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4 Intermediate-Scale Experiments and Field Studies of Dispersants Applied to Oil Spills Because the magnitude, type, and duration of effects on aquatic organisms and ecosystems caused by of] spills depend directly on exposure to toxic components of the oil, the effects of oil are expected to be less if the spill is rapidly diluted by chemical dispersion. A number of experiments at scales larger than normal laboratory size (mesoscale) as well as field studies at sea have been conducted to determine the physical dispersion and the subsurface concentrations of of} components. They and their associated biological effects are reviewed in this chapter. PHYSICAL AND CHEMICAL STUDIES Although laboratory tests can rank various dispersant formula- tions as to their relative effectiveness and can be used to investigate the effects of parameters, such as temperature, water salinity, and of! viscosity, the real test of dispersant effectiveness is a full-sized spit! in a test at sea. However, rigorous sea tests are expensive and difficult to conduct, and results have often been disappointing. An absolute measure of effectiveness in the field would require that a very large set of water samples be taken, covering the entire water mass within which the oil might become dispersed? as well as an accurate measurement of the amount of of} that evaporates from the slick under the field conditions. Very few experiments have 165
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166 USING OIL SPILL DISPERSANTS ON THE SEA attempted this approach, but it is these that provide the most direct evidence of dispersal of oil at sea (Brown et al., 1987~. Some studies have been set up to obtain typical water samples from beneath a dispersant-treated or untreated slick, so as to assess whether the concentrations of of! components exceeded potentially toxic levels. The emphasis of others is to see if dispersants ranked better or poorer by laboratory test would perform in the same order on a larger scale. As will be discussed in Chapter 5, much thought has been given to remote monitoring systems, but there is as yet no standard method for determining effectiveness outside the laboratory. As a result, dispersant operations at spills of opportunity have provided only a limited and ambiguous set of effectiveness data. The compilations of Nichols and Parker (1985) or Fingas (1985) if taken uncritically could be rather discouraging. In many cases, dispersal was observed but could have been due to natural processes—adequate control spins without chemical dispersant were unavailable. In other tests, different observers at the same site reached different conclusions about how much of the slick had been dispersed. The reported effectiveness at any but the most carefully planned field trials is extremely dependent on the types of observations or samples, the location of the observers or sampling devices, and the dispersant application technique. Intermediate-Scale (Mesoscale) Studies Some studies intermediate in size between laboratory and field (microcosm and mesocosm) although not without limitations can provide useful information with greater control and at less expense than a fuB field study (Adams and Giddings, 1982~. An exam- ple is the Controlled Ecosystem Pollution Experiment (CEPEX) in British Columbia, Canada. Two 13-m deep CEPEX plastic* enclo- sures moored at Saaruch Inlet were treated with 3 liters of oil (Green et al., 1982~. In one system, Forest 9527 was added, producing a sta- ble emulsion with average droplet size about ~ am or less (measured by underwater photomicroscopy). *Typical experiments are conducted using plastic (usually polyethylene) enclosures in an open-water area. In one study (Laalce et al., 1984), the function of Ekofisk crude oil suffering different fates was measured using tritium tracers. Only Q0037 percent of the oil was adsorbed in the plastic walls.
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 167 In contrast to expectations, evaporation was inhibited in the dispersed system, which required 10 to 15 days to lose Yolatiles, whereas the undispersed slick lost volatiles in ~ to 2 days. After 7 and 27 days, the amount of of! reaching the sediment increased by about a factor of 10 in the enclosure where dispersant had been used. However, the dispersed oil was more rapidly biodegraded, by a factor of 10, with alkanes essentially oxidized in 15 days, so that in each case only about 0.1 percent of the oil remaining in the water column eventually was found in the sediment (Green et al., 1982~. To test effects of dispersants in a littoral ecosystem simulating the shadow rocky Baltic Archipelago, Linden et al. (1985, 1987) used six pools, 8 m3 each, with a flow-through seawater system. Two pools were exposed to 20 ppm (average initial nominal concentration) North Sea Forties crude oil, two were exposed to 20 ppm oil with Corexit 9550 added, and two served as controls. The differences in biological effects between treatments were attributed to dispersed of] remaining in the water column longer, without adhering to particles or organisms or settling to the bottom. Because seawater flowed continuously through the systems, the dispersed oil was more rapidly washed out. In-a trial using intertidal enclosures, Farke et al. (1985a,b) oiled sand by contaminating inflowing seawater on 12 successive rising tides with ultrasonically dispersed Arabian light crude oil, Finasol OSR-5 dispersant, and an oil-dispersant mixture (ratio O:D of 10:1~. With or without dispersant, average of] concentration in the water (sampled at high tide) was about 10 ppm, and core samples showed that less than 5 percent of the of] penetrated the deeper sediment. Maximum concentration of of! in the top 2 cm of sediment was 560 ppm in both the oil-dispersant experiment and oil-only experiments. After contamination was stopped, the of} concentration returned to baseline values within 4 to 6 weeks. Although the Parke et al. (1985a,b) study using premixed disper- sant and of} does not fully simulate the impact of of} dispersed at sea coming in on the tide, it is closer to that ideal than other intertidal studies in which dispersant was applied after a beach was oiled. In the latter studies, dispersant increased the degree of oil penetration It is clear from this example that dispersed of} is more easily washed out of an enclosed system than untreated oil, and there appears to be evidence that dispersed of} adheres less to particles and sediment than untreated of] (see Chapter 2~.
