Plans to reduce emission of oil prescribe the development of testing techniques to judge effects on the marine environment. In the Netherlands, a technique with the potential to be applied in offshore operations, is based on thermal treatment and grinding of the cutting in order to recover the base oil. The resulting cuttings form a very fine hydrophobic powder with an oil content of ca. 7.5 g kg-1 dry weight. In order to assess the dose-effect relationships and no-effect concentration of these cuttings at the ecological community level, boxcosm experiments were set-up. This study is a joint effort of the Dutch Government and the Dutch E&P Industry (NOGEPA).

Clean marine sediments were inoculated in duplo with 5 concentrations of pretreated cuttings with a 3.2 factor stepwise dilution of concentration. The results were compared with two blancs: one without addition of cuttings and one with addition of cuttings which had been made oil-free by heating at 500 °C. The incubation period was 3 months. The minimal dose of the cuttings was calculated to amount to ca. 1.2 mg oil kg-1 dry weight, the maximal dose to ca. 120 mg oil kg-1 dry sediment in the top 10 cm sedimentlayer. The results, however, showed that the initial background concentration of oil was at least 10 times as high as expected from previous measurements with a mean calculated value of 110 ± 59 mg oil kg-1 dry sediment (n = 10).

A parameter indicative for impact on biological activity in the sediment is the depth of the oxygen present in the sediment and the overlying water. Oxygen concentrations at the sediment/water interface were always near saturation, but the thickness of the aerobic layer differed markedly. In the high-dose boxes the aerobic layer was significantly thinner, problably due to the increased load of organic matter and oil originating from the SCS- cuttings. Furthermore, the oxygen gradient, indicative for respiratory activity, was upto 2.9 times higher compared to the blancs without any SCS-cuttings added. Since oxygen depletion did not occur no harmful effects on the benthic community were to be expected. Besides examining the effects on the natural infauna, the test species Echinocardium cordatum. an oil-sensitive species, was added. The results show that mortality of E. cordatum hardly exceeded naturally induced mortality with increasing dose of the cuttings. At the highest dose added all animals survived. Preliminary results on natural infaunal mortality indicate the same. The impact of oil-containing particles on bottom structure, such as smothering, may have been much less in case of the SCS-cuttings which were rather homogeneously reworked through the sediment toplayer.

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