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(V\:YATC A S S O C I A T E S I N C Utilizing EPA's BIOSCREEN (v.1.4) software program, ATC evaluated the contaminant transport potential of petroleum-affected soil and groundwater at the site. 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). 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. BIOSCREEN computed the mass flux of benzene in the groundwater to the facility well, located approximately 250 feet away from the source, as between 0 mg/day and 1.5 x 10-5 mg/day in 20 years time, taking only the horizontal distance to the facility well into account and assuming a down-gradient mass flux. ATC has estimated that in order to exceed the target concentration of < 0.001 mg/L benzene in the facility well, a very high mass flux of benzene would be needed, which would need to reach the well's screened interval through two intervening layers of clay and another aquifer. This appears to be an unlikely scenario for the project site. RECOMMENDATIONS Based on the evaluation performed by ATC, the project site does not appear to be an immediate and significant threat to human health or the environment. Furthermore, the plume appears to be at a steady state, if not decreasing in size, as 0.001 ac-ft of water of water becomes contaminated and is subsequently remediated each year through due to in-situ biodegradation and other attenuation processes. Because of this, active remediation at the site does not appear to be warranted. Based on confirmation groundwater data, this site should be evaluated for closure under low risk groundwater criteria as defined under No Further Action (NFA) requirements, outlined by the Central Valley Regional Water Quality Control Board's Appendix B of the Tri-Regional Recommendations. w:\17861\reports\modeIing.doc 8