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168 USING OIL SPILL DISPERSANTS ON THE SEA Recent examples of mesoscale studies of physicochemical charac- teristics of dispersed oils include those at the Esso Resource wave basin in Canada (Brown et al., 1987; To et al., 1987~. These studies show that the dispersed of} plume often is highly irregular in shape and nonuniform in concentration. This nonuniformity can lead to serious errors when attempting to estimate dispersant efficiency by analyzing chemical samples from the water column. Further, these mesoscale experiments reemphasize the need to apply dispersants preferentially to the thicker portion of the slick in order to achieve good overall efficiency. American Petroleum Institute Research SpiBs Open-ocean tests sponsored by the American Petroleum Insti- tute in 1978 and 1979 evaluated several factors bearing on dispersant performance and fate: oil type, sea state, and dispersant type and dosage. The studies compared the fates of untreated and chemically treated oil and measured total hydrocarbons in water under slicks (McAuliffe et al., 1980, 1981~. In four spills conducted off New Jersey, Il.7 bb! of Murban and LaRosa crude of} were sprayed with dispersant by helicopter. Murban crude of} changed rapidly when dispersant was immediately applied. A distinct whitish-brown subsurface plume appeared quickly. Over several hours, this plume grew in area and diminished in color and visibility as the dispersed of] diluted. Rough mass-balance calcula- tions, supported by visual and photographic observations, indicated that Murban crude oil was almost completely dispersed (McAuliffe et al., 1980~. The highest total oil concentration measured under the low- viscosity Murban (39° API gravity) crude of! was 18 ppm at ~ m at 23 min after dispersion, decreasing to less than ~ ppm at 6 m after ~ hr. The highest dissolved hydrocarbon concentrations 40 to 50 ppb— occurred in the samples with the highest tote] of] concentrations. After 110 min the highest hydrocarbon ECU to Coo) concentrations were2ppbatlm. When dispersant was sprayed on the fresh I`a Rosa (24° AP] gravity) crude oil, no sudden change was apparent. However, in time this of} became a thin sheen, as contrasted with the thick, black, asphaltic appearance of the thicker of} in the downwind, leading edge portion of undispersed oil. About half the slick was estimated to have been dispersed. The highest total of! concentrations measured in the
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 169 ~ ~ ~ . ~ . ~ ·~ dispersed of! plume were 2 to 3 ppm from the surface through 3 m at 23 mitt after spraying. These concentrations also existed after an hour, and thereafter decreased. The highest dissolved hydrocarbon concentrations were 14 ppb at 47 min. and 9 ppb after 94 min. The two slicks that were allowed to weather for 2 hr before dispersant spraying showed low concentrations of oil in the water. This was probably due to the greater area of the slick and the fact that most of the oil was in the downwind portion of the slick. Since the overall slick was uniformly sprayed, the thick portion where most of the of! resided may have been undertreated. Weathering also would have increased of} viscosities, thereby decreasing dispersant effectiveness. These effects were clearly demonstrated during the 1979 API studies off southern California. In the September 1979 API