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Oil in the Sea III: Inputs, Fates, and Effects
mentation, and shoreline stranding is a concern only where shipping lanes pass close to shore.
Recreational Marine Vessels
Fuel for two-stroke outboard engines is a mixture of gasoline and lube oil in volume ratios varying from 20:1 (5 percent) in older engines to 50:1 (2 percent) in newer models. The bulk of the fuel, gasoline, is comprised of the lighter, molecular-weight fraction (e.g., BTEX) and volatilizes from the surface water. The rate of volatilization is temperature dependent, but the product will remain for several minutes to hours at 15ºC, given the amount of gasoline that is emitted from two-stroke engines in coastal waters and the time of year that they are used (usually during warm periods, when biological productivity is highest). The potential effect from toxins, such as PAH, in unburned gasoline and lubricating oil, on the biota including larvae and phytoplankton is large (see Chapter 5 for greater discussion of potential environmental effects from the release of refined petroleum products). To date there are no published field studies evaluating the effect of gasoline released from the operation of two-stroke engines.
The lube oil mixed with gasoline forms the sheens and slicks that trail behind two-stroke engines during operation. Evaporation and dissolution are the most important fate mechanisms. After two days, nearly 75 percent of the lubricating oil can evaporate at 15ºC (Figure 4-2). A smaller portion of the light lube oil can remain on the surface marine microlayer for longer periods (days) depending on environmental conditions including physical, chemical, and biological processes.
Unlike the discharge of liquid petroleum, hydrocarbons that enter the coastal ocean from land-based sources via rivers have already undergone considerable biogeochemical weathering. Land-based sources result from petroleum inputs to streams and rivers and subsequent transport to surface coastal waters. This transport is selective, with more water-soluble and stable components of the petroleum mixture carried downstream. During riverine transport, the petroleum mixture can undergo further weathering, including evaporation-volatilization and microbial degradation, such that the material reaching the coastal ocean is likely more stable and recalcitrant than the original mixture.
In addition to weathering between the release point and the coastal ocean, the nature of the river transport will play a major role in the magnitude and fate of petroleum products reaching the ocean. A good example is the differential behavior of petroleum transport in the Columbia River and the Chesapeake Bay. The Columbia is a large, relatively fast-flowing river whose plume discharges directly into the coastal Pacific. Petroleum hydrocarbons entering the Columbia River are likely transported rapidly to the coastal ocean, with relatively little retention within the river basin. The Chesapeake Bay, on the other hand, is a shallow, productive, semi-enclosed estuary with a long water residence time and a well-characterized ability to trap eroded solids. Due to its large surface area-to-volume ratio and its relatively high sedimentation rates, the Chesapeake Bay is likely to efficiently transport petroleum hydrocarbons entering from the tributaries. While this trapping reduces the loadings of petroleum hydrocarbons to the coastal oceans, it may result in locally enriched hydrocarbon levels in estuaries and other embayments. (Note that in this report, tidal embayments are included as part of the coastal ocean, so these removal processes in estuaries would be counted as “losses” from the coastal ocean.)
Petroleum hydrocarbons enter the coastal ocean from the atmosphere by wet deposition (scavenging of atmospheric hydrocarbons by precipitation), dry aerosol deposition (transport of marine aerosol particles to the sea surface), and gas exchange. Of these three, it is estimated that gas exchange dominates the total gross loading of hydrocarbons from the atmosphere. Since gas exchange results from the dissolution of gaseous hydrocarbons in sea water, the magnitude of its flux depends on the concentration in the gas phase and the solubility of the hydrocarbon in sea water. Unlike the other sources discussed in this report, atmospheric deposition supplies hydrocarbons somewhat uniformly to the coastal ocean at relatively low loading rates over large areas.
Analysis of the concentrations of petroleum hydrocarbons in the coastal ocean indicates that the surface waters are greatly oversaturated with n-alkanes with respect to the overlying atmosphere. All of the input sources discussed in this report lead in varying degrees to these ambient concentrations in the coastal ocean. Volatilization is the dominant fate process for petroleum hydrocarbons. Terrestrial hydrocarbon loadings (land-based sources) and other nearshore sources support dissolved hydrocarbon loadings in coastal waters that far exceed the loadings in equilibrium with the atmosphere. Hydrocarbon degassing to the atmosphere from coastal water is therefore a major geochemical process.
The behavior and fate of crude oil and refined products in the marine environment are controlled by many different processes that vary considerably in space and time. Physical, chemical, and biological processes all interact to (1) alter oil introduced into the oceans; (2) transport the resulting degradation (weathering) products away from the source; and (3) incorporate the residual substances into compartments of the earth’s surface system. These compartments involve disso