Organotin compounds in precipitation, fog and soils of a forested ecosystem in Germany

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Abstract

Organotin compounds (OTC) are highly toxic pollutants and have been mostly investigated so far in aquatic systems and sediments. The concentrations and fluxes of different organotin compounds, including methyl-, butyl-, and octyltin species in precipitation and fog were investigated in a forested catchment in NE Bavaria, Germany. Contents, along with the vertical distribution and storages in two upland and two wetland soils were determined. During the 1-year monitoring, the OTC concentrations in bulk deposition, throughfall and fog ranged from 1 ng Sn l−1 to several ten ng Sn l−1, but never over 200 ng Sn l−1. The OTC concentrations in fog were generally higher than in throughfall and bulk deposition. Mono-substituted species were the dominant Sn species in precipitation (up to 190 ng Sn l−1) equaling a flux of up to 70 mg Sn ha−1 a−1. In upland soils, OTC contents peaked in the forest floor (up to 30 ng Sn g−1) and decreased sharply with the depth. In wetland soils, OTC had slightly higher contents in the upper horizons. The dominance of mono-substituted species in precipitation is well reflected in the contents and storages of OTC in both upland and wetland soils. The ratios of OTC soil storages to the annual throughfall flux ranged from 20 to 600 years. These high ratios are probably due to high stability and low mobility of OTC in soils. No evidence was found for methylation of tin in the wetland soils. In comparison with sediments, concentrations and contents of organotin in forest soils are considerably lower, and the dominant species are less toxic. It is concluded that forested soils may act as sinks for OTC deposited from the atmosphere.

Introduction

Organotin compounds (OTC) are widely used as fungicides, as stabilizing additives in polymers like polyvinyl chloride, and as antifouling agent in ship paintings (Hoch, 2001). Leaching from soils and landfills (Mersiowsky et al., 2001), weathering of plastics (Quevauviller et al., 1991), and dissolution of ship paintings (Batley, 1996) lead to the release of these compounds into aquatic systems. Sediments of rivers, estuaries and marine systems have been identified as the major environmental sinks of OTC. As a consequence, there is abundant literature on input, distribution and toxicity of OTC mainly focused on the aquatic environment (Hermosin et al., 1993, Weidenhaupt et al., 1997, Arnold et al., 1998, De Mora & Pelletier, 1997).

Nevertheless, current studies indicated that OTC can be re-emitted from the marine environment to the atmosphere by transformation of ionic OTC into volatile species (Amouroux et al., 2000), and methylated OTC have been identified in the natural atmosphere of river estuaries (Tessier et al., 2002). Furthermore, terrestrial point sources like landfills also emit volatile OTC (Feldmann and Hirner, 1995). Thus, emission and atmospheric transport can lead to a re-distribution and deposition of OTC to terrestrial systems. Residues of OTC in soils might affect the population of microorganisms and thus indirectly the biological processes in soils (Kuthbutheen et al., 1989a, Kuthbutheen et al., 1989b). The current knowledge about atmospheric inputs of OTC into terrestrial systems, pool sizes, transport or transformation processes in terrestrial systems is very limited. Runoff from terrestrial ecosystems is one of the major sources of pollutants to freshwater systems, and thus processes in soils may constitute an important but not yet investigated link in the biogeochemical cycle of these compounds.

In case of other heavy metals like Hg, wetlands and other semiterrestrial soils have been identified as compartments of special importance for transformation reactions (Roulet et al., 2001, Weber et al., 1998). Reduction or alkylation resulting in the formation of methylmercury or volatile compounds like Hg0 and dimethylmercury influence strongly the mobility and export of organo-Hg compounds from terrestrial catchments (Wallschläger et al., 1995, Lindberg et al., 2001). Similar processes may occur in these soils for OTC, but are poorly investigated yet.

The objectives of this study were to (i) investigate the input of OTC through precipitation and fog deposition and (ii) determine their contents in upland and wetland soils in a forested catchment in Central Europe, and (iii) to estimate the storage and the relevance of transport and transformation process in the soils.

Section snippets

Site description

The investigation was carried out in the ”Lehstenbach” catchment (4.2 km2 size) in the German Fichtelgebirge mountains, located at an elevation of 700 to 880 m a.s.l. at 50 ° 08′ N, 11° 52′ E (Fig. 1). Mean annual air temperature is 5 °C, and mean annual precipitation is approximately 1150 mm. The catchment is dominated by Norway spruce (Picea abies [L.] Karst.) stands of different age, and 30% of the area are covered with wetland soils of bog and fen type. Upland soils are mainly Dystric

OTC in precipitation

Most of all OTC were found in the precipitation and fog, besides the absence of octyltin species in fog (Table 3). All species had the same pattern: highest concentration were observed in fog and lowest concentrations were usually observed in bulk precipitation. Butyltin species were the dominant OTC in all samples, especially MBT. DMT had the highest concentrations among methyltin species in all samples.

During the 1-year monitoring, the concentrations of total OTC never exceeded 60 ng Sn l−1

Discussion

The concentrations of most OTC in precipitation and fog are rather low in comparison with those measured in estuarine and marine ecosystems (Hoch, 2001). Generally speaking, mono-substituted species are the dominant OTC in precipitation, especially MBT. The dominance of MBT in the precipitation is surprising. Most butyltin species should decompose during atmospheric transport, because they usually are unstable in the presence of sunlight (Maguire et al., 1983, Seligman et al., 1986).

Higher

Acknowledgements

We like to thank the members of the central analytical department of BITÖK for analytical support, especially Gunter Ilgen and Frank Hertel. Christiane Lau and Hui-Ying Chen helped with the sample preparation. We appreciate Thomas Wrzesinsky for providing fog samples and data of monthly fluxes. Financial support came from the German Academic Exchange Service (DAAD) and the German Ministry for Education and Research (BMBF), PT BEO-51-0339476D.

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    Present address: IWW Rhenish-Westfalian Institute for Water, 45476 Muelheim an der Ruhr, Germany.

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