[Risk for the Rostock water supply from the Warnov river by an old deposit at Schwann?]

Accidents of the city gasworks in the end of the fifties at Schwaan, a small town south of Rostock, resulted in an emission of tar and waste water into the river Warnow, from which the city of Rostock is supplied with drinking water about 20 km downstream. The tar contaminated river sediment (old deposit) covering an area of about 200 m2 has a high pollutant inventory: Steam-distillable phenolic compounds 407 mg/kg in the fresh sediment, polycyclic aromatic hydrocarbons (PAH) 400 to 1100 micrograms/l in the aqueous eluate. The mutagenic potential was detected by the AMES-test. As shown by PAH determinations there were no other pollutant sources in the catchment area, in the tributaries or on other places of the river. The mean PAH concentrations of the river Warnow and his tributaries during 1991 to 1994 were found to be in the range lower than 10 ng/l. Further investigations showed a continuous emission of PAH from the tar laugh in small amounts. An additional emission of pollutants (PAH) and their transportation in the river is expected in the case of sediment disturbances. This emission and their possible consequences are difficult to estimate, even though the seston and the humic substances content of the river seems to reduce the PAH risk. Concluding from our results the probability is very low that pollutants were transported downstream to Rostock and pass the water works but may not totally be excluded. Nevertheless from the viewpoint of care a removal of the old deposit is necessary.