Biogeochemistry of trimethyllead and lead in a forested ecosystem in Germany

Lead compounds, especially ionic organolead compounds (OLC), are highly toxic and mobile pollutants strongly affecting many ecosystems. Soil pools and fluxes with precipitation, litterfall and runoff of trimethyllead (TML), one of the dominant ionic OLC in the environment, and Pb-total were investigated in a forested ecosystem in NE-Bavaria, Germany. In addition, ad/desorption of TML to soils was studied in batch experiments and its degradation in soils was investigated using long term incubations. Total soil storage in the catchment was 11.56 mg Pb ha(-1) for TML and 222 kg Pb ha(-1) for Pb-total. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30% of the area. Most Pb-total (>90%) was found in die upland soils. In upland soils. TML was only detectable in the forest floor. The annual total deposition from the atmosphere. estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha(-1) year(-1) for TML and 52 g Pb ha(-1) year(-1) for Pb-total. The contribution of litterfall was 1.5 and 32%, respectively. The concentrations of TML and Pb-total in wet precipitation were: fog > throughfall > bulk precipitation. The annual fluxes with runoff from the catchment was 0.5 mg Pb ha(-1) year(-1) for TML and 2.8 g Pb ha(-1) year(-1) for Pb-total. TML degraded rapidly in the forest floor (Oa horizon) with a half-life (t(1/2)) of 33.5 days. The degradation of TML in Fen (t(1/2) = 421 days) and in the mineral soil (Bw-C horizon, t(1/2) = 612 days) was much slower. Emission of tetramethyllead front wetland soils was not observed during the I year incubation. The adsorption affinity of TML to different soils was Fen > Oa > A greater than or equal to Bw-C The ratio of total soil storages to the present annual input were 3.6 years for TML. TML and Pb-total are still deposited in remote areas even after the use of tetraalkyllead as additives has been terminated for years. The rates of deposition are, however, much lower than in the past. Forest soils act as a sink for deposited TML and Pb-total. TML is accumulated mostly in wetland soils and seems to be stable under anoxic conditions for along time. In upland soils, TML decomposes rapidly Only small amounts of TML are transferred from soils into runoff.

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