Gaseous mercury in background forest soil in the northeastern United States

Although the soil gaseous mercury (Hg) reservoir is an important component of the soil Hg emission process, little is known of the behavior of gaseous Hg in soil pores and the processes governing the ultimate evasion of Hg from soil surfaces. In this study, gaseous Hg in background forest soil in the northeastern United States was examined during 2003 and 2004. This represents the first detailed study of gaseous Hg concentration within the soil profile. Soil gaseous Hg varied seasonally with soil temperature, and highest concentrations consistently occurred in the O- and upper A-horizons. The study revealed clear gradients in gaseous Hg concentrations, which bore striking resemblance to gradients in bound Hg. In addition, measurements of soil Hg flux were correlated with soil gaseous Hg concentrations at 2-cm depth. Taken together, these results indicate that the upper soil layers, rich in bound Hg, act as a source of gaseous Hg that may ultimately be emitted to the atmosphere, although the ultimate point of release is likely closer to the litter layer. Large changes in soil gaseous Hg over shallow layers underscore the importance of fine vertical resolution of measurements, and therefore the utility of flask sampling.