Topographical Influences on the Spatial Distribution of Soil Mercury at the Catchment Scale

Abstract

Inorganic forms of mercury (Hg) can be converted by natural processes into methylmercury, a highly potent neurotoxin that can bioaccumulate in food chains and pose a risk to human health. Although Hg can enter aquatic environments through direct deposition, the predominant source derives from complex terrestrial cycling in nearby ecosystem vegetation and soils. Here we assess the spatial distribution of soil and litterfall Hg within two paired catchments of the Shenandoah National Park: the northwest-facing North Fork Dry Run (NFDR) and the southeast-facing Hannah Run (HR) catchments. Litterfall Hg concentrations were not significantly different between the NFDR and HR catchments. This may be attributable to the speciation of Hg (gaseous elemental Hg) that is involved in leaf-level accumulation. Significant differences in soil organic-layer Hg concentrations were observed between the two study catchments, with NFDR soils having roughly 50 % higher Hg concentrations than those from HR. These differences can be explained by differences in soil N content (and to a lesser extent soil C content) between catchments, as both elements exert a strong control of the amount of Hg bound in soils. We found no evidence that topographic aspect contributes to the spatial variability of soil Hg concentrations in these paired catchments, but did detect an influence from elevation. Soils located near ridges in mountainous catchments can contain relatively high Hg concentrations due to (1) lower turnover rates in soil organic matter pools, (2) enhanced deposition, and (3) limited mobilization of Hg from those areas.