studies off the coast of southern California (McAuliffe et al., 1981), nine separate 10- or 20-bb! releases of 0.90 specific gravity (26.6° API gravity) Pru~hoe Bay (Alaskan North SIope) crude of] took place over 2 days. These tests were unusual in the large number of water samples taken (900), which aDowed contours of subsurface concentrations to be obtained and a more accurate mass-balance made. The slick areas of the 20-bb} of! discharges were 2 to 3 ha (5 to 7 acres) at the start of aerial spraying, 10 to 30 min after release, but increased during the 30-min multipass spraying. The average slick thickness was 0.1 to 0.2 mm initially, but decreased as the slick area increased. The most effectively dispersed 20-bb] slick was sprayed with dis- persant concentrate from a DC-4 aircraft; the results of this test are discussed in detail here. Figure 4-1 shows that the of! concentrations for the first sampling run under the remaining slick and through the dispersed of! plume (five stations were placed along the length of the stick and two across it). The highest dispersed of} concentrations occurred at Station 2 (center of the downwind thicker part of the slick) with an average of 41 ppm at ~ m and 10 ppm at 3 m. A mass- balance was estimated (by layers) by calculating the water volume (as the slick length multiplied by two-thirds of the width) multiplied by the average of] concentration of the seven stations. The chemical analyses were on a weight basis. Correcting for the specific Cavity ~ . ~ ,,; is ,, /^ ^~\ 1 . ~ . ~ ·~ ~ · ~ ~ ~ ~ ~ . ~ tu.9u~ changes the amount of oil discharged from 20 bbl to 18 bbl on a weight basis. It was further assumed that by the time of spraying, 15 percent of the of} had evaporated. Thus the amount of of} in the slick that was sprayed was estimated to be 15.3 bbl. The amount of of} measured in the water column at the various depths (totaled in
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170 USING OIL SPILL DISPERSANTS ON THE SEA o 3 - a) C] 6 9 Stations (if) Time After 13 Spraying (min) Time 1322 1325 13.1 .89 o 6 _ 7.6 ! 1 1 2| — 11.4 1 1.03 - / / — 7.6 / /.58 - ~/Q~ Stations (A Time After 32 35 Spraying (min) Time 1 303 102 1306 54 54 20.5 / 41 ~ 41 / .86 ~ 7.4 29 I / .82,~ .47 _ pR 1309 1315 1317 1.9 2.1 .90 1.69 1.73 J 1.03 J.66 no no no 15 19 @3 ~ 25 27 Amount Percentage of Oil in of Slick Water Dispersed (bbl) in Water 0-2 m 6.05 39.0 (60)* Slick Size 150x500 m g, 2-4 2.54 17.0 (25) Slick Area 7.5 Hectares (1 8.5 Acres) j' 4-7.5 1.42 9.3 (14) Time of Spill 1214-1217 5 7.5-10.5 0.06 0.4 (0.6) Time of Spraying _ 1230-1 250 10.10 66.0 Totals *Percent Distribution by Depth Interval FIGURE ~1 Concentrations (ppm) of oil in water under a 2~bbl crude oil slick that was sprayed i~runediately with dispersant by DC-4 aircraft, September 26, 1979; first sample run. Percentage of slicilc dispersed in water is based on the estimated amount of oil in the sliclc, not the amount of oil dish Bed. Source: McAuliffe et al., 1981. Figure 4-~) was 11.2 bbI, or about 66 percent. Therefore, two-thirds of the of} in the slick (after evaporation) was dispersed, and one-third remained on the surface. Only 0.8 percent of naturally dispersed oil was found under untreated (control) slicks during single sampling runs immediately after the control slicks were released. The highest amounts of naturally dispersed of} generally are found under fresh oil slicks.
