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INTRODUCTION The following report addresses the only remaining concern "the off-site TPHg plume has not been laterally or vertically defined", expressed in the July 18, 2000 Nonconcurrence letter from California Regional Water Quality Control Board —Central Valley Region (CRWQCB-CVR) All other concerns were addressed in the February 20, 2001 "Workplan for Vertical and Lateral Extent Sampling Downgradient of MW12" • The groundwater has not been monitored for all fuel oxygenates and fuel additives, • The status of a domestic water supply well southeast of the site has not been determined; and • The quarterly groundwater morutoring has not been conducted since January 1999 (1 "The off-site TPHg plume has not been laterally or vertically defined". The "Workplan for Vertical and Lateral Extent Sampling Downgradient of MW12" dated February 20, 2001 was approved by Mr Michael Infurna of San Jaoquin County Health on March 19, 2001 Pre-approval of corrective action costs to fullfill the workplan was obtained from the State Water Resources Control Board on April 6, 2001, see Appendix A LOCAL HYDROGEOLOGY The 13 groundwater monitoring wells associated with R603 are screened into the A Zone aquifer, the primary zone of interest, see Figure 3. Depth to groundwater measurements obtained from the R603 wells since 1989 show a semiconfined aquifer at approximately 50 feet bgs predominately flowing to the east, with a relatively flat gradient ranging from 0 001 to 0 003 ft/ft Boring logs generated from the drilling/samplmg for installation of the R603 wells and the Levine-Fricke- Recon(LFR) report "Interun Source Area Remedial Investigation Report Lincoln Center, Stockton, California" dated May 22, 1998 indicate that a gasoline leak from R603 would be transported offsite, downgradient to the east in the aquifer sands located at the 50 foot depth. In the LFR report, Figure 14 - Geologic Cross Section A-A', shows a sandy silt sand aquifer at about 50 feet bgs. This aquifer is referred to as A Zone and extends to approxnnately 75 to 85 feet bgs. LFR defined an aquitard separating the A Zone from the B Zone aquifer as the AIB Aqurtard LFR described the AB Aquitard as overconsolidated clays and silts between 10 and 30 feet thick The B Zone aquifer is approximately 105 to 115 feet bgs. The base of the B Zone is typically found at approximately 145 feet bgs. Below the B Zone is the B/C Aqurtard, a 10 to 30 feet thick, hard and overconsolidated silty clay An A Zone Aquifer test performed by LFR in 1997 on monitoring well MW-IOTA (northwest corner of Pacific and Benjamin Holt Dr) showed transnussivity range from approximately 0 4 to 3.4 square feet per minute. LFR assumed an average aquifer thickness of 25 feet equivalent 2 R-603 May 2001