Mercury and Methylmercury Concentrations and Litterfall Mass in Autumn Litterfall Samples Collected at Selected National Atmospheric Deposition Program Sites in 2016

Mercury (Hg) is a persistent environmental contaminant and can accumulate and concentrate in food webs as methylmercury (MeHg), presenting a health risk to humans and wildlife. Multiyear monitoring and modeling studies have shown that atmospheric Hg in litterfall is an important form of Hg deposition to forests. Annual litterfall consists primarily of leaves with some amounts of needles, twigs, bark, flowers, seeds, fruits, and nuts. Atmospheric Hg accumulates in leaves and reaches an annual maximum concentration at autumn leaf drop. This data set is derived from ambient autumn litterfall samples collected at 23 selected National Atmospheric Deposition Program (NADP) Litterfall Mercury Monitoring Initiative (LMMI) sites located near NADP Mercury Deposition Network sites in deciduous and mixed deciduous-coniferous forests in 11 states in the eastern United States during 2016. The NADP administered litterfall collection at the LMMI sites. The U.S. Geological Survey (USGS) distributed sets of passive litterfall sample collectors to LMMI site operators for systematic retrieval of samples during the 4 to 24 weeks of autumn leaf drop at each site. Samples were processed and analyzed at the USGS Mercury Research Laboratory where concentrations of Hg and MeHg and litterfall dry mass and sample moisture were determined. Hg concentrations were measured in 4 samples collected from each site and MeHg concentrations were measured in one composite sample per site. Litterfall mass and sample moisture were determined for 8 samples per site. Duplicate sets of these sample data were determined for 3 sites in 2016.