Long-term effects of OBM cutting discharges at 12 locations on the Dutch continental shelf

A sampling programme was carried out in 1994 and 1995 covering 12 well sites where OBM cuttings had been discharged 7 to 13 years ago. The locations were selected in areas with different sediment types. In terms of amounts of base oil discharged the discharge loads ranged from 13 to 298 tonnes. The locations included actual production platforms but also abandoned well sites. At each location compendious surveys were carried out, during which 4 stations were sampled at distances between 100 and 2000m from the discharge site. Sediment samples for chemical analysis were collected at all stations and numbers of the OBM-sensitive species Echinocardium cordatum were counted in all samples. Additional macrofauna analyses were performed for 5 locations. The chemical analyses revealed elevated oil concentrations at 5 of the 12 locations, but traces of oil were visually observed in a few samples at another 2 locations. Maximum concentrations, measured over the upper 10 cm layer, were generally in the order of 10-50 mg/kg dry sediment, but at one location a concentration of 200 mg/kg was found, which is comparable to the values found at formerly investigated locations L4-a and P6-b, but considerably lower than at K12-a. This indicates that concentration levels at most locations are generally not as high as found at the locations formerly investigated. Except for one location where oil was found up to 250m from the platform, elevated oil concentrations in surface sediments only occurred at 100m stations. Significant effects on the population densities of E. cordatum were found at 7 of the 12 locations. Further, at 2 locations the abundance pattern of the species was indicative of an effect but there was no statistical significance. Population densities of E. cordaturn appeared to be depressed in the vicinity of two locations where no oil was detected. This indicates that the absence of oil in samples does not necessarily mean that sediment conditions have completely recovered. It can not be excluded that oil is still present in the deeper sediment layers that were not investigated. On the other hand, it is not clear whether remnants of the former discharges are still the actual cause of depressed population densities. Even when sediment conditions have completely recovered, it may take a long time before populations of long living species have recovered. Nevertheless, the consistent response of E. cordaturn shows its particular value as a sensitive indicator of environmental effects caused by the former discharges. At 3 of the 5 locations for which macrofauna analyses were carried out, effects were also found on other components of the benthic community. The occurrence of effects generally seemed to be limited to the 100m station. Although a more extensive sampling programme could have revealed some more subtile effects also at a larger distance from a few locations, this suggests that the zone where persistent biological effects may be expected generally extends within a radius of circa 200 m from OBM discharge sites. 16 refs