Hazards for the Rostock water supply from a tar-contaminated sediment in the river Warnow

By the gas plant at Schwaan (a small city near Rostock), tar and other by-products were released into the river Warnow which supplies Rostock with drinking water 20 km downstream. The tar-contaminated river sediment contained phenols and polycyclic aromatic hydrocarbons (PAH), indicating a mutagenic potential. Investigations in the catchment area and on tributaries showed PAH contents up to about 10 ng/l in the water. The river sediment was found to be the sole PAH source. Although the PAH level is very low, sediment disturbances may produce an increase in concentrations. The consequences of such a worst case are difficult to estimate, even though the content of seston and humic substances in the river seems to reduce the PAH concentrations. Although this lowers the probability that increased PAH concentrations reach the Rostock water supply, the removal of the tar-contaminated sediment from the river is strongly recommended.