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 171 Time 1331 1334 1339 1344 1350 83 4.35 8.56 7.5 .69 O : 1 1 1 1 1 ll 1 ~ 3.21 1.14 ~ 1.~ ~ 12.1 1.46 _ , ~ 2.60 ~ 1.70 9.22 ~ 6.~ I \ — ~37 J 10.10 J ~ I / 3~ ·: j ~~ | I 6 ~ .07~4 / 9 - 02 . .05 \ \ · / /- Stations () ~ (hi) ~ ~ Time After 41 44 49 54 60 Spraying (min) Amount Percentage of Oil in of Slick Time 1354 1359 Water Dispersed 3.55 .82 (bbl) in Water - - a) 9 1 Stations (A Time After 1:04 Spraying (hr:min) 1.70 1.73 _ ~~m _ .07 1.28 2.38 0-2 m 4.74 31 (35)* Slick Size 162 x 840 m 2-4 3.68 4-7.5 4.42 7.5-10.5 0.79 Totals 1 3.6 24 (27) 29 (32) .18 — l 1:09 *Percent Distribution by Depth Interval 89 5.2 (5.8) Slick Area 13.6 Hectares (34 Acres) Time of Spill 1214-1217 Time of Spraying 1 230-1 250 FIGURE 4-2 Concentrations (ppm) of oil in water under a 2~bbl crude oil slick that was sprayed immediately by DC-4 mrcra£t, September 26, 1979 (day 1~; second sample run. Source: McAuliffe et al., 1981. Figure 4-2 shows the results of a second sampling run through the slick about an hour after spraying. During this time? the slick had elongated as the remaining wind-driven surface slick separated from the dispersed oil plume. The highest concentration, found at 1 m at Station 2 after 15 min (Figure 4-1), occurred from O to 6 m at Station 4 after S4 min. The dispersed oil had diluted by mixing downward. The mass-balance calculation at that time provided an estimate of about 89 percent of the unevaporated oil dispersed. A third sample run was started 3 hr after spraying, as a transect
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172 USING OIL SPILL DISPERSANTS ON THE SEA At 3 6 18 4 4 Time 1555 1558 1604 1622 1626 1630 1 .53 1 .38 .18 .23 .19 .99 o 1 - Q 6 9 Stations Time After Spraying (hr:min) /.4r2 i`_ .64 ·03 .06 .05 / .06 0> am / .05 .25 / /~p .05 .06 .05 .05 I ~ / 1 .11 \ .13 .14 .11 .14 0., _ .03 .06 .05 .05 = . 1 1 1 ,, ,_ 1 / I .06 / .85 1~ 2.26 in_ a ·.5' <7_ .1 1 .49 ~ \` 1 3:05 3:08 3:14 3:32 3:36 3:40 Near Drogue Under Downwind Surface Slick Slick Length 2,000 m FIGURE 4-3 Concentrations (ppm) of oil in water under a 2~bU crude oil slick that WE sprayed immediately by DC-4 aircraft, September 26, 1979 (day 1~; third sample run through small downwind slick and then near drogue where dispersion occurred. Source: McAuliffe et al., 1981. from under the separated teardrop-shaped slick back to the drogue that was following the dispersed oil plume in the water (Figure 4-3~. The distance was about 2 km. Samples taken at the drogue had concentrations of 1 to 2 ppm through 6 m and 0.5 ppm at 9 m. These concentrations represent the further dilution of the dispersed of] with time. The results of these very elaborate field studies produced some of the few quantitative mass-balances on dispersed oil that have ever been obtained. Although a large number of samples were collected, there can still be errors in the calculated amounts of dispersed oil.* ~ An independent check of dispersant effectiveness can be made based only on the observed concentrations in the water. An average O.l-mm-thick ~lick, if completely dispersed and uniformly mixed in 1 m of water, would produce a concentration of 100 ppm, 33 ppm in 3 m, and 17 ppm in 6 m. The sample stations, with the higher concentrations shown in Figures ~1 and ~2, approach these values if the oil measured at greater depths is added back to the shallower depths. The measured concentrations would be even closer to the theoretical concentrations if they were corrected for volume of oil to weight of oil, percentage of evaporation, and an estimate of the amount of slick · . remammg.
