A Dutch programme was carried out to study the distribution of discharged washed and unwashed oil-containing cuttings in the sediment and waterphase and their effects on selected test species in the Dutch part of the North Sea.

The distribution of OBM cuttings in sediment around drilling locatons was related to the total amount of oil discharged in the watercolumn. Increased oil contents were found over a range of 1000 to 5000 m in residual current direction. Total oil contents remained high at locations where unwashed OBM cuttings were discharged. At one location oil was found in high concnetrations upto 6 years after discharge of OBM cuttings, especially due to the persistence of the oil fractions "Other peaks" and "UCM". Washing of OBM cuttings resulted in lower amounts of oil discharged, lower concentrations in the sediment in the vicinity of the platform (upto 100 m), equal concentrations at 250 m but higher concentrations upto at more than 500 m off the platform when compared with unwashed cuttings and the respective amount of oil discharged. However, within one year, degradation of oil seems to have taken place at one of the washed sites at 5000 m.

During discharge in the watercolumn, oil was taken up by Mytilus edulis from washed cuttings as well as from unwashed cuttings. Leaching of oil from the sediment into the waterphase was measurable in the field for only a limited period of time, less than a year after discharge.

Several sediment-inhabiting species were tested on their response to oil contamination. Four species tested in experimental boxcosm set-ups, Fchinncardium cordatum, Amohiura filiformis,Laois koreni and Nucula turgida responded to elevated contaminaton levels by increased mortality. Boxcosm experiments supported the conclusion derived from the results of field studies on the macrofauna distribution around drilling sites, i.e. the NOEC-value should be estimated in the order of 1-10 mg oil kg-1 dry sediment [12]. For boxcosms taken at unwashed and washed sites one year after discharge this was confirmed. Preliminary results of a boxcosm experiment with sediment where discharge of unwashed OBM cuttings had taken place 6 years before, the NOEC-value seemed to have increased. Experimental boxcosms enable assessment of possible adverse effects of experimentally treated drilled cuttings or other toxic substances on the (macro)benthic infauna. Adverse effects were also seen at one of the washed sites using a short-term bioassay test with the sediment-inhabiting amphipod Bathyporeia sarsi.

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