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CONSULTANTS, INC. GEOTEC H N ICAL ■ ENVIRONMENTAL ■ MATERIALS Project No. E8323-06-01 August 23, 2007 Mr. Mike Infuma �^ San Joaquin County Environmental Health Department �^ 304 East Weber Avenue, Third Floor Stockton, California 95202-2708 Subject: ADDITIONAL INVESTIGATION WORKPLAN RANCH MARKET G Fy 23569 SOUTH SANTA FE ROAD RIVERBANK, CALIFORNIA USTCF CLAIM # 15211 Dear Mr. Infuma: Geocon has prepared this Additional Investigation Workplan for the Ranch Market site located at 23569 South Santa Fe Road in Riverbank, California (Figure 1). This workplan was prepared in response to the Geocon recommendations contained in the July 20,2007,Groundwater Monitoring and Remedial Status Report, First Quarter 2007, and approved by the San Joaquin County Environmental Health Department(SJCEHD)on July 27, 2007. A copy of the SJCEHD approval letter is attached. INTRODUCTION In May 1991 an unauthorized release was discovered when three underground storage tanks (USTs) containing gasoline were removed from the site. Subsequent subsurface investigations partially defined the extent of petroleum impacts in soil and groundwater, resulting in the installation of groundwater monitoring wells MW-1 though MW-9. The contaminants of concern associated with the release are total petroleum hydrocarbons as gasoline (TPHg) and benzene. Toluene, ethylbenzene, xylenes, and methyl tertiary butyl ether(MTBE) are also present in groundwater within the impacted area, however theses concentrations, in most instances, are below drinking water maximum contaminant levels (MCLS). In 2000 and 2002, Secor International, Inc. (Secor) constructed soil and groundwater remediation systems at the site. The remediation effort included the use of soil vapor extraction (SVE), air sparge, and ozone injection remediation technologies. Ten soil vapor extraction wells, one air sparge well, and 20 ozone sparge wells were constructed at the site in conjunction with the soil and groundwater cleanup effort. After several years of active soil and groundwater remediation contaminant concentrations in shallow depth groundwater located within 60 to 100 feet of the former UST excavation appear to have reached asymptotic levels. While past remediation efforts have reduced the originally estimated contaminant mass in soil and groundwater by as much as 75 percent, contaminant concentrations in groundwater nearest the former UST excavation have remained near current concentrations for the past several years, while concentrations in monitoring well MW-7 (located approximately 235 feet south of the former UST excavation) have increased during the same time period(see Table 1). 6671 Briso Sheet 0 Livermore,CA 94550-2505 ■ Telephone(925) 371-5900 ■ Fax(925) 371-5915