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7ontinue SGYENS.t016LL M75 decay rates of benzene, it is estimated 12 years ould be required to attain cleanup levels Groundwater monitoring woulduring t period The program BIOSCREEN was used te movement and biodegradation of benzene in the zone 4 aquifer under the influence of advection, dispersion, sorption and biodegradation. Benzene was used as the modeled chemical because it is the most soluble and mobile of the BTEX compounds. BIOSCREEN uses the Domenico (1987) analytical solute transport equation model to predict chemical concentrations. The results of modeling are presented in Appendix A The purpose of the modeling was to 1) predict the future extent and concentration of a dissolved contaminant plume, 2) to assess the possible risk to potential downgradient receptors 3) and to provide technical support for intrinsic remediation ' Benzene concentrationr p edications were made for 10, 20 and 30 years after a petroleum release The results of modeling indicate that the dissolved BTEX groundwater plume will not extend more than 300 feet beyond the source area and that it is very unlikely that the plume will pose a risk to potential downgradient receptors Evaluation Effectiveness This alternative would be effective in protecting human health and the environment There are no human receptors nearby who would be exposed to the contaminated groundwater during the period it is undergoing remediation There are no surface water bodies nearby that could be impacted by this groundwater Groundwater in this area moves slowly (6 7 feet per year) and as a result, is not likely to impact distant receptors The effectiveness of this alternative will be ensured by monitoring If monitoring shows this alternative to be ineffective, additional remediation options could be employed without much risk due to the slow groundwater migration rates. The BTEX compounds comprise one of the most optimum contaminant classes for bioremediation BTEX degrades readily under correct conditions; these favorable conditions are present at the site Implementabdity This alternative would be the least intrusive for this very high traffic (both vehicle and pedestrian) area with access constraints This alternative would be easy to implement and is anticipated to require only two more wells and no other construction which would impact local services Implementation would require development of a long-term monitoring program and performance of 5-year reviews Groundwater sampling and monitoring of the contaminant levels in the groundwater would be conducted for an approximate 10- to 12- year period The 5-year reviews would examine the current plume location and determine if any new downgradient receptors are present which may require an alternate course of action. Cost For cost estimation purposes 10 years of groundwater monitoring was assumed The overall cost of this option was estimated to be $273,000 Costs are shown in Appendix B, Table 2 UPROJECTS\SAN10Q W WA=ONPL 38