- Biogeochemistry of Organotin and Organolead compounds in a Forested Catchment in NE-Bavaria, Germany (2004)
- Organotin-compunds (OTC) and Trimethyllead (TML) have a higher toxicity than their corresponding inorganic forms and may affect the functioning of ecosystems. Little is known about their behaviour and fate in the terrestrial environment. The goal of this thesis was to investigate the biogeochemistry of OTC (methyltin, butyltin and octyltin compounds) and TML in a forested catchment, especially their input and output budget. The occurrence of OTC, TML, Sntotal and Pbtotal in the atmosphere, soils, precipitation, and runoff in a forested ecosystem in NE-Bavaria, Germany were investigatedand the inputs and outputs in the solute phase determined. In addition, their ad-desorption and transformation (degradation) in forest soils was studied using batch experiments and long term incubations, respectively. OTC and TML concentrations in the gas phase during April to June 2003 was on average 110 pg Sn m–3 and 0,34 pg Sn m–3. Tri-, di-substituted and octyl species were the dominant OTC in the gas phase. In aerosols, only butyltin compounds, dimethyltin and monomethyltin (<500 pg Sn m–3) were found and mono-substituted OTC predominated. For OTC, the washout factors were in the order: mono- >= di- > tri-substituted OTC, and the gas / particle partition coefficients were mono- >> di- >> tri-substituted OTC. Aerosol particles serve as a sink for OTC in the atmosphere, especially for monomethyltin and monobutyltin. From August 2001 to August 2002, the concentrations of tin and lead compounds in precipitation were in the order: fog > throughfall > bulk precipitation. Average concentrations of OTCtotal ranged from 57 ng Sn l–1 in fog to 5.8 ng Sn l–1 in bulk precipitation. The concentrations of Sntotal were between 490 ng Sn l–1 in fog and 140 ng Sn l–1 in bulk precipitation, on average. Average concentrations of TML were 1 ng Pb l–1 in fog and 0.1 ng Pb l–1 in bulk precipitation and 9.6 and 0.76 micro-g Pb l–1 in case of Pbtotal. The annual total deposition from the atmosphere, estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha-1 yr-1 for TML and 52 g Pb ha-1 yr-1 for Pbtotal. The contribution of litterfall was 1.5% and 32%, respectively. The annual total deposition of OTCtotal was 172 mg Sn ha–1 yr–1, with 26% represented by litterfall. The total deposition of Sntotal was 4.9 g Sn ha–1 yr–1, of which 4.1% was litterfall. The annual flux with runoff from the catchment was 0.5 mg Pb ha-1 yr-1 for TML and 2.8 g Pb ha-1 yr-1 for Pbtotal. The annual runoff of OTCtotal and Sntotal amounted to 25 mg Sn ha–1 yr–1 and 2.4 g Sn ha–1 yr–1, respectively. The mass balance showed high retention of tin and lead compounds in the catchment. The total soil storage of OTCtotal and Sntotal in the catchment were 6.7 g Sn ha–1 and 1.9 kg Sn ha–1. Total soil storage in the catchment was 11.6 mg Pb ha-1 for TML and 222 kg Pb ha-1 for Pbtotal. The dominance of mono-substituted compounds in precipitation is well reflected in the concentrations and storages of OTC in both upland and wetland soils. 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 Pbtotal (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. OTC degraded slowly in soils with half-lives (t½) estimated from 0.5 to 15 years. The degradation rates in soils were generally in the order mono- >= di- > tri-substituted OTC. Decomposition rates of OTC in the forest floor were higher than in wetland and mineral soils. TML degraded rapidly in the forest floor (Oa) with a t½ of 0.09 years. The degradation of TML in Fen (t½ = 1.2 years) and in the mineral soil (Bw-C, t½ = 1.7 years) was much slower. Emission of tetramethyltin and tetramethyllead from wetland soils was not observed, suggesting little Sn and Pb methylation in the wetland soils. The adsorption and desorption isotherms for all species and soils were linear over the concentration range of 10–100 ng Sn, Pb ml–1. The strength of OTC adsorption correlated well with the carbon content and cation exchange capacity of the soil and was in the order mono- > di- > tri-substituted OTC and butyltin > methyltin compounds. The adsorption and desorption showed a pronounced hysteresis. The ratio of total soil storages in the catchment to the present annual input was 3.6 years for TML, reflecting the rapid degradation of TML in forest soils. The ratios of OTC soil storages to their present annual deposition ranged from 4.3 to 400 years. These high ratios reflect probably the high stability, low mobility of OTC in soils and the variation of the annual total deposition.