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Oil in the Sea III: Inputs, Fates, and Effects
waters contain elevated concentrations of radionuclides, metals, volatile organic aromatic compounds, monoaromatic hydrocarbons, light alkanes, higher molecular weight aromatic hydrocarbons, ketones, phenols, and organic acids. The environmental effects that may result from oil and gas production in a field depend greatly on the characteristics of the receiving environment. For example, there was a decreased abundance of fouling organisms, particularly barnacles, from the surface to a depth of about 3 m on a platform leg immediately below the produced water discharge located 1 m above the water surface (Howard et al., 1980). Produced water discharges, however, are usually dispersed to some degree. If discharged into the ocean, the produced water dilutes rapidly so that no impacts are ascribed to salinity. In more confined estuarine waters, produced water discharges form dense, saline plumes that move along the bottom sediments, but the resulting elevated water column and interstitial sediment salinity levels are within the range of tolerance of euryhaline estuarine organisms. In shallow, more confined areas with high suspended sediment loads or fine-grained sediments, medium molecular weight hydrocarbons and metals can absorb to particles and be deposited. Measurable effects are most likely in shallow waters, areas of restricted flow and dispersion, water with a high concentration of suspended particulates, and areas of fine-grained anaerobic sediments.
Effects of Production Discharges in Estuarine Waters
U.S. regulations now prohibit most discharges of produced waters from platforms to state waters of Texas and Louisiana, although the phase-out is not yet complete and some exceptions are provided, for example the highly dispersive distributaries of the Mississippi and Atchafalaya Rivers. The discharge of treated produced water from several offshore platforms at shore-based facilities is still permitted in Upper Cook Inlet, Alaska. The discharge of produced waters into estuaries and shallow coastal waters continues globally in developing fields (e.g., Nigeria, Angola, China, Thailand), and the effects of produced water discharges may still linger where the practice has been discontinued (Rabalais et al., 1998).
The effects of produced water discharges in estuaries have been studied extensively in Texas and Louisiana. For example Mackin (1971) surveyed estuarine benthic communities in eight Texas bays receiving produced water effluents. He reported no effects in two bays, minor localized effects in several other bays, and a zone of severely depressed fauna up to 106 m from submarine outfalls in Trinity Bay, Texas and a zone of enhanced faunal abundance and diversity down-current from there. Mackin (1971), however, conducted no chemical analyses. Armstrong et al. (1979) repeated these studies in Trinity Bay to correlate the benthic community effects with the distribution of hydrocarbons in sediments. In shallow waters of 2-3 m, they demonstrated the impacts of high concentrations of hydrocarbons, in this case sediment naphthalene concentrations of 4 to 8 ppm up to 1200 m from the platform, with corresponding severely depressed benthic fauna.
It was not until the mid to late 1980s that more extensive, systematic studies of the effects of produced water discharges in estuarine waters were conducted. While most surface water disposal was terminated on January 1, 2000, except within the distributary channels of Louisiana’s major rivers (the Mississippi and Atchafalaya), it is prudent to review the results of these studies for several reasons. First, this disposal method was practiced in coastal Louisiana and Texas and at one time accounted for 2,500,000 bbl/d of discharges into estuarine waters with the potential in some areas for long-term accumulation of contaminants and subsequent reintroduction to the environment (Boesch and Rabalais, 1989b; Rabalais et al., 1991a,b; Rabalais et al., 1998). Second, accidents associated with current disposal methods (pipelines and barges) will have similar results. Third, surface water disposal in estuarine waters still occurs elsewhere in the world.
Boesch and Rabalais (1989a), Neff et al. (1989), St. Pé (1990), Rabalais et al. (1991a,b), Steimle & Associates, Inc. (1991), Mulino et al. (1996) studied the effects of produced water discharges in estuarine waters of Louisiana. Where suitable measurements were made, the eventual fate of the dispersed produced water and the effects on benthic infauna could be explained by the volume of the discharge, the concentration of the various constituents, and the sedimentary regime, physical structure, and hydrology of the receiving environment (Boesch and Rabalais, 1989; Rabalais et al., 1991a). Dilution of water-soluble contaminants was influenced primarily by the volume of the receiving waters, the current velocity, and the potential for resuspension of sediments. Dispersion of sediment-adsorbed contaminants was influenced by the bed shear stress, sedimentation rates, and the grain size distribution of the surface sediments. The dilution potential of the environment was high for erosional environments with high current speeds and low for depositional environments, with intermediate potential for environments with periodic resuspension (storm-related) and deposition. There were no documented effects on the benthic community due to elevated salinities, because the overlying water and sediment interstitial salinities were within the range of the euryhaline organisms found in these habitats. Volatile hydrocarbons in the water column density plume that disperses across the sediment bed varied from nil to as high as 130 mg/L; alkylated PAH in bottom sediments reached concentrations from 2 to 40 ppm with one value of 100 ppm (Rabalais et al., 1991a,b). The potential for accumulation to depth in depositional environments exists (some sediments contained 30 ppm alkylated PAH at 35 cm depth) (Rabalais et al., 1991a). Produced water source contaminants persisted in surface sediments for two years after cessation of the effluent, as did benthic community effects, and persisted for as