SITE INFORMATION AND CORRESPONDENCE

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Mr. Paul Supple Arco Station No. 6100 25775 Patterson Pass Road Page 5 of (SAWI Report), vapor wells VW-18, VW-19 and VW-20 did not penetrate the sand/silt contact that yields gasoline and water in MW-1. EHD believes that the site is characterized well enough at this time to conclude that: • The fuel release(s) from the UST system on the site has significantly impacted subsurface soil and perched ground water, • The vertical extent of impacted soil on the site is as great as 105 feet bsg and is as great as 201 feet bsg offsite, • Regional first ground water onsite has not been assessed and therefore the impact of the release(s) on first ground water is not known, • The plume is not stable due to migration of perched water and free product, • The documented migration direction, demonstrated by the free product in MW-1, is southeasterly, toward the adjacent shallow alluvial canyon which may be a recharge area for ground water migrating into the Central Valley immediately on the east, • The lateral and vertical extent of impacted soil and ground water toward the southeast is not known, and • MTBE is a contaminant of concern, therefore • The fuel release on the site poses a significant threat to ground water and should be remediated. EHD believes the threat to ground water posed by the release(s) on the site should be immediately reduced by taking the following actions: • Utilize the soil vapor extraction system that has been installed on the site to reduce the contaminant mass in the vadose zone. EHD notes that a soil vapor extraction (SVE) test was performed by Resna, Inc. in 1991, and SVE was evaluated to be an effective remedial method for the site. As feasibility studies and a corrective action plan have not yet been completed, operation of the SVE system should be implemented as an interim remedial action; • Free product removal from MW-1, also an interim remedial action, should continue; • The lateral and vertical extent of the offsite impact of the release(s) should be assessed as expeditiously as possible. EHD again notes that the lower sand encountered in MW-1 may not have been encountered in vapor wells VW-18, VW-19 or VW-20 as shown on cross section B-B'. The assessment should include determining the down-gradient extent of free product, its preferred migration route and its impact on soil and ground water.