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t. %Iwo 7.0 DISCUSSION The simulation assumes that the contaminant mass has not yet been released, and the results predict that the highest impact will occur in the first year after release. In terms of measurable impact to groundwater,the sum of the predicted concentration for the first year of each polygon was used to estimate the total impact. The estimated first year concentration in water was calculated using the following formula.- 1!`year volume in grams * ft3 0:264189 gal * 1000 m Area impacted in ft' 7.48 gal Liter gram Thus; 29.0 g 1 ft' * 0.264189 gal * 1000 MR = 0.59 mg/L 200 ftz* 9ft, * 7.48 gal * Litre 1g In reality, we know that the release has already occurred, and the first year has already passed. We can therefore compare the model results to the laboratory data from the soil samples that were collected in 1999 to assess the accuracy of the model predictions. While the reporting limits of diesel are 1 ppm in soil, the predicted concentration of 8.7 mg/kg at a depth of 25 feet was not realized if one assumes that the release probably began 10 to 20 years ago. Further, it is unlikely that the predicted concentration of 22 mg/kg will be reached.fourty-five years after the release. Hence, it is apparent that the model does not accurately describe the true situation at the site. There are a number of possible reasons why the model overestimates the impact of the release. Most of these can be attributed to missing or imprecise data that are used in the simulation, but some are also likely to be due to erroneous assumption's. One likely source of error is'the'estimated mass of contaminants in place. The mass is a function of both the volume of the impacted area and the distribution and concentration of hydrocarbons within that area. Neither of these variables is well known, and both were probably overestimated. Using a smaller mass of contaminants would have yielded a lower impact to both the soil and the groundwater. Part of the error may stem from the assumption that liquid phase dispersion can be ignored. This would result in higher dissolved concentrations and lower travel times. It is known that even though diesel has relatively low solubility, if will normally diffuse through the subsurface as a solute in soil moisture and groundwater. The model also ignores in-situ degradation of the hydrocarbons. This is a serious flaw in the model, and contradicts other approaches that assume that passive remediation(natural degradation)plays an important role in the destruction of hydrocarbon contaminants. Another possible source of error is the groundwater recharge rate,which was based on an estimate of the hydraulic conductivity. In groundwater flow modeling, the hydraulic s