SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Ken Gillies Gillies Trucking 3931 Newton Road, Stockton Page 2 of the Modesto Formation will be cleaned by repeated rising and falling of groundwater elevation and that the resulting impacted groundwater will be cleaned by sorption in the various Riverbank units. Based on these results and a reported decrease of diesel- impacted groundwater concentrations from 190,000 parts per billion (ppb) to approximately 10,000 ppb over seven years, UCE re-recommended site closure as a low-risk site. UEC also concluded that the mass of TPH-d impacting soil, is slightly less than 42,500 pounds. The EHD has several concerns with the work performed and the conclusions presented. With a mass on the order of 42,500 pounds of TPH-d in soil of the Modesto subjected to periodic inundation by groundwater, the EHD is concerned about the potential impact to more than 30 water supply wells within 2,000 feet of your site identified by your consultant. The leaching test was designed to utilize impacted soil samples from the Modesto Formation, but as no impacted samples were recovered from the two soil borings, one soil sample identified as Modesto aquitard and three Riverbank soil samples were spiked with diesel fuel to a concentration of 100 milligrams per kilogram (mg/kg)to conduct the test. The EHD is concerned with the results and interpretation of the leaching test as conducted for several reasons: • Intuitively, fresh diesel will have more shorter chain hydrocarbons than weathered diesel, and therefore a somewhat higher solubility in water; • The residence time of the spiked diesel on the soil samples is significantly less than that for the impacted site soil, which may limit absorption and adsorption into microscopic pores in mineral grains and into such naturally occurring organic matter as is part of the soil; and • TPH-d concentrations in some of the Modesto Formation samples near the base of the Modesto sand unit ranged up to nearly 20,000 mg/kg (GT-22) in contrast to the 100 mg/kg spiked samples; • The concentrations of TPH-d in soil samples collected from GT-22, approximately 6.5 years after GT-10 was drilled were not significantly lower than those in the GT-10 samples despite at least 3 additional years of immersion in groundwater; and • The results of the leaching test as performed implies a nearly 100% leaching potential for diesel in the diesel-impacted soil, which in turn indicates an extremely high risk associated with the nearly 42,500 pounds of diesel estimated to be in soil on the site. A low-risk from impacted soil left on site at site closure is more commonly associated with a very low leaching potential for the impacted soil. UEC implies that the reduction of dissolved TPH-d in GT-10 water samples from nearly 190,000 ppb in 1999 to approximately 10,000 ppb in 2006 validates the leaching model and results. The EHD notes that the 190,000 ppb detected in GT-10 water in March 1999 declined to 48,000 ppb in June 1999 and declined to 7,000 ppb by October 1999. The September 2000 sample had 180,000 ppb TPH-d, but three months later the water had 1,000 ppb TPH-d. The 10,300 ppb TPH-d in GT-10 in October 2006 has increased to 33,000 ppb in March