ARCHIVED REPORTS_PROPOSED REMEDIATION AUGUST 2009

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Mr.James L. Barton August 31, 2009 Page 2 removal rates have been below one ppd for four of the past six months (March 2009 through August 2009), with a maximum removal rate in this time period of 1.6 ppd. The low mass removal rates correspond with decreased dissolved phase hydrocarbons in the groundwater at the Site. With groundwater hydrocarbons being non-detect in the area that is being sparged, continued air sparging is no longer warranted. Dissolved Groundwater Concentration Trends Prior to the July 31, 2008 startup of the upgraded AS/SVE system, several monitoring wells located in proximity to former source areas (Figure 2) (MW-2, MW-3, MW-81D, MW-11) had concentrations of TPHg and benzene above cleanup objectives. In response to the upgraded AS/SVE system operation, dissolved concentrations of TPHg and benzene have decreased to below the laboratory detection limits of 50 micrograms per liter(µg/L)for TPHg and 0.5 µg/L for benzene in all four of these wells. Concentration trend plots for MW-2, MW-3, MW-8D, and MW-11 are included as Figures 4 through 7. Concentration data trends comparing current data to April 2008 pre-upgrade data are summarized below: • MW-2—TPHg concentration in the third quarter 2009 is below the 50 µg/L detection limit for the first time since 2001. Benzene has been below the detection limit of 0.5 µg/L for two consecutive quarters (2Q09 and 3Q09). In April of 2008, TPHg and benzene concentrations were 15,000 µg/L and 350 µg/L, respectively. • MW-3—TPHg and benzene concentrations have been below the detection limits of 50 µg/L and 0.5 µg/L, respectively, for two consecutive quarters (2Q09 and 3Q09). In April 2008,TPHg and benzene concentrations were 32,000 µg/L and 4,500µg/L, respectively. • MW-8D—TPHg has been below the detection limit of 50 µg/L for three consecutive quarters (1 Q09 through 3Q09). Benzene has not been detected for five consecutive quarters (3Q08 through 3Q09). In the fourth quarter 2008, benzene was non-detect at an elevated reporting limit of 5 µg/L. In April 2008,TPHg and benzene concentrations were 7,300 ug/L and 1,800 µg/L, respectively. • MW-11 —TPHg and benzene concentrations have been below the detection limits of 50 Itg/L and 0.5 µg/L, respectively for four consecutive quarters (4Q08 and 3009). In April 2008, TPHg and benzene concentrations were 5,200 µg/L and 1,300 µg/L, respectively. Recommendation Based on the low influent concentrations and mass removal rates currently achieved by the AS/SVE remediation system, coupled with the non-detect dissolved hydrocarbon concentrations observed in recent groundwater monitoring, AECOM recommends shutting down the AS/SVE remediation system for rebound testing at this time. The proposed rebound test is outlined below: • With approval from the CVWB, shut the system down to allow the subsurface environment time to reach an equilibrium state. • Following a 6-week shutdown, sample groundwater monitoring wells MW-2, MW-3, MW-8D, and MW-11 and analyze for TPHg by EPA Method 8015 and benzene,toluene, ethylbenzene, and total xylenes (BTEX) by EPA Method 8260. • If 'significant' rebound is not observed in the monitoring wells, leave the system shutdown for an extended rebound period extending into the first quarter of 2010 to allow collection of the First Quarter 2010 groundwater monitoring event samples and evaluation of data for rebound. For the purposes of this test, `significant' rebound is defined as 10%of pre-startup TPHg and benzene AECOM Environment I AECOM