as part of the standard FIA Phase 3 Soil Quality Indicator program. Field protocols include the measurement of the thickness of the forest floor and the collection of the entire forest floor found within a 30-cm diameter sampling frame. We removed approximately 0.1 g of the sample for plots in our region of interest, and these were sent to two different laboratories for Hg analysis by cold-vapor atomic absorption. The two laboratories calibrated their instruments against common Hg standards. We found good agreement between samples analyzed at both laboratories. Observations of mercury concentrations were joined with the Forest Inventory and Analysis Database to assign basic location information and associated inventory data. There is a strong gradient of mercury storage across the United States. Once the data are spatially detrended, foresttype group remain significant predictors of Hg storage; conifer species tend to store more mercury than hardwood species.

1*US Forest Service, Phone: 651.649.5191, Email:
2US Geological Survey