Soil Mercury Sequestration and Re-Mobilization and Relationships to the Global Cycling of Mercury.

Soils serve as the key link between atmospheric deposition of mercury (Hg) and mobilization to streams, lakes, and oceans. Hg pollution impacts wildlife and humans through consumption of Hg-laden fish and atmospheric deposition is the main pathway for Hg input to remote ecosystems. Soils act as important intermediate reservoirs for atmospheric wet and dry deposition, and soil storage efficiently removes Hg from active cycling. However, the semi-volatile nature of elemental Hg also causes partial re-volatilization back to the atmosphere and introduces significant uncertainty to model predictions of this movement. Currently, such re-emissions of previously deposited Hg are estimated to account for up to 60% of current atmospheric Hg loads. These legacy emissions therefore can lead to continued cycling of past pollution and contribute to Hg exposure that is delayed in time and displaced geographically from primary emissions. Photochemical reduction processes are known to play a critical role in re-mobilizing soil Hg, but the underlying mechanisms and dynamics of gaseous Hg within the soil profile and the influence on re-emissions are not well understood.

This presentation will emphasize the highly dynamic cycling of Hg within soils, with an emphasis on sequestration of atmospheric Hg and surface-atmosphere exchange processes. We will provide a global review of soil Hg sequestration and soil-atmosphere exchange studies and summarize observed patterns and limitations in re-emission estimates. We will also show results that characterize the evolution of gas-phase Hg within soil profiles to provide better understanding of source and sink areas of Hg in soils.