Survey of nine organotin compounds 203 1

MBT DBT TBT MPT DPT TPT

Fig. 5 . Schematic representation of average OT patterns in zebra mussels at the locations used to build the models: Model (A) built from three locations that have only TPT sources nearby. Model (B) is a less strict model of eight locations with TPT sources and possi- bly some additional TBT inputs. Model (C) was built from eight lo- cations near to TBT sources.

ing" model (C) included only location 33 besides the locations used to build the model.

CONCLUSIONS

Zebra mussels collected from different parts of The Neth- erlands all showed considerable body concentrations of TBT, TPT, and their degradation products. Concentrations of TBT ranged from 0.006 to 11.5 pg/g and TPT from 0.028 to 3.2 pg/g. The widespread presence of TPT is somewhat sur- prising and supports the theory that TPT is rather easily transported by air.

The study of variations in TPT concentrations with time in zebra mussels close to a potato field revealed a n un- expected late increase in body concentrations. This may in- dicate that dead potato foliage and/or dust ending up in ditches is a more important cause of TPT introduction in the water than is regular spraying. The decrease of TPT body

burdens after the spraying season is rather slow. This together with the accumulation potential makes the zebra mussel an excellent biomonitor for studying long-term variations in TPT levels in the environment. The values found for tri- OT/di-OT ratios, and the metabolic effects identified by principal component analysis, also suggest that TBT is more easily depurated by zebra mussels than is TPT.

Both TCT and FBTO are much less used as pesticides than are the other OTs. Consequently, they were detected in a few samples only and d o not seem to pose real environmental threats.

Finally, using the zebra mussel as a biomonitor to study variations in environmental OT concentrations has provided a comprehensive insight into the quality of fresh water in The Netherlands. Results can be interpreted in terms of local use and persistence of the compounds studied. Therefore, we ad- vocate the future use of zebra mussels for monitoring OT res- idues in the freshwater environment and for evaluating the effects of legislation.

Acknowledgement-The Institute for Inland Water Management and Waste Water Treatment (RIZA, Lelystad, The Netherlands) is ac- knowledged for its support at various stages of this research. Zebra mussels were kindly provided by RIZA, the provinces of Friesland, Groningen, Noord-Holland. and Utrecht, the Zuiveringsschappen Limburg, Rivierenland (Tiel), Hollandse eilanden en waarden (Dordrecht), Amstel en Gooiland (Hilversum), Oost Gelderland (Doetinchem), and Drenthe (Assen). Other participants included the Hoogheemraadschappen Rijnland (Leiden), Uitwaterende sluizen (Edam), Delfland (Delft), West Brabant (Breda), and the consultancy agencies Grabowski en Poort (The Hague), Vallenduuk (Boxtel), AquaSense (Amsterdam), and Witteveen en Bos (Deventer). Finally, the joint technical service Oost Brabant (Boxtel) and the water stor- age corporation Brabantse Biesbosch (Werkendam) are acknowl- edged for their kind participation. Also, Pim Leonards, Joris van Kesteren, and Bert van Hattum are gratefully acknowledged for their advice and assistance during this project. This work is part of a doc- toral thesis and is supported by a grant from the University Stimu- lation Fund of the Free University (USF grant 89-27).

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