Oil contaminated sediments in the North Sea: environmental effects 20 years after discharges of OBM drill cuttings

Abstract

From Summary and Conclusions: “1. Platform L13-fe, erosion area: Slight contamination levels occurred up to at least 100 m, but to less than 250 m from the platform. Oil seems to have disappeared for the greater part after 18 years. 2. Platform L13-fe, erosion area: A clear effect on the benthic community could only be observed at the station nearest to the platform (40 m). Here the fauna was substantially impoverished. 3. Platform L13-fe, erosion area: The sea urchin Echinocardium cordatum seemed to be affected in its abundance up to 250 m from the platform. 4. Platform K12-a, transition zone: The sediment conditions seem to have considerably improved compared to the first 7 years after the discharges but slightly elevated oil concentrations occurred up to at least 100 m from the platform. Patchiness in the distribution of discharged material may have camouflaged the presence of local higher contamination levels. 5. Platform K12-a, transition zone: At 35 m from the platform the benthic fauna was clearly affected. The fauna composition at this station was different and substantially poorer than at stations further away from the platform. 6. Platform K12-a, transition zone: At 100 m the benthic community seemed to be only slightly affected. 7. Platform L4-a, sedimentation area: At stations 250 m and further away from the platform contamination seems to have decreased to values below background level. 8. Platform L4-a, sedimentation area: At 100 m oil concentrations in the superficial sediment layers (0 – 10 cm) had decreased to values just beyond background level. However in the deeper sediment layers no decrease was observed. 9. Platform L4-a, sedimentation area: At 100 m the fauna seemed to be not or hardly affected, in spite of the presence of oil in the sediment. 10. Platform L4-a, sedimentation area: At 45 m from the platform the sediment was severely contaminated. The oil concentration (32 g per kg dry sediment) was higher than ever measured near L4-a. The chromatogram of the sediment sample was different from those usually found near old OBM locations and indicated that the oil was not or hardly weathered. This has led to the idea that the oil could be rather ‘fresh’. However, no recent source of contamination is known. It is argued that most probably anaerobic conditions have inhibited bacterial degradation and weathering of the OBM base oil, that was discharged here 19 years ago. 11. Platform L4-a, sedimentation area: At 45 m from the platform the benthic fauna was severely impoverished. Most species that were common or even abundant further away from the platform were absent or occurred in very low densities. 12. Location G13-1, sedimentation area, abandoned well site: A measurable effect of beamtrawl fishing on the distribution of discharged material could not be demonstrated. As a consequence of the very patchy distribution of the discharged material, a much more extensive sampling program would be necessary to detect such an effect. 13. After 20 years, oil contaminated surface sediments (top 10 cm) generally occur up to somewhere between 100 m and 250 m from platforms. The concentrations measured are slightly beyond background level, but give an underestimation of the contamination present, since oil usually accumulates in the deeper sediment layers. 14. Measurable effects on the macrobenthic community are generally limited to an area within 100 m from platforms after 20 years. 15. At the species level effects may occur in an area within 100 m to maximally 250 m from well sites.