Acute toxic effects of produced water in relation to chemical composition and dispersion

The total discharge of produced water in the Norwegian sector of the North Sea is expected to increase to 90 × 106 m3 within the year 2000, with nearly all of this originating from oil production. Produced water from three oil fields in this sector showed large differences in chemical composition and toxicity towards four test organisms (Skeletonema costatum, Mytilus edulis, Abra alba, Crassostrea gigas). Values of EC50 for these organisms ranged from 0.2 to ca 30% of produced water in the test medium. Biodegradation of the produced water changed the chemical composition and generally reduced the toxicity. Model data for dispersion combined with toxicity estimates indicates that acute toxicity should be expected only in the immediate vicinity of the outlets, while at a distance (i.e. > 2 km) toxic effects are considered negligible.