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1986 Edition, January 1, 1986

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Tidal Area Dispersant Project Fate and Effects of Chemically Dispersed Oil in the Nearshore Benthic Environment

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Product Details:

  • Revision: 1986 Edition, January 1, 1986
  • Published Date: July 1986
  • Status: Not Active, See comments below
  • Document Language: English
  • Published By: American Petroleum Institute (API)
  • Page Count: 234
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:


The Tidal Area Dispersant Project was designed to compare the chemical fate and ecological effects of nearshore chemical dispersion and onshore mechanical cleanup of a nearshore oil spill. The information gained from this experiment was expected to help answer the question of whether less overall ecological harm is caused by nearshore dispersion of floating oil or by on shore mechanical cleanup.

The recent availability of low-toxicity oil spill dispersants has resulted in renewed interest in their use to mitigate the effects of nearshore oil spills, Dispersing an oil slick at sea, in deep water, removes the oil from the surface of the water. This avoids impacts associated with floating oil. The dispersant transfers the impact of the floating oil from the shoreline, where impacts can be severe and long lasting, to the water column, where weathering, biodegradation and dilution greatly reduce adverse effects. The net result expected is little or no acute impact and no long term damage.

In deep water the advantages of dispersant use are clearly apparent; the potential disadvantages are not obvious. In the nearshore environment, where the greatest volume of oil is spilled, the potential disadvantages of dispersant use must include the interaction of the cloud of dispersed oil with bottom sediments and with the community of animals which inhabits them.

In the present study, the consequences of dispersing and not dispersing nearshore oil slicks were compared quantitatively in afield experiment carried out on August 19, 1981. A major goal of the experiment was to elucidate impacts resulting from exposing intertidal and subtidal sediments and their animal communities to transient, relatively high concentrations of dispersed oil.

The object of the chemistry program was to look at the chemical fate of the two oil treatments in the water column, intertidal sediments, intertidal animal tissue, and subtidal sediments (including the surface flocculent layer).

The object of the biological program was to compare the ecological effects of of dispersed and non-dispersed oil in intertidal and nearshore subtidal habitats. In the intertidal habitat ecological effects were studied at three levels of integration: community structure, whole animal physiology and biochemical processes. In the subtidal habitat only infaunal community structure was studied.

The scenario on which the experimental design is based envisages a moderate (1000 gallons/acre) spill of crude oil into the nearshore environment. The choice is whether to disperse the oil to protect a nearby low energy depositional area. The experiment consisted of the release of two identical spills of Murban crude oil; one was dispersed, the other was untreated and was cleaned up by mechanical means.

The undispersed oil was not pretreated to simulate weathering. It was released several hundred feet offshore during a falling tide and was carried to the beach by the wind. It was stranded on the beach during a time when the water table in the beach was falling so as to result in maximal penetration of oil into beach sediments.

The dispersed oil was premixed with 10% v/v of a widely available glycol ether based dispersant. It was released during the hour preceding high water. Two small boats with mixing gates provided maximal exposure of the intertidal benthos to the resulting cloud of dispersed oil.

The experiment was carried out in Long Cove, Searsport, Maine. Three test areas, as comparable as possible, were established in the upper part of Long Cove. They were laid out so that in SW wind conditions the reference area is "upstream" of the undispersed oil area which, in turn, is "upstream" of the dispersed oil area.

Five subtidal sampling plots were established which received no direct treatment. They were laid out in such a way as to lie beneath the expected path of the cloud of dispersed oil.

The major impact of the undispersed oil was expected to be on the intertidal sediments and organisms. The major impact of the dispersed oil was expected to be on organisms in the water column. Two major questions involved the impact of the dispersed oil on the subtidal environment and the degree to which dispersion of the oil would affect its weathering rate.

Following dispersion, considerable and rapid loss of the one and two ring aromatic compounds was observed. These are the compounds most associated with the acute toxicity of petroleum. This means that the dispersed oil lost toxicity as it diffused downward. Thus benthic animals were exposed to oil that was much less toxic than it had originally been.

It was also observed that as the dispersed oil was mixed downward from the surface the weathering by physical processes proceeded more slowly than near the surface. Thus in the chemical dispersion of an oil spill it may be advantageous to use mixing techniques that retain the dispersed oil as near the surface as possible to permit the maximal transfer of volatile hydrocarbons from the emulsified state to the vapor state.

A major finding of the sediment chemistry was that there was no tendency for the dispersed oil to become incorporated either into intertidal sediments or into subtidal sediments. This is in contrast to the ready incorporation of the untreated oil into intertidal sediments and its persistance there.

It was observed that the uptake of petroleum hydrocarbons by two bivalve molluscs, Mya arenaria and Mytilus edulis, was significant lylower in those animals that were exposed to dispersed oil than in those that were exposed to undispersed oil. This observation suggests that dispersed oil is less bioavailable to filter feeding animals than is undispersed oil. In animals exposed to either oil treatment the rates of hydrocarbon loss (depuration) were similar, i.e., the treatment of the oil did not affect the rate of depuration.

Neither oil treatment was observed to affect scope-for-growth (SFG) in either Mya arenaria or Mytilus edulis. The dispersed oil treatment was found to have no effect on the activity of glucose-6-phosphate dehydrogenase (G-6-P) or aspartate aminotransferase (AAT). A t the same time, exposure to undispersed oil was found to elevate the activity of both G-6-P and AAT. Elevated values of both enzymes were found for up to 10 weeks following the oil discharge in the area receiving undispersed oil. By the following summer (1982) enzyme levels were similar in all treatment areas. The species composition and community structure of animal communities represent an equilibrium condition with respect to the communities' physical and biotic environment. Changes in either can lead to changes in species composition and community structure. Typically, stressed communities tend to have fewer species and to be less diverse. As environmental stress increases, species which are not very good competitors, but which are very resistant to environmental stress, may begin to become important members of the community (opportunistic species).

Results of the intertidal infaunal analysis have shown that exposure to dispersed oil had no discernable effects on species composition, population density, diversity, or evenness. Exposure to undispersedoil caused mortality of bivalve spat, reduced diversity, reduced evenness, and increased population densities of five species of opportunistic worms. Impacts of undispersed oil continued into the next summer. These impacts are similar to those observed at real-world oil spill sites.

Results of subtidal infaunal analyses have shown that exposure to dispersed oil may have caused a transitory increase in the population density of an opportunistic oligochaete at the station closest to the dispersed oil discharge. Oil leaching off the shore contaminated by the undispersed oil discharge caused much greater and more long lasting impacts in the subtidal region than did the dispersed oil, At the stations nearest to the undispersed oil discharge the effects seen were similar to those observed in the intertidal habitat contaminated with undispersed oil, i.e, the density of opportunistic species increased; evenness and diversity decreased. Further offshore, where the toxicity of the leached oil had decreased, the density of a normally occurring species increased in apparent response to the increase in bacteria resulting from the degradation of undispersed oil leaching off the contaminated beach. Further offshore, in ten meters of water, no effects were observed.