Alternate Title

Opportunistic Sampling at a Deep-water Synthetic Drilling Fluid Discharge Site in the Gulf of Mexico


Two opportunistic benthic surveys were conducted at an offshore semisubmersible oil drilling rig located in 565 m of water on the continental slope of the Gulf of Mexico to determine the extent of synthetic-based drilling fluid (Petrofree LE) concentrations in surrounding sediments and the composition of the associated macrofauna and megafauna communities. Sediment concentrations of Petrofree LE ranged from 89 to 198,320 μg/g in surficial sediments (0-2 cm) and from 4 to 85,821 mg/g in the 2-5 cm stratum. The highest Petrofree LE concentrations were located 50-75 m northeast of the discharge site, a phenomenon that may have been related to surface and midwater currents in the vicinity of the rig. Although no direct quantitative measures of in situ degradation are available, high concentrations of Petrofree LE relative to discharge periodicity suggest lower than anticipated rates at this deep-water site. Between July 1997 and March 1998, the densities of polychaetes and gastropods increased sharply in the study area. In March, polychaete (primarily dorvilleids) density, gastropod density, and Petrofree concentrations were all significantly higher northeast of the drill site compared with southwest. Polychaete and gastropod densities northeast of the drill site were roughly 3,600 and 3,000 times higher than those reported in eastern and western areas of the northern Gulf of Mexico at similar depths, respectively.