Assessment of recent mercury trends associated with the National Atmospheric Deposition Program Mercury Litterfall Network

Abstract

The National Atmospheric Deposition Program established the Mercury (Hg) Litterfall Network in 2007 to assist with estimating changes in dry deposition of Hg. These measurements represent primarily gaseous elemental mercury (Hg⁰) taken up by foliage actively during the growing season through stomata. Hg deposition is driven by litterfall mass; thus concentrations are a better indicator of trends. Previous work assessed trends from 2007 to 2014 from 27 locations in the eastern U.S. and found that litterfall total Hg concentrations declined. Here, data from the same area representing 2017 to 2021, 2013 to 2021, and 2007 to 2021 were compiled. For the first two time periods no significant trends in litter concentrations were observed; however, values measured at locations impacted by local/regional sources had higher concentrations and showed increasing trends, but these were not significant. Using all sites for which data were available from 2017 to 2021, total Hg concentration in litterfall for 2017 to 2018 was significantly greater than 2020 to 2021. Using all data from 2007 to 2021 Hg concentrations in litter have declined, as have precipitation concentrations. In general, from 2013 to 2021 Mid-Atlantic, East Coast, and Mid-Western concentration in foliage declined due to controls on sources; while the Great Lakes Region and Southeast did not change. Methylmercury was measured in litterfall at all locations. MeHg concentrations generally declined from 2007 to 2021, but have not changed since 2017. However, concentrations for 2021 were higher than for 2020 for most sites. Methylmercury in litterfall has been demonstrated to bioaccumulate in terrestrial ecosystems raising concerns for songbirds.