ENVIRONMENTAL-EFFECTS OF A DISCHARGE OF DRILL CUTTINGS CONTAMINATED WITH ESTER-BASED DRILLING MUDS IN THE NORTH-SEA

A field-monitoring program has been carried out to assess the environm ental effects associated with drill cutting discharges at a drilling l ocation in the North Sea, after drilling with ester-based muds. The st udy included a baseline survey just before drilling started and three postdrilling surveys, 1, 4, and 11 months after termination of drillin g. During these surveys, ester concentrations and macrofauna densities were determined at distances between 75 and 3,000 m from the well. Fo r the first and second postdrilling surveys, ester concentrations rang ed between 2 and 4,700 mg . kg(-1) dry sediment. During the third surv ey, esters were detected up to 200 m, with concentrations between 1 an d 250 mg . kg(-1) dry sediment. Between 500 and 3,000 m, ester concent rations were below detection level. The half-life was estimated as 133 d. This value reflects the composite effects of biodegradation and se diment relocation. During the second postdrilling survey, a reduced ab undance as well as a reduced species richness of the macrofauna was ob served up to 200 m from the well. Also, the presence of Capitella capi tata was an obvious sign of disturbed sediment conditions in this area . A few species showed abundance gradients over the whole sampling tra nsect and occurred in reduced abundance up to 500 or 1,000 m compared to the 3,000-m reference station. The third postdrilling survey clearl y revealed effects at 75 to 200 m from the well. At distances greater than or equal to 500 m, persistent effects could no longer be demonstr ated. Although the results of the present study show that effects had not completely disappeared after 1 year, there are signs of recovery o f the macrobenthos, because a reasonable number of species had recolon ized the area in the vicinity of the well site, particularly at 200 m, in fairly high numbers.