FIGURE 2-5 Relative average, annual input (1990-1999) of petroleum hydrocarbons (kilotonnes) to North American (A) and worldwide (B) marine environment from sources associated with petroleum extraction.

Inputs from oil and gas extraction are restricted to the coastal and offshore oil and gas production areas, namely the Gulf of Mexico, Alaska, southern California, and Canadian Maritimes (Fig. 2-6). Over 90 percent of the inputs from extraction activities are from produced water discharges (Fig. 2-5), which release low but continuous amounts of dissolved components and dispersed crude oil (Utvik, 1999). Because dilution and weathering are important mechanisms for reducing the concentrations of toxic components in produced water, the elimination of coastal discharges in most North American waters has significantly reduced the overall potential effects of these inputs. Similar discharges continue, however, in production fields of developing countries.

Spills (of mostly crude oil) from platforms comprise 5 percent of the total inputs from extraction activities. The amount of oil released into the coastal zone for the Canadian Maritimes, eastern Gulf of Mexico, and southeast Alaska are remarkably similar, ranging from 2.2 to 2.5 tonnes per year. Oil input from platform spills into California coastal waters is very low, at 0.4 tonnes per year. The highest amount of oil from platform spills is for the northwestern Gulf of Mexico, with inputs into coastal waters reaching 81 tonnes per year and representing 92 percent of the total amount of oil spilled from platforms into North American coastal waters. Spills into state waters are about twice the input from spills in offshore waters. Spills from platforms are generally small, averaging 3.5 tonnes (1,000 gallons), yet small spills can have significant impacts under some circumstances.

Extraction activities do not appear to be an important source of PAH. Less than 2 percent of the 5,000 tonnes of PAH entering the marine environment comes from petroleum extraction (Table 2-4). Extraction activities release about 0.07 thousand tonnes of PAH to marine waters in North America, with 74 percent associated with produced water discharges. Most of the remaining PAH inputs resulting from extraction activities are from atmospheric deposition.

Inputs from extraction of petroleum are composed primarily of components of crude oil, although they also include minor spills of refined products from equipment and vessel operations associated with oil platforms (see Chapter 3). The single largest input from extraction activities is from produced waters that contain the dissolved compounds (mostly single-ring aromatic hydrocarbons) and dispersed oil that cannot be separated from the water extracted with the oil from reservoirs (see Table 2-2). The dissolved compounds are also relatively volatile, so a significant fraction of the oil from produced water is removed rapidly by volatilization and evaporation, particularly when released to open, well-mixed waters. The finely dispersed oil droplets stay suspended in the water column and undergo microbial degradation or are sorbed onto suspended sediments that are then deposited on the seabed (Boesch and Rabalais, 1989a). Elevated levels of contaminants in sediments typically extend up to 300 m from the discharge point. Produced water discharges increase with reservoir age; thus these inputs may increase over time unless more produced water is re-injected or better treatment technologies are developed.

Except for very rare blowouts, spills from platforms are generally small and make landfall only when spilled close to shore or inshore. Impacts would be greatest in coastal or inland areas where numerous small spills result in chronic exposure during the life of the field. Oil production in coastal waters only occurs in Louisiana, Texas, and Alaska, although most of it is in Louisiana.

The environmental effects that may result from oil and gas production in a field depend greatly on the characteristics of the receiving environment (Rabalais et al., 1991a, Rabalais et al., 1992). Measurable effects are most likely in

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