Pools and fluxes of mercury and methylmercury in two forested catchments in Germany

Mercury (Hg) and methylmercury (CH3Hg+) are global pollutants, but little i nformation is available on rates of atmospheric input, distribution and mob ility in soils and catchments of central Europe. The objectives of this stu dy were to investigate input and output fluxes of these compounds in a deci duous and a coniferous catchment in NE Bavaria (Germany), and to estimate p ools and mobility of total Hg (Hg-tot) and CH3Hg+ at the catchment scale. B ulk precipitation, throughfall, litterfall and runoff were collected biweek ly from April 1998 to April 1999. Several soil profiles were sampled to est imate pools of Hg compounds in the catchments. In both catchments highest c ontents of Hg-tot were found in the Oa layer of the forest floor (up to 500 ng g(-1)) acid the soil storage of Hg-tot calculated for a soil depth of 6 0 cm was approximately 890 g ha(-1) in the coniferous and 190 g ha(-1) in t he deciduous catchment. Highest contents of CH3Hg+ in upland soils were obs erved in the Oi layer of the forest floor, and soil storage of CH3Hg+ was 4 .35 g ha(-1) in the coniferous and 0.59 g ha(-1) in the deciduous catchment . The annual total deposition of Hg-tot (total deposition not measured dire ctly but calculated from throughfall and litterfall) was 541 mg ha(-1) year In the coniferous and 618 mg ha(-1) year(-1) in the deciduous catchment. T otal deposition rates of CH3Hg+ were 3.5 and 2.6 mg ha(-1) year(-1). The co ntribution of litterfall to the total deposition of Hg-tot and CH3Hg+ was 5 5% in the deciduous catchment. In the coniferous catchment, the contributio n of litterfall to total deposition was only 29% for Hg-tot, but 55% for CH 3Hg+. By far the largest proportion of the deposited CH3Hg+ and Hg-tot rema ined in the catchments (85% in the coniferous, 95% in the deciduous). As co mpared to remote Swedish catchments, deposition and output via runoff of Hg -tot were higher, but deposition and output of CH3Hg+ were lower in our cat chments. In contrast to other studies, the annual budget revealed no differ ences in the mobility between the two species at the catchment scale. Howev er, temporal patterns of the runoff fluxes and converse gradients of CH3Hg and Hg-tot contents in the forest floor indicated differences in mobility on shorter time scales. (C) 2000 Elsevier Science B.V. All rights reserved.