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 173 Table 4-1 summarizes the effectiveness of dispersant sprayed on seven slicks and the estimated percentage of of} naturally dispersed under the two control slicks. Table 4-l shows the effectiveness of aircraft versus boat spraying, a comparison of two different disper- sants applied in the same manner, and immediate spraying versus a 2-fur delay. Aerial spraying of the fresh oil slick was more effective than boat spraying (60 and 78 percent versus 62 percent with less dispersant aerially applied). The reduced effectiveness of boat appli- cation may be due principally to mixing the dispersant concentrate with seawater (with an induction system to 2 percent concentration) before spraying. Dispersant H (Corexit 9527) was 5 to 6 times more effective than dispersant ~ (unidentified) (62 versus 11 percent) in boat spraying of just the thick of} part of the fresh slick. (Labo- ratory testing with both fresh and weathered Pru~hoe Bay crude oil showed about this same difference.) Oil that was on the water for 2 hr prior to spraying was not as effectively dispersed, probably due to increased viscosity from greater Toss of volatile hydrocarbons by evaporation. Boat spraying the entire slick uniformly was very ineffective compared with spraying just the portion of the slick that contained most of the oil. Too little dispersant was sprayed on the thick areas, too much on the thin ones. This difference in effective- ness is in accord with the basic concept that the dispersant should be sprayed where most of the of! exists. Table 4-1 also shows that generally the higher the rate of dispersant application, the greater the amount of oil dispersed. Another APT-sponsored study of dispersed versus untreated oil was conducted at Long Cove, Searsport, Maine. The results are presented later in this chapter. Protecmar The French Protecmar program's studies emphasized middIe- scale field tests with two boats because such tests are more realistic than laboratory tests and less expensive than fur-scale offshore tests (Bocard et al., 1981, 1984, 1987; Desmarquest et al., 1985~. In one test, 45.5 bib of light fuel of! was treated with undiluted Dispolene 325 sprayed from an airplane. About half the of} was dispersed within 4 hr. the thicker areas slowly disappeared after 7 hours, and only a scattered sheen remained after 20 hr. In a similar experiment with light fuel oil in the Mediterranean Sea, several dispersants were sprayed from aircraft and boats (Bo-
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174 USING OIL SPILL DISPERSANTS ON THE SEA TABLE 4-1 Effectiveness of Dispereant Treatments During the 1979 Southern California Studies Treatment Estimated Percentage of Dispersant Applieda Percentage of Slick Dispersed in Water Sprayed immediately by plane, day 1, dispereant H Sprayed immediately by plane, day 2, dispersant H Sprayed after two hours by plane, day 1, dispersant H Thick oil part of slick sprayed immediately by boat, dispernant H Thick oil part of slick sprayed immediately by boat, dispereant J Entire slick sprayed immediately by boat, dispersant H Entire slick sprayed after two hours by boat, dispereant H Untreated oil 4.9 3.6 4.0 8.7 8.4 1.5 1.5 78i 16b 60 ~ 3.5b 45 62 11 8 5 0.8 ~ 0.4c aEstimated percentage of dispereant applied to thicker part of oil slick, Estimated to contain 90 percent of the oil. —The mean of the first two sampling rune through the immediately aerial sprayed slicks: day 1 first sample run, 66 percent; second sample run, 89 percent. -The mean of one run through each of the two untreated control clicks. SOURCE: McAuliffe et al., 1981. card and GateHier, 1981~. For both chemical and natural dispersion, infrared analysis of water samples detected 0.! to 0.5 ppm concen- trations of oil. Subsurface concentrations obtained in some of the Protecmar tests are summarized in Table 4-2. Several general con- clusions were derived from the Protecmar tests: . The trend of dispersant effectiveness observed was similar to the United Kingdom's standard test (Labofina/Warren Spring I'aboratory), but with more viscous oils, different dispersants can hardly be distinguished. · The French standard test (also a version of Labofina) does not account exactly for the variation in dispersant efficiency with oil · ~ VlSCOSlty.
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 175 TABLE 4-2 Subsurface Oil Concentrations After Dispereant Application in the Proteemar Studies (Maximum Significant Values in ppm) 1 m 2 m|2.5 m - Trial t1 t2 t3 t1 t2 t3 Protecmar 1 2 1 Proteemar 2 1 1 Protecmar 3 Slick A 1 60 5 1 3 3 Slick B 4 3 3 5 Protecmar 4 Sprayed by 17 4 4 1 Traces Traces Canadair CL215 All other slicks 7 4 4 3 Traces Traces Protecmar 5 3 Traces 3 Traces Protecmar 6 Helicopter 1 2 (1 hr Traces Traces 30 man) Ship 4 2 NOTE: The columns t, t, and t3 are sampling times corresponding, respectively, to 0 to 30 min. 3 to 3 or 3~) min. and 6 to 7 hr after dispereant application. SOURCE: Bocard et al., 1987. · The results of the IFP dilution test (Chapter 2) agree with those obtained at sea when mixing is applied to the treated of} slick. Ranking of dispersants for a given of} viscosity is the same as in the field test (Desmarquest et al., 1985) North Temperate and Arctic Tests A number of field trials have been conducted to test effects of dispersants in north temperate coastal and arctic habitats. In 197S, in three experiments at Victoria, British Columbia, oil was spilled in a semiprotected coastal area and restrained with a boom (Green et al., 1982~. Ten percent Corex~t 9527 was applied by ship using the Warren Spring Laboratory system. Fluorometric monitoring of water samples showed as much as 75 percent dispersal. The highest oil concentration was 1 ppm, which decreased to background levels (less than 0.05 ppm) within 5 hr. in agreement with the field tests cited above. Previous tests in CEPEX enclosures showed that microbial oxidation of dispersed Canadian North Slope of} occurred at least 10 times faster than undispersed oil, and this may have been a factor in the rapid disappearance of the oil (Green et al., 1982~. The results
