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Remedial Action Plan RE: 425 W. Larch Street, Tracy, CA October 1 , 2008 W&A Project No. FTIM-08-001 Page 6 of 11 5 EVALUATION OF REMEDIAL ALTERNATIVES The purpose of this RAP is to identify a remedial technology, or a combination of technologies, capable of mitigating petroleum hydrocarbon impact in the vadose, capillary fringe, and saturated zones beneath the site. Commercially available and commonly used remedial technologies were evaluated based on site-specific conditions for technical feasibility, implementability, regulatory acceptance, and cost-effectiveness. The objectives of the remedial altemative(s) chosen will be to reduce hydrocarbon impact in groundwater and soil. Certain site specific conditions listed below were taken into consideration in evaluating the remedial alternatives. • The diesel and gasoline USTs have been removed from the site. • The site is underlain by predominately fine grained soils to approximately 26 to 28 feet bgs. • The majority of the petroleum hydrocarbon impact appears to remain within the fine grained soils located at the water table interface (approximately 8 to 12 feet bgs) . • Fluctuations in groundwater elevations appear to impact concentrations of petroleum hydrocarbons (creating a smear zone) detected in monitoring well samples, indicating that significant residual mass is adsorbed to soil. • Primary chemicals of concern (COCs) of concern are MtBE and gasoline-range organics (GRO). BTEX constituents are limited to the eastern end of the property, in the vicinty of wells MW-2 and MW-3R. • Groundwater pump test (November, 2006) data indicated that pump and treat appears to be a viable remedial alternative. Table 2 presents the assumptions and costs for the remedial alternatives compared in this section. 5.1 Monitored Natural Attenuation (MNA) i Natural attenuation is a combination of physical, chemical, and biological processes that, under favorable subsurface conditions, can reduce the dissolved-phase hydrocarbon mass without active remediation. Natural attenuation is primarily dependent on subsurface biochemical and geochemical processes to aid in natural biodegradation, volatilization, and dispersion of petroleum hydrocarbons. The advantages of natural attenuation are that the overall project costs can be relatively low, there is no disturbance to the site or adjacent properties, and no remediation wastes are generated. The time required for petroleum hydrocarbon concentrations to meet clean-up goals by attenuation is generally shorter if active remediation is implemented. Significantly elevated concentrations of petroleum hydrocarbons are present in wells MW-2 MW-3R, MW-4, MW- 19, MW-21 , MW-22, MW-23 , and MW-24. Given the current attenuation rates observed at the site to date (1996 to 2008), natural attenuation does not appear to be the most appropriate remedial alternative at this time. If an MNA approach were to be implemented, the cost of groundwater monitoring, sampling, and reporting for a period of 20 years is estimated to be approximately 568"00. These costs assume quarterly monitoring and sampling of the existing 26-well network. The period 20 years is an estimate only-since actual attenuation rates and projections cannot be made on the data set available at this time. WIp EEIELU E ASSOCIATES,PTES, INC. INEFIELD nye OCIATEs Ings