SITE INFORMATION AND CORRESPONDENCE (2)

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Ranch Market 2 - 10 October 2016 23569 South Santa Fe Road Riverbank, San Joaquin County During third quarter 2000, which was the first sampling event in which MW-7 was sampled, and was prior to the implementation of remediation at the Site, benzene concentrations in MW-5 and MW-5D were 80 ug/L and 5.0 ug/L, respectively, while benzene in MW-7 was 320 ug/L. These data, along with the historical documentation of a former UST at the 23659 Santa Fe Road property, corroborate the potential presence of a separate source impacting MW-7. The Central Valley Water Board is taking the above information into consideration, and upon further evaluation of the possibility that a second release occurred from the former Barrera's Market, we may open a new case related to this release. 2. Hydrocarbon concentrations appear to be declining in all Site wells except MW-1, in the source area. Concentrations of TPHg, benzene, toluene, ethylbenzene, xylenes (BTEX), and MTBE were all below laboratory reporting limits in May 2013. However, concentrations of TPHg and BTEX have increased incrementally over the past three (3) years, with concentrations of TPHg and benzene at 1,400 micrograms per liter (ug/L) and 71 ug/L, respectively, which are above the Water Quality Objectives (WQOs) of 5.0 and 1.0 ug/L, respectively. MTBE in MW-1 is below the WQO of 5.0 ug/L. Additional groundwater monitoring is needed to observe if the increasing trends continue. All Site wells should be gauged during each semi-annual sampling event, however, due to limited remaining Cleanup Fund budget for this case, semi-annual groundwater sampling should be limited to wells MW-1, MW-2, MW-3, MW-5, MW-5D, and MW-7. Domestic well DW-3 is to be sampled on an annual basis during first quarter. Hydrocarbon concentrations in wells MW-4, MW-6, MW-8, and MW-9 have been below laboratory reporting limits during recent sampling events. Please submit third quarter 2016 groundwater sampling data in a semi-annual Groundwater Monitoring Report due 30 October 2016. Please also perform the first quarter 2017 groundwater sampling event and submit the data in a semi-annual Groundwater Monitoring Report due 30 April 2017. While the Report includes domestic well groundwater analytical data only from DW-4 for the most recent event, in future Groundwater Monitoring Report, please provide all historical domestic well analytical data. 3. It does not appear that the Site meets the Low Threat Closure Policy (LTCP) Vapor Intrusion criteria. While benzene concentrations in groundwater are less than 100 ug/L, and the vertical separation between surface and impacted groundwater is greater than 10 feet, TPHg concentrations in the top 10 feet of soil are greater than 100 milligram per kilogram (mg/kg). TPHg was reported in sample "#3 West End" at 225 mg/kg, collected at 9.5 feet bgs in 1991 from the west side of former UST#3. While this data is old, and soil vapor extraction (SVE) has removed 18,324 pounds of TPHg, no subsequent soil samples have been collected in this area shallower than 15 feet bgs. Additionally, vapor samples collected from the influent stream of the SVE system in April 2007, just before the system was shut down, were reported with a benzene concentration of 0.15 parts per million by volume (ppmv). Using Eurofins online unit conversion calculator, this equals 479.2 micrograms per cubic meter (ug/m3), which exceeds the LTCP Table 1 residential threshold for benzene of 85 ug/m3. The residential threshold has been used for comparison as it is believed that a section of the Site structure is used in a residential