SITE INFORMATION AND CORRESPONDENCE_FILE 3

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• Page 1 of 2 Vicki McCartney [EH] From: Vicki McCartney [EH] Sent: Thursday, January 24, 2013 4:35 PM To: Daren Zumberge; 'Ami Adini' Cc: Shayan Simantob; Elie Bales; 'Gus Osterkamp'; Gabriele Baader; Nuel Henderson [EH] Subject: 425 Larch Road in Tracy, CA Dear Ms. Daren: As the responsible party for the Frontier Transportation property at 425 Larch Road, Tracy, California, on 23 January 2013, you requested a discussion with me regarding your site and proposed corrective actions for the site. Today, I returned your call along with Nuel Henderson, PG. You stated your concern with the pace of work on your site and utilizing the remaining funds efficiently from the Underground Storage Tank Cleanup Fund (USTCUF)to complete corrective actions on your site and achieve site closure; you solicited the opinion of the San Joaquin County Environmental Health Department(EHD) on which course of action would be most effective for this site. Prior to returning the phone call, EHD personnel reviewed the site for closure consideration to determine if the site could be closed under the recently adopted Low Threat Closure Policy (LTCP)of the California State Water Resources Control Board (SWRCB). The EHD found that the site does not meet all the LTCP criteria for closure under the policy. The deficiencies noted included the following: 1. The plume of impacted groundwater is not fully delineated in the down-gradient direction; 2. Plume length exceeds 250 feet, therefore methyl tertiary-butyl ether(MTBE) concentrations cannot exceed 1,000 micrograms per liter(Ng/L), but in the most recent sampling event dated 18 September 2012, MTBE concentrations were as high as 7,470 Ng/L; 3. Five down-gradient water supply wells have been identified as potentially being threatened by the plume of impacted groundwater, the closest being within 1,000 feet of the known down-gradient extent of the groundwater plume; and 4. Excess benzene concentrations in soil gas have been identified as a potential health risk through vapor intrusion. The SWRCB, through its Five-Year Review, last recommended (October 2011)that the EHD direct active remediation be implemented on the site to achieve Water Quality Goals in a timely manner. Based on the last recommendation from the SWRCB and the review of the LTCP criteria, the EHD is of the opinion that additional remediation of the source area should be implemented as quickly as possible to address the issues noted above and achieve site closure. Review of the initial 30-day dual phase extraction (DPE) pilot test shows that the test had positive itivea sulresults; over 2,000 pounds of contaminants[total petroleum hydrocarbons as gasoline (TPH-g)]were and the mass extraction rate declined from over 250 pounds of TPH-g per day to less than 10 pounds per day. With an approximately 10-foot vadose zone (probably increased somewhat during active groundwater extraction) it would intuitively seem that there is a fairly limited contaminant mass stuck (sorbed)to soil to extract, and extracting over 2,000 pounds with the decline of mass extraction rate noted above suggests to the EHD that much of the extractable contaminant(through vapor extraction) has been removed. If this is so, then it would seem that only a limited amount of additional extraction would be needed to reduce the sorbed mass to the extent practicable; this leads the EHD to the opinion that implementation of the second 30-day DPE pilot test may be adequate to address the vadose zone sorbed contaminant mass. Dissolved contaminant concentrations in some wells increased following the initial 30-day DPE pilot test, but this may be due to the groundwater extraction (GWE) portion of the DPE drawing more intensely impacted groundwater to the extraction points. GWE is generally not the preferred method for addressing sorbed contaminant mass in the saturated zone, but as MTBE is the contaminant with excessive dissolved concentrations and MTBE has little sorption to soil and is one of the most mobile contaminants, the GWE portion of the DPE may prove to be effective at reducing the concentrations of this contaminant 1/25/2013