ARCHIVED REPORTS XR0013118 - Laserfiche WebLink

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Site Closure Request Pombo Property June 14,2005 NaWl The results of historic soil and groundwater sample analyses are provided in Tables 1 and 2. 11. Discuss concentration.and mass changes over time, and current concentrations of contaminants remaining in groundwater at the site. Active remediation has not been implemented at the site. The off-site groundwater plume has been delineated,with concentrations ranging from low-to non-detected. Grab groundwater samples taken from two borings reported no-detectable results for BTEX compounds in one(GMX-6)and low concentrations in the second(GMX-29: 0.6 µg/L benzene,2.6 pg/L toluene, 1.6µg/L ethylbenzene, and 4.0 pg/L xylenes),all well below WQO concentrations. TPH compounds were not analyzed for in GMX-6;concentrations of 140 pg/L TPHg,660µg/L TPHd, and 160 µg/L TPHd(following silica-gel cleanup)were reported for GMX-29. Low concentrations of two PAHs were detected for both grab groundwater samples. 12. Isoconcentration contour maps. An isoconcentration contour map is provided on Figure 4. 13. Summary of remedial methods used to clean up the site. Active remediation has not been implemented at the subject site. Petroleum-impacted soil was excavated and removed from western portion of the site during site development of a stormwater retention basin in August 2001. Excavated soil was reused at an adjacent parcel as fill beneath an asphalt paved parking lot. A summary of excavation activities and Geomatrix's October 8,2001 confirmation sampling is captured in a letter to the DTSC, "Response to Comments-January 2, 2002 Letter",dated May 9,2002 (Geomatrix,2002)and included in Appendix C. 14. Discussion of whether background is unattainable using best available remediation methods. Active remediation has not been implemented at the site. Though excavation of petroleum-impacted soil is an effective remediation method,soil and groundwater concentrations of petroleum hydrocarbons exhibited at the site do not appear to be migrating and the presence of petroleum hydrocarbon concentrations in soil and shallow groundwater at the site should not pose an unacceptable non- carcinogenic or carcinogenic risk to future on-site residential receptors or unacceptable human health risk to current or future commercial workers at the site. 15. Discussion and estimate of contaminant mass remaining in soil and groundwater versus contaminant mass removed or destroyed by soil excavation or remedial actions. Remedial action has not been implemented at the site,however petroleum-impacted soil was removed from the western portion of the property to 15 feet bgs during construction of a 50,000 square foot storm water retention basin in August 2001. For the petroleum-impacted soil remaining on-site,results of the updated HHSE(Geomatrix,2001)indicate the presence of petroleum constituents measured in soil and shallow groundwater at the site should not pose an unacceptable non-carcinogenic or carcinogenic risk to future on-site residential receptors or unacceptable human health risk to current or future commercial workers at the site. The Geomatrix report is included as Appendix C. 16. Assumptions,parameters, calculations,and the model used in any risk assessments. Assumptions,parameters,calculations,and the model used in the Geomatrix"Results of the Updated Human Health Screening Risk Evaluation"are provided as Appendix C. 17. Assumptions,parameters, calculations and the model used in fate and transport modeling. Fate and transport modeling was not performed. 18. Rationale why conditions remaining at the site will not adversely impact water quality,human health and safety. 6