ARCHIVED REPORTS XR0007489 - Laserfiche WebLink

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� y c � B R I A may originate from a former ARCO service station located approximately 300 feet upgradient of the subject site In June 2000, MtBE was detected in four out of seven monitoring wells located at the former ARCO station in concentrations ranging from 20 6 ppb to 3,790 ppb In April 1997, groundwater samples collected from site monitoring wells were analyzed for halogenated volatile organic compounds(HVOCs),and VEW-3 continued to be analyzed for HVOCs quarterly until July 2000, when groundwater monitoring at the site ended HVOCs were detected in Wells MW-2, MW-3, MW-4, RW-1, RW-2, and VEW-1 Trichloroethylene (TCE), tetrachloroethylene (PCE) and cis-1,2-d►chloroethylene (DCE) were detected at maximum concentrations of 9 33 ug/L, 25 ug/L, and 0 63 ug/L, respectively It appears that the HVOCs originated from an offsite source Although there is no soil sample data for HVOCs in the vicinity of the former waste oil tank,the distribution of HVOCs in groundwater at the site indicate that the former Shell waste oil tank is not the source Groundwater samples collected from the wells nearest the former waste oil tank did not contain detectable HVOCs Wells MW-1,MW-5,VEW-1,VEW-2,and VEW-4 provide a non-detect boundary between the former waste oil tank and service bays and the HVOC plume Regulatory Status Review and Recommendations The site appears to meet the CRWQCB criteria for a low-risk groundwater designation As outlmed ►n Appendix B of the "Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sates" a low-risk groundwater case has the following general characteristics o Contaminants remaining in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants, o Separate phase product has been removed to the extent practicable, o No existing water-supply wells,deeper aquifers,surface waters,or other receptors are threatened by pollutants remaining in the aquifer, and o The total pollutant mass remaining in the groundwater is decreasing at predicted rates and neither creates,nor threatens to create,a risk to human health and safety or future beneficial use(s)of the aquifer Each of the low-risk groundwater case characteristics, as they relate to the site, is discussed below Contaminants remaining in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants. Mass balance calculations representing site conditions prior to remediation indicate site soil and groundwater contained approximately 2,754 pounds of TPHg (Attachment A) Soil and groundwater analytical data indicate the bulk of this original hydrocarbon mass was removed by SVE and GWE TPHg and BTEX concentrations have decreased • to near or below detection limits in all of the site monitoring wells While MtBE and chlorinated