Wet and dry deposition of total mercury and methylmercury at an unpolluted site in Puerto Rico
James Shanley1, Martin Risch2, Robert Brunette3, Willam H. McDowell4 and Grizelle Gonzales5
Wet deposition of mercury (Hg) at an unpolluted site in windward northeastern Puerto Rico is comparable to the highest levels in the continental USA. Here we update the existing seven-year record after three years of measurements at a relocated station, provide estimates of dry Hg deposition based on throughfall and litterfall, and quantify methylmercury (MeHg) in wet and dry deposition. In 2006-2007 (the most complete record), annual wet Hg deposition averaged 27.9 µg m-2. High wet Hg deposition is attributed to scouring of global pool Hg from the upper free troposphere by rain near the tops of high convective clouds, as well as high rainfall amounts (2855 mm y-1). In 2014, after years of incomplete record due to logistical difficulties, the station relocated 11 km west to a NADP site in August 2014. The new station has less rainfall due to its lower elevation (360 vs. 480 m). The measurement period included a significant drought in 2015, and the devastating hurricanes of September 2017, which shut down the station for several weeks. Wet Hg deposition was 15.9 µg m-2 with 2034 mm rainfall in 2015, and 30.9 µg m-2 with 3347 mm rainfall in 2016. Weighted average Hg concentration decreased slightly from 9.8 ng L-1 at the original site to 8.7 ng L-1 at the new site. Preliminary wet and dry deposition measurements did not overlap well in time and space, but suggest that dry Hg deposition is a major fraction of the total. For calendar year 2014, litterfall Hg deposition (representing primarily dry deposition of Hg0) at the NADP site was 38.0 µg m-2. Throughfall Hg deposition (representing primarily dry deposition of Hg2+) for 9 months of overlapping measurements at the original site in 2006-2007 was 54% greater than wet Hg deposition. The percentage total Hg as MeHg was 0.06 % in rainfall, 0.56% in throughfall, and 0.31% in litterfall. Combined, these measurements suggest that dry Hg deposition is considerably greater than wet (similar to findings in temperate forests) and that relative input of (or conversion to) MeHg in the forest canopy is considerably less than published values.
1U.S. Geological Survey, jshanley@usgs.gov 2U.S. Geological Survey (retired), mrrisch@usgs.gov 3Eurofins - Frontier Global Sciences Inc, Bobb@frontiergs.com 4University of New Hampshire, Bill.McDowell@unh.edu 5USDA Forest Service, ggonzalez@fs.fed.us