ARCHIVED REPORTS XR0001439 - Laserfiche WebLink

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• Work Plan for Additional Subsurface Assessment,Former Unocal Station 40123 , October 5, 1999 survey of the parking lot to determine if any abandoned USTs existed beneath the site, and exploratory borings to collect soil and groundwater samples The underground survey detected no large metal anomalies, and concluded that there were no USTs beneath the site However, hydrocarbon-impacted groundwater was detected in the northern portion of the site - A Phase I Environmental Site Assessment report dated December 16, 1994, was prepared by Enviros Inc for Unocal The study identified twelve sites in the vicinity of the subject site that had impacted soil and groundwater None of the sites were located in the immediate upgradient direction from the subject site Two of the sites identified were former service stations that were located across from and downgradient of the subject site a former Shell Oil service station at the northwest corner of 11th Street and Central Avenue (operating from approximately 1949 to 1962), and a former service station (possibly Chevron) at the northeast corner of 11'h and Central Avenue (operating from approximately 1913 to at least 1962) In May 1995, nine exploratory soil borings were drilled and three groundwater monitoring wells (U-1 through U-3) were installed by Pacific Environmental Group, Inc (PEG) at the subject site Exploratory test pits were dug and two abandoned USTs were encountered at the northern property boundary, beneath the 11'h Street sidewalk In November 1996, the two abandoned 550-gallon USTs were removed A third UST that was encountered during the removal activities was abandoned in place by grout injection One groundwater monitoring well (U-4)was installed in December 1996 in the location of the removed USTs to evaluate soil and groundwater conditions On October 7, 1998, PEG drilled and installed two groundwater monitoring wells (U-5 and U-6) and one soil boring (B-4) Soil samples collected from the borings had detectable concentrations of total petroleum hydrocarbons as gasoline(TPHg)(220 parts per million (ppm) at 11 feet bgs in U-6) and total petroleum hydrocarbons as diesel (TPHd)(21 5 ppm at 10 feet bgs in B-4 and 12 3 ppm at 11 feet bgs in U-6) Benzene, methyl tert-butyl ether (MtBE), lead, or volatile organic compounds (VOCs) were not detected in the soil samples Grab groundwater samples had concentrations of TPHg (968 parts per billion (ppb) in U-6) and TPHd (115 ppb in B-4 and 312 ppb in U-6) Benzene, MtBE, or lead were not detected in the groundwater samples Groundwater monitoring and sampling has been conducted quarterly since June 2, 1995, in wells U-1, U-2, and U-3, since March 3, 1997, in well U-4, and since October 22, 1998, in well U-5 and U-6 In the latest event on June 14, 1999, concentrations of TPHg(I 10 ppb in U-1 and 1,400 ppb in U-4), TPHd (720 ppb in U-4), benzene (1 3 ppb in U-1 and 5 5 ppb in U-4), and MtBE (2 9 ppb in U-1, 5 6 ppb in U-2, and 2 5 ppb in U-6) were present Also, the halogenated volatile organic compounds (HVOCs) cis-l,2-dichloroethene (0 67-19 ppb), tetrechloroethene (2 1-6 2 ppb), and trichloroethene (0 55-2 5 ppb) were detected Depth to water during this monitoring event ranged from 9 33 to 10 45 feet bgs and groundwater flow direction was to the north- northwest, which is consistent with available historical data SCOPE OF WORK To further evaluate the distribution of residual petroleum hydrocarbons in soil and groundwater downgradient of the subject site, GR proposes to advance one soil boring to approximately 12 • 240073 03 2