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Response to Delay No Further Action Required Decision And Closure Request Moore Petroleum,Inc (Frank's Exxon), 14800 West Frontage Road,Manteca,California Page 2 groundwater zone at the site The concentrations of TPHg, 1,2-DCA and MTBE in the shallow groundwater have been reduced to below 50 parts-per-billion(ppb), below 5 7 ppb and below 16 ppb, respectively (Appendix B, Table 2), since the commencement of the remedial activities on site The results of the fate and transport study indicate that the newly installed domestic well on site, and the proposed new domestic well off site, will not be adversely impacted by the subsurface environmental conditions at the subject site Apex therefore requests that "No Further Action" letter be issued to the subject site 1. THE AMOUNT OF REMAINING CONTAMINATION ON SITE Apex has performed a mass balance calculation to estimate the residual petroleum hydrocarbons remained in the subsurface soil and groundwater at the subject site The results of the calculation indicate that remaining constituents in the groundwater are 0 068 pounds of MTBE, 0 032 pounds of 1,2-DCA and 0 56 pounds of total petroleum hydrocarbons, as diesel (TPHd) There are also 0 01 pounds of 1,2-DCA remaining in soil at the subject site Please refer to Appendix B for detailed mass balance calculation and assumptions 2. CONTAMINANTS THAT MAY IMPACT THE ON-SITE DOMESTIC WELL In October 1997, the site owner replaced the former domestic well at the site with a new well The new well was completed to 360 feet below ground surface (bgs) The well is screened from 340 feet bgs to 360 feet bgs, and was sealed from ground surface to 220 feet bgs Water samples were collected from the new domestic well after the completion of the well in November 1997 No petroleum constituents were detected in the water samples Fate and Transport Modeling for Domestic Wells On and Off site The computer model used in the groundwater chemical fate and transport study for the subject site is RiskPro AT123D (AT123D) developed by General Sciences Corporation, a subsidiary of Science Applications International Corporation The AT123D model is an analytical groundwater transport model It computes the spatial concentration distribution of chemicals in the aquifer system and predicts the transient spread of a contaminant plume through a groundwater aquifer The fate and transport processes accounted for in the AT123D model are 1) Advection of the chemical with the water flowing through the aquifer, 2)Dispersion of the chemical, including hydrodynamic dispersion and molecular ddTiision, 3) Adsorption, and 4) Decay The AT123D model was used to estimate the impact to the domestic wells on and offsite, as a result of the subsurface environmental conditions at the subject site The domestic wells included in the study are the old domestic wells both on and off site (assuming they were not S� replaced with the new wells), and the newly-installed/proposed deep wells on and off site The JY C\ PROJECTIE'S\ERA02 009-Moore Petroleum,Manteca\ResponsetoRegionalBoard\Response doc