Experimental studies of the effects of drilling discharges

Abstract

The long-term effects of diesel and four different low-toxicity oil-based drilling-mud cuttings on the chemistry and benthic fauna of a marine sediment were compared. Eighteen tanks (mesocosms) containing beach sediment were deployed in a control and five treatment, three replicate experimental set-up. The drill cuttings used had total oil concentrations ranging from 6 to 15% and were added to the tanks in quantities calculated to be representative of hydrocarbon levels measured in sediments 400-500 m from platforms in the North Sea. Selected parameters monitored at varied intervals throughout the experiment were: redox profiles, sulphide concentrations, hydrocarbon concentration and meiofaunal abundance. Numbers of macrofaunal organisms evacuating the sediments in the seven days after treatment application were also recorded. Redox measurements showed all sediments other than controls to be significantly reduced, but no differences were observed between treatments. Sediments became most reduced after 3 months and showed recovery thereafter, approaching control values after 15 months. Sulphide levels peaked at about 3 months and declined thereafter, but were still elevated above control levels after 15 months. Highest concentrations were recorded in treatments with the highest oil content. Mesobenthic meiofaunal abundance was significantly reduced in all cuttings treatments, but effects from individual treatments were indistinguishable. Epi/ endobenthic copepod abundance increased markedly in all low-toxicity treatments, and particularly in those with lower oil content, but remained similar to control levels in diesel treatments throughout the experiment. In the seven days after treatments, sediment evacuation rates by Tellina were highest in diesel and tended to reflect oil concentrations in low-toxicity treatments. The results show that in equal oil concentrations diesel and low-toxicity oil-based drilling-mud cuttings have indistinguishable effects on sediment chemistry, but that, even after 15 months of weathering, diesel-based cuttings are demonstrably more toxic to benthic fauna.