Studies on the environmental effects of production water and drill cuttings from North Sea offshore oil installations
Some of the environmental effects of production water and drill cuttings from North Sea, offshore oil installations were examined. Particular attention was focussed on changes induced within marine bacterial populations and the importance of deleterious or beneficial effects resulting from these changes. The effects of 'Forties' production water on planktonic bacteria were assessed using large, enclosed experimental ecosystems, as part of a large multidisciplinary investigation of the general effects of the effluent on marine organisms from a number of trophic levels. Production water concentrations were selected to simulate conditions 500-1000 m from a large oil production platform. Planktonic bacteria responded within 3 days of effluent addition with 10-fold increases in bacterial biomass, which resulted from an increase in either total bacterial numbers or average bacterial cell size. These changes were induced by high concentrations in the effluent, of labile, non-petrogenic organics (primarily organic acids) and inorganic nutrients (nitrogen and phosphate). High rates of hydrocarbon degradation were also detected. Biodegradation was probably an important mechanism for removal of these organic pollutants from the water column. An onshore, marine sediment tank system was used to compare the fate and effects of water, diesel oil and alternative ('low-tox') oil based drill cuttings on marine sediment during a 12 month exposure period. The cuttings were dispersed across sediment surfaces to produce continuous layers 0.5 to 1.0 cm deep. Oil based cuttings were also added to sediments at a lower dosage (1/10 of high dose) to produce discontinuous layers of cuttings. Their relative impact was mainly assessed in terms of changes in sediment microbiology and chemistry, although the effects of cuttings on other benthic organisms were also considered and their acute toxicities to Crangon crangon were determined. Changes in the hydrocarbon contents of deposited cuttings were investigated to provide information on oil composition and rates and mechanisms of oil loss. The overall impact of cuttings on the sediments resulted from a combination of direct toxicity, physical smothering and indirect effects of organic enrichment. The intensity of these effects was dependent on the type of cuttings and density of deposits. Water based cuttings proved least damaging to the sediment system and showed little evidence of organic enrichment and a very low level of direct toxicity. Sediments treated with this material were initially affected by smothering (shown by changes in sediment reducing conditions) but returned to control conditions within 12 months of treatment. Sediments treated with oil based cuttings were more severely affected. Both types of oil based cuttings stimulated microbial activity within underlying sediments resulting in increased reducing conditions, high sediment sulphide concentrations and increased numbers of aerobic-heterotrophic and sulphate-reducing bacteria. In high dosage tanks these effects of organic enrichment disrupted growth of epibenthic animals (tunicates). Low dosage treatments induced less severe, localised intensification of sediment reducing conditions which appeared to have little effect on tunicate populations. Toxicity tests and changes in numbers of macrofauna within the sediment suggested 'low-tox' oil based cuttings were more toxic than diesel oil based cuttings. Their high toxicity probably resulted from a change in the composition of the low toxicity mud during drilling operations, or concentration of toxic components in discharged cuttings. The effects of direct cuttings toxicity were particularly important in low dosage tanks, in which the effects of organic enrichment were less severe. It was concluded that smothering and organic enrichment of sediments heavily contaminated with oil based cuttings were not influenced by the type of base oil in the original drilling mud. Use of low toxicity muds which significantly reduce cuttings toxicity may help to reduce the overall impact of drill cuttings on more lightly contaminated sediments (beyond the areas of organic enrichment). The rate of oil loss from deposited cuttings was dependant on physico-chemical and biological factors and was also affected by the density of cuttings deposits. The initial, rapid loss of oil during and immediately after dispersal of cuttings over the sediment surfaces resulted from oil dissolution and suspension in exchanged seawater and possibly evaporation from seawater surfaces. Oil losses were proportionally greater from the more dispersed cuttings in low dosage tanks. There then followed a period of slower oil loss by physical and biological weathering. Chemical analyses of cuttings provided no evidence of hydrocarbon biodegradation in high dosage oil based cuttings deposits, where this activity was probably suppressed by anaerobic conditions. Less than 10% of added oil was lost from these high dosage deposits during 12 months of exposure while low dosage deposits lost up to 50%. Hydrocarbon analyses indicated that some oil was lost from cuttings in low dosage tanks by biodegradation.