ARCHIVED REPORTS - Laserfiche WebLink

Home Browse Search

' Pacific Mini Mart � � Page 2 of 3 4511 Pacific Avenue, Stockton, CA October 29, 2008 bsg either were not impacted or were not analyzed. Therefore, the only points of comparison that can be used to evaluate the effectiveness of the SVE system are from the two adjacent borings B-4 (advanced in October 1992) and B-21 (advance in October 2007). Comparing the TPH-g concentrations at 25 feet bsg, B-4 contained 1,677 mg/kg and .B-21 contained less than.1.0 mg/kg, which suggests that the SVE system effectively removed the contamination from the vadose zone soil above 35 feet in the B-4/8-21 area. AGE made no recommendation concerning what to do with the SVE system which is currently not in operation. in the SRCR, AGE recommends preparation of a closure summary report when dissolved petroleum hydrocarbon concentrations in groundwater samples from vapor well VW-1 have achieved compliance with water quality objectives (WQOs) or when levels have been reduced to the point that WQOs can be achieved by natural attenuation. The EHD believes that the site is not ready for closure consideration at this time. The analytical results from the confirmation boring soil samples from saturated soil below 35 feet bsg showed contaminant concentrations exceeding the SFBRWQCB ESLs for leaching potential into groundwater and dissolved contaminant concentrations exceeded the Central Valley Regional Water Quality Control Board (CVRWQCB) WQOs. Groundwater contamination in MW-5 may be rebounding since the shutdown of the GWE system in April 2006, as shown by rising concentrations of TPH-g, which increased • from less than 50.0,ug/I in April 2006 to 8,500 ug/I in October 2006. During March 2008, TPH-g concentrations in MW-4, MW-5, and VW-1 were 200 pg/l, 3,900 pg/I, and 900 pg/I, respectively. It appears that the groundwater plume is defined laterally cross gradient by MW-1, MW-3, and MW-6, upgradient by MW-7, and downgradient by MW11A, MW-1.1 B, MW-12 A and B, and MW-13 though MW-18 with no detections of the constituents of concern since October 2006. The EHD believes that least two more quarters of monitoring data (4th quarter 2008 and 1st quarter 2009) will be needed to evaluate contaminant concentration trends before closure consideration. The sampling frequency for unimpacted monitoring wells can be reduced. As discussed by telephone with your consultant on June 18, 2008, total ` petroleum hydrocarbons quantified as diesel (TPH-d) can be eliminated from the suite of . constituents for routine sample analysis., If the site contaminant plume is stable and declining in extent and concentrations in the absence of active remediation, this site may be a good candidate for closure. If site conditions appear to be favorable for closure after the next two quarterly monitoring events, a closure summary report may be submitted to the EHD for review. The closure report should address residual contamination issues, including an estimate of the contaminant mass remaining verses the contaminant mass removed from remediation activities, fate and transport modeling, vapor intrusion potential, the projected time to return to background conditions, and why the site can be . considered a low-risk site despite locally exceeding SFBRWQCB ESLs and CVRWQCB WQOs. Site closure is dependent on favorable resolution of these issues and confirmation that the dissolved plume is stable and declining without active remediation. If site conditions are not favorable for closure, additional remediation may be needed, to achieve WQOs in a timely manner. In summary, the EHD directs at least two more quarters of monitoring data (4th quarter 2008 and 1" quarter 2009), the sampling frequency for non-critical monitoring wells may be reduced if requested and justified by your consultant. Total petroleum hydrocarbon quantified as diesel (TPH-d) can be eliminated from the suite of constituents for routine sample analysis. 081029directive