Monitoring the long-term environmental impact of washed and unwashed OBM drill cuttings discharged on the Dutch continental shelf, 1990 : sediment analysis and bioaccumulation

This study deals with monitoring of long-term environmental impact of washed and unwashed oil based mud (OBM) drill cuttings discharged on the Dutch continental shelf in 1990. Two drilling locations were selected for this study, namely K-12-a (unwashed discharge) and L-5-5 (washed discharge). Oil concentrations in the sediment around the drilling locations K-12-a and L-5-5 seem to develop different patterns. Whereas at location L-5-5 a steady decrease of oil content was observed over a period of 2 years, no decrease was seen at location K-12-a over a period of 6 years. At both sites oil contents were enhanched up to 250 m in residual current direction. At K-12-a levels of oil exceeding background values were measured at 5000 m in residual current direction. Accumulation of weathered oil at K-12-a by a test organism, Echinocardium cordatum (sea urchin), may have caused an alteration in dosis-response ratio compared to previous years. This suggests that toxicity and/or biological availability decreases on the long-term.