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204 USING OIL SPILL DISPERSANTS ON THE SEA Coral Reefs Admired throughout the world for their beauty and biological productivity and diversity, coral reefs are generally considered a habitat worth protecting from oil spill damage. As with seagrass communities, there is concern that dispersal of an of] slick in shallow water near or above a core] reef might cause greater damage than the untreated oil, because of the higher concentrations of hydrocarbons introduced into the water column. Legore et al. (1983) studied the effects of untreated light Arabian crude of} and chemically dispersed of} (20:1 Corexit 9527) on corals that were submerged 1 m at low tide. The individual plots (2 by 2 m) were boomed with the skirts (3.5 m) extending to the bottom, even at high tide. The initial nominal of! concentrations for the 24-hr exposures were that of a slick 0.25-mm thick. Thus, the dispersed of! concentration (including dissolved hydrocarbons) would have been 250 ppm if mixed uniformly. The slick remained on the surface except for of} that may have adhered to the wads of the boom. In 5-day experiments, of} equivalent to a 0.-mm thick slick was added initially and the same amount added on days two, four, and five. One-day treated corals showed normal appearance and tolerated the short exposures. Following 5-day exposure, coral recovered more slowly from seasonal bleaching. There was also some long-term re- duction in growth for both of} treatments. The exposures used are high and were produced by restricting water flow by the boom skirts. A study to simulate the effects of an of} spill moving over a coral reef with and without the use of dispersants was done at Bermuda (Cook and Knap, 1983; Dodge et al., 1984, 1985a,b; Knap, 1987; Knap et al., 1983, 1985; Wyers, 1985; Wyers et al., 1986~. Corals were exposed both in the laboratory and on the reef to Arabian light crude dispersed with 1:20 Cored 9527 or with 1:10 BPl1OOWD. Some corals were taken to the laboratory and then returned to their home on the reef for recovery studies; some were actually oiled on the reef using an enclosure. Exposures were at levels of 11 to 23 ppm for periods of 6 to 24 hr and were similar for of] alone and dispersed oil. There were no significant differences between the of} and dispersed of} treatments in tissue rupture, contraction or swelling, mesenterial filament extrusion, or pigmentation loss. Most of the observed stress occurred during dosing. Recovery began 24 hr after oiling and was complete in less than a week. One of the few apparently synergistic effects was reduced pho- tosynthesis of the zooxanthellae (symbiotic algae) within the coral
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 205 resulting from an 8-fur exposure to 19 ppm dispersed of} and in- hibited synthesis of lipids, particularly wax esters and triglycerides (Cook and Knap, 1983~. Oil or dispersant alone had no such effect. Carbon fixation was restored within 5 hr after exposure ceased, and lipid synthesis returned to normal within 5 to 24 hr. The smaller droplets in the chemically dispersed oil did not adhere to the corals in contrast to the physically dispersed of} droplets, some of which were found on coral a few weeks after exposure to 20 to 50 ppm of] alone. In summary, when corals were exposed to of} and dispersed of! under field conditions, exposure to hydrocarbons was greater in dispersant-treated plots than in untreated oiled plots. However, no effects were observed on growth of corals exposed either to of] or to dispersed of} for 24 hr and measured ~ year after the exposure. Corals exposed for 5 days to the of} or dispersed of} showed reduced growth in comparison to the controls. "Exposure to dispersed of} also appeared to delay recovery of corals under stress by cold tempera- tures" (ASTM, 1987, based on Birkeland et al., 1976 and Fucik et al., 1984). As with other organisms and habitats, the primary factor is ex- posure to the water-soluble fraction of the oil. When toxicities are based on such analytical measurements, there are no major differ- ences between physically and chemically dispersed oil. Knap et al. (1985) concluded that "in the long term, Diploria strigosa appears relatively tolerant to brief exposures to crude of} chemically dispersed in the water column." However, these experiments were limited to only one coral species, and other reef organisms, such as crustaceans, echinoderms, and other invertebrates, are known for their sensitivity to both chemically and physically dispersed oil. Therefore protection of the entire reef community, perhaps by dispersal of the of} offshore, is a priority. The American Society for Testing and Materials has published a standard guide for the use of chemical dispersants in the vicinity of coral reefs (ASTM, 1987~. It recommends the following: · Whenever an of] spin occurs in the general vicinity of a coral reef, the use of dispersants should be considered to prevent floating of] from reaching the reef. · Dispersant-use decisions to treat of! already over a reef should take into account the type of of} and location on the reef. · Coral reefs with emergent portions are high-priority habitats for protection during of] spins.
