A toxic exposure and mortality model was developed which estimates time, duration, and concentration of exposure to toxic substances and the resulting (acute) mortality. This model will become part of the Natural Resource Damage Assessment Model for Great Lakes Environments (NRDAM/GLE), which is being developed for the Department of the Interior for use in Type A assessments under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA). The physical fates part of the NRDAM/GLE supplies concentration fields in three dimensions and over time. The exposure model (part of the biological effects model of the NRDAM/GLE) uses Lagrangian particles to trace the movements of fish and their eggs and larvae. The time history of each particle determines the time of exposure to toxic concentrations and the average concentration of exposure over the time period. The acute mortality model assumes that mortality is a lognormal function of concentration determined by the LC50 and the normal equivalent deviate, and that LC50 is a log-log function of time of exposure. Corrections for temperature, hardness, and pH are also made. Mortalities are summed through the time period during which concentrations remain toxic and used to estimate effects on the populations.
The exposure model may be used in a model of any aquatic system to estimate both mortality and bioconcentration by aquatic biota, since the model estimates the actual concentrations seen through time. Since biota do not remain stationary in space and concentration fields change over time, this model system allows estimation of the actual exposure of biota. This is a unique approach to toxicity modeling which allows more realistic estimations of toxic response than could be predicted using static models.