Long-term effects of OBM cuttings discharges in the sandy erosion area of the Dutch Continental Shelf

Abstract

From the Summary and Conclusions: “1. Elevated oil concentrations (up to 300 mg-kg"1 dry sediment) were found up to 100 m (residual current direction) from the platform. At 250 m the chemical analyses did not show elevated concentrations, but traces of oil were visually observed during the fieldwork. Elevated Ba concentrations confirmed the presence of discharged material at this station. 2. At 100 m oil was found to a depth of at least 20 cm in the sediment. At 250 m Ba concentrations were highest in the upper 10 cm, but also at 15-20 cm the concentrations were substantially elevated, indicating that much of the discharged material is stored in the deeper sediment layers. 3. In the residual current direction no traces of discharged material were observed at distances >500 m. In perpendicular directions no traces were found atvdistances >250 m. 4. An accumulation of biological effects was found at 100 m and 250 m (res. curr. direction) from the platform. The fauna was substantially impoverished at these stations, even at relatively low oil concentrations (250 m). 5. Adult Echinocardium cordatum were absent up to 500 m from the platform. 6. At 25 m from the platform the fauna composition was different from all other stations, but not really impoverished. It is suggested that increased turbulence around the platform legs may have caused increased erosion of the seabed so that old banks of shells and shell fragments were uncovered, which were colonized by a macrofauna community with a different composition. 7. Based on the above findings it is concluded that environmental effects around an OBM location in the erosion area of the Dutch Continental Shelf were still detectable