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206 USING OIL SPILL DISPERSANTS ON THE SEA · The use of dispersants over shallow submergent reefs is gener- ally not recommended, but the potential impacts to the reef against impacts that might occur from allowing the of} to come ashore should be weighed. · Dispersant use should be considered to treat oil over reefs in water depths greater than 10 m if the alternative is to allow the of} to impact other sensitive habitats on shore. · Dispersant use is not recommended to treat oil already in reef habitats having low-water exchange rates (e.g., lagoons and atolls) if mechanical cleanup methods are possible. Mangroves Mangroves grow along tropical and subtropical shorelines. As intertidal forests, they are important protectors of shorelines from storms and currents, they provide habitat and nursery areas for many organisms, some of which are commercially important, such as lobsters, prawns, shellfish, and finfish (Teas, 1979), and they con- tribute to the overall productivity of tropical marine environments. Their importance as nursery areas and nutrient sources cannot be overstated. Experiments on mangroves are relevant to how the organic frac- tion of sediment and suspended particulate matter interacts with oil and dispersed of] (Getter and Ballou, 1985; Getter et al., 1985~. A mangrove study begun in 1978 in Panama, indicated that exposure of a mature red mangrove forest to of} and dispersant resulted in many of the effects observed in the laboratory and at other spill sites: changes in growth, respiration, transpiration, and uptake of petroleum hydrocarbons (API, 1987~. These effects were reduced at the site treated with of! and dispersant compared to the site treated with oil alone (Getter and Ballou, 1985; Getter et al., 1985~. In the Panama study two similar mangrove sites were selected and boomed, and mangrove trees, seagrasses, and associated biota were surveyed. One site received 380 liters (2.4 bbl) of Prudhoe Bay crude oil, and the other site received the same quantity of oil mixed with 19 liters (5 gal) of Corexit 9527. This oil-dispersant mixture was formulated to simulate of] dispersing on the water within or immediately adjacent to the mangrove forest. Thus, the dispersed oil concentrations in the water were presumably higher than if the oil had been dispersed before reaching the mangroves.
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INTERMEDIATE-SCALE EXPERIMENTS AND FIELD STUDIES 207 Oil premixed with dispersant did not readily adhere to sedi- ments or algal mats; it lightly coated prop roots and was evenly distributed on the forest floor within two tidal cycles. The dispersed oil was mostly removed from surface sediments and algal mats within 1 week, after which only a noticeable sheen remained. Similar results were obtained in the BIOS and Long Cove studies. The three stud- ies should be distinguished from those that treated oiled intertidal sediment directly with dispersant. Although less of} was retained in the mangrove peat or organic sediments when dispersant was used, the proportion of aromatic compounds retained was greater. Dispersed oil was taken up much more rapidly by mangrove seedlings, but it did less long-term harm because tidal flushing rapidly removed the dispersed of} from the surfaces of the sediments a.n ~ algal mats. However, some undispersed of} accumulated on the wrack at the high-tide swash line. Untreated of! formed a slick over the enclosed area and eventually was pushed well into the mangrove forest by waves and rising tides, and was retained there, contaminating substrate, prop roots, beach wrack, intertidal algal mats, crab burrows, and depressions in the forest floor. Sediments and prop roots at the outer edge of the forest were cleaned of heavy of} contamination after several tidal cycles, but interior areas remained heavily oiled after 1 week. Heavily oiled wrack was evident throughout the site, especially at the high-tide swash line. Inspection of the sites 120 days later confirmed these observa- tions. Where untreated oil contacted the mangrove site, 60 to 70 percent of the plants were dead or defoliated. Where the equal vol- ume of chemically dispersed of} came ashore, there was no evidence of of] Ed very little damage (5 percent defoliation). This is one of the more dramatic examples of how chemical dispersal of of! can diminish the impact compared to untreated of} (Getter and Ballou, 1985~. In Malaysia, another study involving mangrove trees, using of! and Corexit 9527, gave variable results between of] versus oil plus dispersant (Hoi-Chew, 1986; Hoi-Chaw and Meow-Chan, 1985; Lai and Feng, 1985~. Treatments were most toxic to seedlings. Most trials showed either no difference or that oil alone was more toxic than dispersed oil. Mortality was found to be related to surface deposition and uptake. The largest accumulation of of} components occurred in leaf tissue (Lad and Feng, 1984, 1985~. In experiments to find how to save mangroves that were already
