Assessment of Dry Deposition and Foliar Leaching of Mercury and Selected Trace Elements Based on Washed Foliar and Surrogate Surfaces

Abstract

Throughfall and litterfall are the primary deposition pathways of Hg to forested ecosystems, yet the source of this Hg (soil or atmospheric) remains unclear. Leaf samples were harvested from live trees and used in leaf washing experiments to assess whether the recovered Hg originated from surface deposits (wash-off of dry deposition) or internal sources (foliar leaching). The behavior of Hg was compared to eight other elements also found in leaf wash with known external (Cd, La, Ce, Pb) or internal (Mn, Rb, Sr, Ba) sources. These experiments indicated that wash-off of dry deposition occurred rapidly, while foliar leaching occurred continuously during the entire wash period (up to 12 h). To isolate dry deposition, surrogate Teflon surfaces exposed for a 98 h dry period were washed and compared to washed foliar surfaces. Computed dry deposition velocities for both surfaces were within a factor of 2 for Hg, Cd, La, Ce, and Pb, suggesting that dry deposition was the primary source of these elements from the washed foliage. Deposition velocities varied widely (3−18 times higher from the foliar surfaces) for Mn, Rb, Sr, and Ba, most likely due to the leaching behavior exhibited by these elements. These experiments show that dry deposition of atmospheric Hg is likely the largest source of Hg in throughfall.