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v ASSOCIATET C S I NC . • The initial run of the first-order reaction model indicated that the plume was between approximately 120 feet and 135 feet in length. This approximation closely resembles the actual site conditions observed in December 1997, extending from the southern-most of the former USTs to the north. Therefore the longitudinal dispersivity (8.4 ft) calculated by the model was not adjusted to modify the plume length. • BIOSCREEN matched the observed plume length reasonably well with the first-order reaction model. Ending concentrations (for 1997) appeared to be a good match to actual site conditions. Field observations are not available for the early stages of the plume's development. The instantaneous reaction model predicted higher initial concentrations and a much shorter plume length of approximately 20 feet. The December 1997 plume was estimated by BIOSCREEN to contain 0.015-kg of benzene. BIOSCREEN also estimated that the plume under a no-degradation scenario would contain 0.055-Kg benzene. Therefore, BIOSCREEN indicates that 0.04-Kg, or 73%, of the benzene mass that has left the source since 1986 has biodegraded. The majority of the contaminant mass in the source estimated to be in place in 1986 is still there in 1997 (14.0-Kg versus 13.9-Kg, or 99.3% remaining) and in 2002 (14.0-Kg versus 13.8-Kg, or 98.6%). This data is shown on Figure 7. • BIOSCREEN estimated the current plume contains approximately 0.034 ac-ft of petroleum impacted water with 0.001 ac-ft of water being affected as it flows through the source. Because the plume appears to be at a steady state, if not decreasing, 0.001 ac-ft of water of water becomes contaminated each year with at least the same amount being remediated each year due to in-situ biodegradation and other attenuation processes. • The BIOSCREEN model was extended from 11 years to 36 years, to provide an estimate of the plume concentration 20 years from now. The size of the model was also extended to 250 feet to represent the distance from the former USTs to the facility well. Examining the first-order reaction model results with these parameters shows that the extent of the detectable plume has not changed significantly, while the concentrations estimated within the source and the plume have decreased somewhat. The instantaneous reaction model shows similar results. CONCLUSIONS In order to evaluate the level of risk residual petroleum hydrocarbons at the site pose to human health and the environment, ATC evaluated contaminant transport potential of affected soil and groundwater at the site. w:\17861Veports\rmode1ing.doc 7