Abstract. The 96-h toxieity of four organophosphates (thiometon, disulfoton, malathion, and demeton-S-methyl, the oxygen analogue of thiometon) in the freshwater bivalve molluse Dreissena polymorpha was tested using different nominal concentrations ranging between 6 and 50 mglL. No mortalities were observed in musseis exposed to malathion and demeton-S-methyl (26 mglL and 6 mglL, respectively), and at the lowest eoncentrations of thiometon and disulfoton (6 and 10 mglL, respectively). At higher thiometon and disulfoton concentrations, mortalities occurred. At the highest concentrations of 50 mg thiometonIL and 30 mg disulfotonIL, mussei mortalities of 88 and 93%, respectively, were determined. Organophosphate concentrations of up to a factor 10 times higher than in the ambient water were found in exposed musseis, irrespective of whether they were alive or dead. The search for organophosphate metabolites via GCIMS analysis of mussei tissue extraets was negative, suggesting lacking or low oxidative activation of the insecticides used. The mollusc is highly resistant to toxie effects of organophosphate insectieides and their biologieal aetive oxygen analogues.On November 1, 1986, a pestieide storing facility at Sandoz in Basel (Switzerland) caught fire. In order to extinguish the fire, water was pumped at high volurnes from the nearby river Rhine. This water found its way back into the river Rhine due to overflowing sewage canals (Capel et al. 1988; Güttinger and Stumm 1992). Along with the fire-extinguishing water, pesticides stored in the affeeted facility were swept into the river. Among these pesticides were organophosphate insectieides, mainly thiometon and disulfoton, mereurial-based fungieides, and herbicides. Consequently, the river Rhine ecosystem
*Present address: Department of Environmental and OccupationalHealth, University of Pittsburgh, Pittsburgh, PA 15238, USACorrespondence to: C. Dauberschmidt faced high water eoncentrations of these pesticides. Measured peak eoncentrations elose to the source of contamination were 500 J.1glL and 600 IJ.-glL for thiometon and disulfoton, respectively. These pestieide coneentrations severely affected insect, erustaeean, and fish populations of the Rhine ecosystem. In particular, eels (Anguilla anguilla L.) appeared to be highly susceptible to organophosphate intoxication, as fish of this speeies were found dead up to 400 km downstream of the souree of contamination (Güuinger and Stumm 1992).Based on the ehemieal eharaeteristies of these compounds, it was assumed they would undergo quick hydrolysis and biodegradation, and would not remain in the Rhine sediments (EAWAG 1987). However, up to 3 1/2 months following the incident, eopious concentrations of unhydrolyzed parent compound of the major contarninants, thiometon and disulfoton, were found in the Rhine river sediment near the site of original eontamination (430 ppm and 3,000 ppm, respectively, Institute of Toxicology, unpublished data).In addition to the initial fish deaths, unusually high numbers of dead diving...
