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' No Further Action Request Boyett Petroleum,419 S.Main Street,Manteca October 7,2005 ' Page 4 11. Best Available Technology (BAT) used. Contaminant concentrations in groundwater have been ' reduced by natural attenuation to below the California primary drinking water Maximum Contaminant Levels (MCLS). Active remediation methods have not been employed at the site. 12. Reasons why "background" was/is unattainable using BAT. Condor anticipates that natural ' attenuation will decrease the remaining contaminant concentrations to background levels over time. Using active remediation methods to further reduce the existing contaminant concentrations would not be economically feasible. ' 13. Mass balance calculations of substance treated versus that remaining. Condor estimates the mass of MTBE in groundwater in October 2003, based on the results of the groundwater investigation conducted between October 20 and 23, 2003, was approximately 3.89___,pounds. MTBE was detected in ' the groundwater samples collected from monitor wells MW-9 and MW-11 on June 30, 2005, at concentrations of 1.1 µg/L and 3.1 µg/L, respectively. Both wells are screened between clay beds that are present at depths of approximately 25 and 50 feet bgs. MTBE was not detected at or above the ' laboratory reported detection limits in well EW-1, which is also screened between the two clay beds. MTBE was not detected in any of the groundwater samples collected from the remaining monitor wells, all of which are screened above the upper clay. The distance between MW-11 and EW-1 is ' approximately 50 feet. Using this distance as the assumed radius of the MTBE plume, and an average thickness for the plume of 50 percent of the former plume, Condor estimates the remaining mass of MTBE in groundwater at the site is approximately 0.03_pounds. This represents a reduction of 3.86 pounds in the mass of MTBE in groundwater at the site since October 2003. This is a 99 per~ ' reduction in mass. Mass calculation sheets are in Appendix H. 14. Assumptions, parameters, calculations, and mode used ed in risk assessments, and fate and transport modeling. Not applicable. Risk assessment and fate and transport modeling have not been ' . performed. 15. Rationale why conditions remaining at the site will not adversely impact water quality, health, or other beneficial uses.BTEX and TPH-G have not been detected in groundwater samples collected at the site since September 2002_ Concentrations of MTBE in groundwater samples collected from the monitor wells have decreased from a high of 27,000 µg/L in June 2001 to 3.1 }Lg/L in June 2005. The reduction in BTEX, TPH-G, and MTBE concentrations is likely due to intrinsic bioremediation. ' Intrinsic bioremediation will likely continue to occur within the aquifer at the site to further reduce the mass of contaminants remaining in groundwater. The concentrations of MTBE detected in the groundwater samples collected from the monitor wells in June 2005 were below the MCL of 13 µg/L. i16. WET or TCLP results.Not applicable. These analyses were not performed. ' 4.0 CONCLUSIONS It is Condor's opinion that the site has been adequately characterized and remediated and does not pose a ' threat to water supply wells, deeper drinking water aquifers, surface water, or other sensitive receptors. Condor recommends the site be considered for closure. 5.0 LIMITATIONS ' This NFAR was prepared under the direct supervision of a Professional Geologist in the State of California. The standard of care for all services performed or furnished by Condor is commensurate with ' u %ui% auu Jrtitlt int utttat tly uo�u uy u ni�ut�tt �t itt� iitvirviuit%u�at Nivt�oa�vtt Nru�La�tur, uiiUs l oiuutaa 1 CONDOR 1