The Gulf of Mexico Comprehensive Synthetic Based Muds Monitoring Program used a combination of physical, chemical, and biological measurements to survey fourteen drill sites on the continental shelf and continental slope of the Gulf of Mexico where cuttings generated during drilling with synthetic based drilling muds (SBM) had been discharged. The three continental shelf and three continental slope sites selected for more intensive investigation are the focus of this paper. The objective of the program was to document the distribution of cuttings in drill site sediments and to characterize the areal and temporal extent of chemical and biological changes due to cuttings discharges. None of the drill sites had large cuttings piles such as have been observed in the North Sea. Chemical and physical measurements in sediments collected during two surveys one year apart at a subset of eight of the drill sites showed that deposition of discharged SBM cuttings solids was limited to a radius of less than about 250 m from the discharge location. Mean synthetic based fluid chemical (SBF) concentrations in near-field sediments decreased during the year between sampling surveys. Benthic ecology parameters were measured at the three continental shelf discharge sites and sediment toxicity to amphipods was measured at the three continental shelf and three continental slope discharge sites. The benthic community within 250 m of the site with the highest SBF concentrations in sediments had reduced benthic faunal abundance and diversity. Benthic communities were only slightly disturbed at the other two sites. Amphipod survival exceeded 75% in sediments collected within 250 m of most of the drill sites on the continental shelf and slope, indicating that most SBM cuttings-contaminated sediments, even those close to the drill sites, were not toxic. A sediment quality triad (SQT) analysis, used to characterize the impacts of drilling discharges on sediment quality at the three continental shelf sites, identified a gradient of decreasing disturbance with distance in sediments collected within 250 m of the drill sites compared to sediments > 3000 m away. There was strong evidence that chemical and ecological recovery of sediments near SBM cuttings discharges was progressing during the year between the sampling surveys.

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