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208 USING OIL SPILL DISPERSANTS ON THE SEA oiled, Teas et al. (1987) treated mangrove trees with of} and after ~ day washed them with high-pressure seawater or with a nonionic water-based dispersant. Oil killed many of the trees (within 30 months) whether or not they were spray-washed the next day. To simulate the case in which the mangrove forest was impacted by oil dispersed) offshore, some plots received of} plus glyco] ether- based dispersant. These plots did not show significantly more deaths than untreated control plots. The results of this study imply that it is not possible to save trees already oiled by washing them with dispersant, but that if the of} is dispersed offshore, the trees can be protected. In summary, mangroves can be protected from of} damage by dispersing the of} offshore, but if the untreated of} comes into the forest it can cause serious harm that may require 20 years or more for mature tree regrowth. SUMMARY Boehm (1985) in summarizing the BIOS studies concluded, in agreement with Mackay and Hossain (1982), that chemical dispersion reduced the affinity of of! for solid particles as long as the dispersant- oi} micelIar association persisted. Further, dispersal reduces the probability of an of! mass coming ashore, and often reduces the long- term impact of that which does reach the shore or intertidal zone. This is borne out by most of the field studies reviewed above (Table 4-7~. Dispers~nts never increased sorption of of} to sediments and in a few experiments decreased sorption of of} to organisms. The distinction must be made between dispersed of} coming ashore, which generally did not penetrate sediment, and dispersant applied to an oiled intertidal sediment, where treated of} usually penetrated more deeply than the untreated oil. In subtidal areas, use of dispersants may increase toxicity to benthic fauna and plants, at least in the short term, but may reduce long-term effects of oil. In some intertidal areas, such as mud flats, dispersed oil coming in with the tide had little or no effect, while untreated oil caused longer-term effects (e.g., Long Cove, Searsport, Maine study). Once oil has penetrated salt marshes, the best ap- proach is to leave it alone. Dispersant applications in a marsh show little benefit, as is the case with oil stranding on a beach. Benthic organisms respond differently to oil over the short versus
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214 USING OIL SPILL DISPERSANTS ON THE SEA the long term. Dispersed of! may occur in sufficient concentrations to cause immediate mortality for various groups, including commer- ciaDy important shellfish. Reproduction and feeding behavior may also be negatively affected. Bioaccumulation and tainting occur to a greater extent in the short term in filter feeders when dispersants are used, since concentrations in the water are elevated. Over the Tong term, more bioaccumulation of of} occurs in deposit feeders when dispersants are not used. In general, it appears that intertidal and subtidal macroaIgae (seaweeds) can be damaged by heavy oiling as measured in labo- ratory studies but are often not damaged in field oiling situations. Dispersant use would not increase damage and, based on one study in the United Kingdom (Crothers, 1983), might decrease it. Exper- imental observations and results are scarce and often contradictory; in some field experiments, dispersed of} had no effects on Scolds, whereas in other exposures, red algae suffered tissue damage and inhibited growth when chronically exposed to dispersed crude oil. Only first-generation dispersants have been shown to cause major effects on seaweeds on shorelines. Effects on vascular plants vary depending on species and habi- tat type. For subtidal seagrasses and vegetation in salt marshes, dispersant use directly on vegetation or in shadow waters with low circulation may increase the harmful effects of of! because of in- creased hydrocarbon concentration in subsurface waters and subse- quent absorption by plants. Dispersants are not especially effective in preventing damage to oil-covered plants in low-energy salt marsh environments. Tropical mangroves may be protected if the oil is dispersed be- fore an untreated slick strands within the mangrove forest. This is especially true for mature mangroves. Short-term toxicity to indi- vidual organisms within the mangrove ecosystem may be higher, but con~munitv recovery is enhanced bv the oil being dispersed prior to O ~ —-— — ~ ~ entry. On coral reefs the use of dispersants, if it is able to reduce exposure to oil, wiB benefit the reef in the long run even though there may be short-term deleterious effects on photosynthesis of symbiotic algae within the coral and on other reef organisms. Use of dispersants does not appear to affect homing of salmon.
Representative terms from entire chapter: