Monitoring the long-term environmental impact of washed OBM drilling cuttings discharged on the Dutch continental shelf, 1991 : sediment analysis

This study deals with monitoring the long-term environmental impact of washed oil based mud (OBM) drilling cuttings discharged on the Dutch continental shelf in 1991. Sediment samples were taken along a transect around the drilling location L-5-5 and analysed for their oil and barium concentrations. This location was previously monitored in 1989 and 1990. The oil contamination of the sediment had improved slightly in 1990 compared to 1989. In 1991, no further improvement was observed. Concentrations of oil exceeded background levels at 250 m in the residual current direction, whereas barium was spread out further, up to 500 m in the residual current direction. The oil:barium ratio, indicative of oil degradation, showed only a significant decrease in the first year after discharge. Comparison with long-term studies at other drilling locations where unwashed OBM drilling cuttings were discharged, it can be concluded that both short-term and long-term exposure to oil is reduced at washed locations.