some of the limits of cleanup are. For example, in hydraulic washing in biologically-rich areas, water tem-peratures should not exceed 40℃ (Mauseth et al. 1996). Some shoreline chemical cleaners are more toxic than oils, others much less so (sensu Fingas et al. 1995). Thus, in biologically-rich areas, only the least toxic ones should be used. Some bioremediation agents contain toxic materials so their use should be restricted in bio-logically-rich areas. On the other hand, burning heavily-oiled marshes, while very destructive to vegetation, can quickly reduce the risk of oiling to migratory birds and still result in rapid and complete recovery of the marsh (Mendelssohn et al. 1995). Thus, each method has its limits.
The real question during a response is "How Clean is Clean Enough?". That has to be decided by consensus, but that consensus should now include an evaluation of the sensitivity of various shorelines and the natural processes that will ultimately clean up the remaining oil.
4.0 Bioremediation
What is bioremediation and what is its role in shoreline cleanup? Bioremediation is our attempt to accelerate the natural biodegradation of oil in the environment. Biodegradation is the process by which bacteria chemically degrade specific compounds in oil. Compounds amenable to natural or enhanced degradation include alkanes and polycyclic aromatic hydrocarbons (PAH's). Compounds that are not amenable to enhanced degradation (in the time frame of a response or restoration activity) include asphaltenes and waxes. Of these, the PAH's are of most ecological concern because they are toxic and carcinogenic. Therefore, an effective bioremediation activity is not necessarily one that removes the oil, per se, but one that substantially accelerates degradation of the PAH'S.
Until the EXXON VALDEZ oil spill, many believed that it was necessary to add oil-degrading bacteria to the sea coast to accelerate the degradation of oil. Many people also believed that using microbes, oil could be degraded in a few days. In the laboratory, the culture and addition of oil-degrading bacteria to oil does, in many cases, accelerate the degradation of oil in controlled experiments, even over a period of a few days. However, the sea coast is not a controlled experiment. Weather, currents, wind, waves, rainfall, and microbial competition and predation all act to limit the usefulness of adding bacteria or other materials to an oil spill. Numerous shoreline oiling experiments in Alaska, Canada, the lower US and Europe have all shown that oil degrading bacteria are present everywhere along the coast and that the primaryfactors limiting biodegradation are nutrients (N,P), oxygen (in anaerobic marsh soil) and the oil itself (some are more degradable than others) (Swannell et al. 1996 ; Venosa et al. 1996 ; Hoff 1993 ; Hoff et al. 1995). Therefore, most active researchers today support the concept that to accelerate biodegradation it is necessary to supply only those factors that are limiting : in many cases this will be nothing. In a few cases it may be necessary to add nutrients (nitrogen, phosphorus) or oxygen (via chemical or physical aeration; Swannell et al. 1996). The best way to determine what is limiting is to measure existing concentrations of nutrients and oxygen in oiled shoreline sediments. For example, Venosa et al. (1996) suggest that in beach sand, soluble nitrogen may be limiting if it is below 2 mg / L in the sediment. In that case, nitrogen could be supplied continually in a dissolved state (as described by Venosa et al.1996) or in "slow-release" forms such as prills, pellets or briquettes.
Shoreline bioremediation experiments conducted by Venosa et al. (1996 ; also cited in Mearns et al. 1997) on a sandy beach in Delaware (USA) demonstrate the attributes of successful bioremediation and how it compares to natural processes (Figure 3). Using a randomized-block design they subjected inter-tidal plots of Nigerian oil to no treatment, nutrient treatment only, and treatment with oil-degrading bacteria-plus-nutrients. The experiment lasted 3 months. Half the oil was removed from the shoreline every 28 days simply due to physical washing and weathering processes. Alkanes and PAH's in the oil that remained on the shoreline were degraded naturally (with no nutrient or bacteria added) with additional half-lives of about 28 days. Added nutrients doubled the rate of degradation of alkanes (from 28 to 14 days ; and increased the rate of degradation of PAH's by about 50% (Figure 3C and D)). Addition of oil-degrading bacteria did nothing. In fact, within a few days of simply adding the oil, Venosa et al. (1996) found that natural concentrations of alkane- and PAH-degrading bacteria were about one million organisms per gram of sand (Figure 3E and F). In addition, 4 out of 5 bioassays revealed that regardless of treatment, oiled beach sediment rapidly lost its toxicity to microbes, sea urchin larvae and shrimp embryos (Figure 3G through J). However, even a low weathered oil concentrations, the oiled sediments were still mod-erately toxic to amphipods (Figure 3K).
