The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Oil in the Sea III: Inputs, Fates, and Effects
shallow waters, areas of restricted flow and dispersion, water with a high concentration of suspended particulates, and fine-grained anaerobic sediments (Boesch and Rabalais, 1989 a,b; St. Pé, 1990). There are clear effects of produced water discharges on estuarine waters, sediments, and living resources in inshore production fields where the receiving environment is not conducive to the dispersion of the effluent plume. In the United States, studies of their effects (Boesch and Rabalais, 1989 a,b; St. Pé, 1990; Rabalais et al., 1991a) led to the prohibition of produced water discharges into coastal waters in the late 1990s. In shallow shelf waters, hydrocarbons from produced water accumulate in bottom sediments, and the diversity of benthic fauna may be reduced up to 300 m from the outfall (Rabalais et al., 1991a,b; also see Chapter 5; Table 5-7). Measurable effects occur around offshore platforms but, except for artificial reef effects (sedimentary changes or changes brought about by a cuttings pile), such effects are usually localized (Rabalais et al., 1993; Kennicutt et al., 1996a; Montagna and Harper, 1996). It is noted, however, that discharge of oil-based drill cuttings has never been permitted in the U.S. and was recently prohibited in the North Sea. Beyond some contamination of organisms by petroleum, there is little convincing evidence of significant effects from petroleum around offshore platforms. Where oil-based drill cuttings are discharged, there are more readily evident effects of sediment contamination and benthic impacts to much greater distances from the platforms (up to 1 to 2 km) (see Chapter 5). Although directed studies have identified some specific sublethal effects of long-term oil and gas development (Kennicutt et al., 1996a, b; Street and Montagna, 1996), the most significant unanswered questions remain those regarding the effects on ecosystems of long-term, chronic, low-level exposures resulting from discharges and spills caused by development activities. Unique features of deep-sea communities and the relative lack of understanding of these communities may make them more vulnerable to production activities. As the reservoirs age, the volume of produced water discharges from existing production facilities will significantly increase. The ecological impact of increasing rates of produced water discharge in both nearshore and new deep-water habitats is not clear. It will be important to consider these increases in future monitoring programs.
Another component of crude oil that is released during petroleum extraction consists of the VOC that occur as gases at ambient temperature and pressure, and thus escape to the atmosphere. Atmospheric deposition from extraction activities accounts for 4 percent of the total extraction-related inputs. Because VOC inputs are estimated using production volume, inputs are largest for the areas of highest oil and gas production. Inputs into coastal waters are about an order of magnitude lower than into offshore areas (see Table 2-2).
Only 0.2 percent of the VOC released to the atmosphere are estimated to be deposited into surface waters, when very conservative assumptions are used. This input is overwhelmed by hydrocarbon outgassing from the oceans. Therefore, impacts from VOC deposited at relatively low, uniform rates over large areas of ocean are likely to be small. Still, the fate and potential effects of VOC inputs to marine ecosystems are poorly understood; thus, there are unanswered questions about the concentrations and duration of VOC in the microlayer, the bioavailability of such volatile compounds, and their toxicity.
Transportation of Petroleum
The five major sources of petroleum hydrocarbon discharges into the marine waters by transportation activities include pipeline spills, tank vessel spills, operational discharges from cargo washings, coastal facilities spills, and gross atmospheric deposition of VOC releases from tankers (Table 2-2, Figures 2-7 and 2-8).
DeLuca and LeBlanc (1997) estimate that there are approximately 23,000 miles of pipelines that carry petroleum hydrocarbons in North America. Pipeline spills can occur as petroleum hydrocarbons are transported from the source to refineries and from refineries to the consumer (see Chapter 3 for greater details). The total input of petroleum hydrocarbons to the marine environment by spills from pipelines to North American waters is estimated to be 1,900 tonnes per year (Fig. 2-7).
The volume of crude oil spilled from pipelines in coastal areas is double that spilled in offshore areas, increasing the potential impacts because weathering and fate processes will not reduce the risks of exposure from such spills of crude oil (see Chapter 3). Accidental spills by pipelines are more common in coastal waters because production first occurred in the coastal regions and many of the pipelines are approaching 30–40 years old. It is highly probable that accidental spills from coastal pipelines will continue into the future as these pipelines age further unless steps are taken to ensure the integrity of this important system. The efforts of the Office of Pipeline Safety, under the Pipeline Integrity Management Program, are timely and appropriate for reducing these risks.
Because of numerous regulations and technology advances in vessel construction (i.e., double-hull tankers, new construction materials, and vessel design) spills from tank vessels have been reduced significantly during the past decade, even though the tanker fleet has increased by some 900 vessels to a total of 7,270 in 1999. Spills greater than 34 tonnes in size represent less than 1 percent of the spills by number but are responsible for more than 80 percent of the total spill volume (see Chapter 3 for greater details). In North American waters, vessel spills have been reduced consider