other hazardous materials were developed for use in Natural Resource Damage Assessments (NRDA) under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) and the Oil Pollution Act of 1990 (OPA) (French and French, 1989; Reed et al., 1989; French and Reed, 1991; French, 1991; French et al., 1996; Reed et al., 1996). There are presently two models in regulation: the Natural Resource Damage Assessment Model for Coastal and Marine Environments (NRDAM/CME, French et al., 1996) and the Natural Resource Damage Assessment Model for Great Lakes Environments (NRDAM/GLE, Reed et al., 1996).

The NRDA models simulate spreading and shoreline stranding of oil. The amount of oil remaining on the shoreline is a function of oil viscosity and shoreline type. Stranded oil is assumed to be removed by waves and other physical processes at a constant rate. The holding capacities and removal rates are based on data collected after spills. Impacts on intertidal habitats, such as salt marshes, mangroves, macroalgal beds and coral reefs, are assumed to be a 100 percent loss if a threshold thickness (dose) is exceeded for any interval of time. The threshold is based on observational data for salt marsh impacts (French et al., 1996).

Modeling Impacts of Surface Oil to Wildlife

Wildlife (birds, mammals, and reptiles) are primarily impacted by direct exposure to floating oil, ingestion of contaminated prey or depletion of food resources. Impacts via a loss of food resources are included in the NRDAM/CME (French et al., 1996), under the assumption that wildlife are food-limited and a proportionate loss of wildlife biomass would result from lost prey production because of a spill. Models used to assess impacts of oil on wildlife populations are summarized in Table 5-2.

In evaluating the wildlife impacts of the Exxon Valdez, Ford et al. (1996) used experimental bird drift and loss rates to estimate the percent of oiled animals that would reach a beach and be stranded. Oiled and dead birds are scavenged and may sink at sea. The percent stranded is related to the trajectory of the carcasses. Ford et al. (1996) used reverse trajectory modeling to determine where beached animals originated, and the percent loss estimates from the drift experiments, to estimate a total kill.

In the NRDAM/CME (French and French, 1989; French et al., 1996), wildlife, oiled and killed, are a function of area swept by surface oil, dosage, and vulnerability. Wildlife are assumed to move randomly within the habitats they normally use for foraging. The dose is estimated from the oil thickness, path length through the oil, and the width of a (swimming) bird. A portion of wildlife in the area swept by the slick is assumed to die based on the probability of encounter with the slick, dosage, and mortality once oiled. Estimates for these probabilities are derived from information on behavior and field observations of mortality after oil spills.

French and Rines (1997) performed hind-casts on 27 oil spills to validate the wildlife impact model. The results showed that the model is capable of hind-casting the oil trajectory and shoreline oiling, given (1) accurate observed wind data following the spill, and (2) a reasonable depiction of surface currents. Since winds and currents are the primary forcing variables on oil fate, obtaining accurate data on these is very important to the accuracy of any simulation. The accuracy of the impact model is primarily dependent on the accuracy of the wildlife abundance data for the time and location of the event. In the validation study, regional mean abundances from literature sources were assumed.

In nearly all cases, impact information for a spill consists primarily of counts of rescued or dead wildlife. Model validation is necessary to illustrate where the model predicts reasonable estimates of impacts on wildlife. Modeling results show that the wildlife impact algorithm in the model is valid when input data on abundance are accurate (French and Rines, 1997). In a few cases, the model estimated more birds killed than were observed. These cases were for species impacts not normally assessed or reported. Even in cases where large efforts were made to recover oiled wildlife, such as following the Exxon Valdez, it is well recognized that many oiled animals are lost at sea or scavenged and not counted directly as oiled. Small and less visible species and

TABLE 5-2 Models Used to Assess Impacts of Oil on Wildlife



Sea birds—Oil spill trajectory model and oil vulnerability index

Samuels and Lanfear, 1982

California sea otter—Sea otter movements and oil spill trajectory model

Brody, 1988

Sea birds and marine mammals—Oil slick encounter and subsequent mortality

Ford et al., 1982; Ford, 1985

Gray and bowhead whales—Oil spill impacts

Reed et al., 1987a; Jayko et al., 1990

Fur seal model—Simulated population processes and mortality due to oiling

Reed et al., 1987b; French et al., 1989; Reed et al., 1989

Sea birds—Estimate numbers oiled from strandings of oiled animals on beaches

Seip et al., 1991; Ford, 1987; Ford et al., 1996

Exxon Valdez—Experimental bird drift and loss rates to estimate the percent of oiled animals that would reach a beach and be stranded.

Ford et al., 1996

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement