Toxicity of sediments from around a North Sea oil platform: are metals or hydrocarbons responsible for ecological impacts?

Discharges of contaminated drill cuttings have caused appreciable ecologica l change of the benthos adjacent to many oil and gas platforms in the North Sea. Many platforms have large piles of cuttings lying beneath them and th ese probably present the greatest potential hazard to the environment durin g platform decommissioning and removal. There is, however, a lack of consen sus on which aspects of drill cuttings are responsible for the adverse ecol ogical effects. This hinders risk assessment of management options. Here we report data on the toxicity of sediments from around the North West Hutton platform to the amphipod Corophium volutator, the polychaete Arenicola mar ina and the Microtox (R), acute test system. Sediment was acutely toxic to Corophium out as far as 600 m from the platform. Sediment from 100 in from the platform remained acutely toxic to Corophium when 3% contaminated sedim ent was mixed with clean sediment. A 10% dilution of this sediment also inh ibited Arenicola feeding almost completely. Sediment elutriates did not inh ibit Microtox light output, but organics extracted by dichloromethane were very toxic. Fifteen minute EC50 values were as low as 0.25 mg ml(-1) and we re strongly correlated with hydrocarbon concentrations. Metal concentration s in whole sediments were correlated with their toxicity to Corophium but t he relationship was much weaker when data on dilutions were included. Excep t at sites immediately adjacent to the platform, metal concentrations were well below ERL values from the literature, so were too low to explain sedim ent toxicity. Toxicity of sediments to Corophium was closely correlated wit h their hydrocarbon content, even when tests on dilutions were included in the analysis. We conclude that hydrocarbons are the most significant cause of toxicity in these sediments contaminated with oil based drill cuttings a nd that polar organics, sulphide, ammonia and other water soluble substance s are of much lower significance. Applying OSPAR guidelines to our data on the toxicity of cuttings pile material to Corophium data would give a maxim um allowable concentration of 0.03% in clean sediments. The Microtox data i ndicate that sediments from deeper in the pile would require an even greate r dilution than this. (C) 2001 Elsevier Science Ltd. All rights reserved.