Mercury and trace metal wet deposition across five stations in Alaska: controlling factors, spatial patterns, and source regions
Authors: Pearson Christopher , Howard Dean , Moore Christopher , Obrist Daniel 
Published: December 2018

Abstract. A total of 1,360 weeks of mercury (Hg) wet deposition data were collected by the State of Alaska Department of Environmental Conservation and the U.S. National Park Service, across five stations covering up to eight years. Here, we analyze concentration patterns, source regions, and seasonal and annual deposition loadings across these five sites in Alaska, along with auxiliary trace metals including Cr, Ni, As, and Pb. We found that Hg concentrations in precipitation at the two northern-most stations, Nome (64.5° N) along the coast of the Bering Sea and the inland site of Gates of the Arctic (66.9° N), were significantly higher (average of 5.3 ng L−1 and 5.5 ng L−1, respectively) than those at the two lowest-latitude sites, Kodiak Island (57.7° N, 2.7 ng L−1) and Glacier Bay (58.5° N, 2.6 ng L−1). These differences were largely explained by different precipitation regimes, with higher amounts of precipitation at the lower latitude stations leading to dilution effects. Highest annual Hg deposition loads were consistently observed at Kodiak Island (4.80 +/− 1.04 µg m−2), while lowest annual deposition was at Gates of the Arctic (2.11 +/− 0.67 µg m−2). Across all stations and collection years, annual precipitation overwhelmingly controlled annual Hg deposition, explaining 73 % of the variability in observed annual Hg deposition. Our analyses further showed that annual Hg deposition loads across all five Alaska sites were consistently among the lowest in the United States, ranking in the lowest 1 to 5 percent of over 99 monitoring stations. Detailed back trajectory analyses showed diffuse source regions for most Hg deposition sites, which were almost identical with precipitation origins, suggesting global or regional Hg sources. One notable exception was Nome where we found pronounced differences between precipitation and Hg source origins with increased Hg contributions from the western Pacific Ocean downwind of East Asia. Analysis of multiple trace elements from Dutch Harbor, Nome, and Kodiak Island showed generally higher trace metal concentrations at the northern station Nome compared to Kodiak Island further to the south, with concentrations at Dutch Harbor falling in-between. Across all sites, we find two distinct groups of correlating elements: Cr and Ni and As and Pb. We attribute these associations to possibly different source origins, whereby sources of Ni and Cr may be derived from crustal (e.g., dust) sources while As and Pb may include long-range transport of anthropogenic pollution. Neither Hg nor any of the other trace elements analyzed, consistently associated with these two groups of elements, suggesting largely diffuse source origins. Calculations of enrichment factors (i.e., elemental enrichment compared to the upper continental crust) show low enrichment for Cr and Ni which is in support of a predominantly crustal source. High enrichment factors for Pb and Se are indicative of anthropogenic or additional natural sources for these elements. For most other elements including Hg, enrichment factors fell in-between these groups showing no clear source attribution to either crustal or anthropogenic source origins.