Monitoring Polyeyclic Aromatic Hydrocarbon (PAH) Bioavailability Near Offshore Produced Water Discharges

摘要

Produced water is the largest volume waste associated with production of oil and gas on offshore platforms. Produced water that meets effluent limitations for total oil and grease contains low concentrations of petroleum hydrocarbons, low molecular weight organic acids, and several metals. The U.S. EPA has expressed concern that some of these chemicals could be bioaccumulated to potentially toxic concentrations by marine organisms in the receiving waters of offshore produced water discharges. This paper presents the results of an intensive monitoring study to determine if marine animals bioaccumulate polycyclic aromatic hydrocarbons (PAHs) from produced water discharges to offshore waters of the Gulf of Mexico. Two platform pairs, each consisting of a produced water-discharging platform and a nearby non-discharging reference platform were selected for monitoring in offshore waters (90 to 260 m depth) of the northern Gulf of Mexico. Produced water, ambient sea water, two species of bivalve mollusks, and several species of fish were collected from each platform pair. Forty PAH analytes, including parent and alkyl PAHs, were analyzed in all samples. The two discharging platforms discharged an average of 1.75 million L/d and 1.13 million L/d of produced water containing 30 to 43 g/L (ppb) and 24 to 51 g/L of PAHs, respectively. More than 90 percent of the PAHs were naphthalene and alkylnaphthalenes. The two bivalves, jewel boxes and thorny oysters, collected from legs of the platforms, contained up to 800 ng/g dry wt (ppb) total PAHs in their soft tissues. Muscle tissues of fish collected from the platform pairs contained <1 to 120 ng/g dry wt total PAHs. The dominant PAHs in the bivalve tissues were alkylnaphthalenes, fluorenes, phenanthrenes, and dibenzothiophenes, all characteristic of a petroleum source. A few individual PAHs were present at a significantly higher concentration in tissues of thorny oysters, but not jewel boxes, from one of the produced water-discharging platforms than from the paired reference platform. The relative concentrations of different PAHs in oysters was different than that in the produced water. The evidence for bioaccumulation of PAHs from produced water by shellfish was considered weak and inconclusive, because bioaccumulation of petroleum-derived PAHs was demonstrated in only one instance, and the source(s) of the PAHs was unclear. Concentrations of individual PAHs in fish muscle were low. Higher concentrations of individual PAHs were detected with similar frequency in fish from the reference and discharging platforms. Thus, the fish were not bioaccumulating PAHs from the produced water discharges. Bivalves are more suitable than fish for monitoring bioaccumulation of petroleum hydrocarbons near offshore produced water discharges.