Mercury in forest canopy throughfall water and its relation to atmospheric deposition

Abstract

Deposition rates of Hg to forested land areas have been studied for 13 mo in two small forested catchments in south-western Sweden. Monthly sampling of forest canopy throughfall water in a mature Norwegian spruce stand and precipitation to an open field was carried out. Hg deposited to the catchments via litter fall was also considered. Integrated monthly samples of both precipitation and throughfall water were collected by 6 and 45 separate collectors, respectively, designed for an unbiased Hg sampling. The average Hg concentration in precipitation during this period was 13.0 ± 4.0 ng Hg L-1. The results show that annual dry deposition of Hg to a spruce stand in throughfall water can be 50% of the wet deposition, 4 to 7 compared to 12 g Hg km−2 yr−1, respectively. A strong seasonal trend in Hg deposition via throughfall water is also present, with increased levels during the growing season and with a monthly maximum in August. Further, the dry deposition measured in throughfall water seems to be negligible during part of the winter.