SITE INFORMATION AND CORRESPONDENCE_CASE 1

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14800 W. Frontage Rd (Hwy. 99) - - 2`-- - •----�w4 September 1998 Manteca, San Joaquin County 'µ In response to the request by Regional Board staff, APEX submitted the 7 July 1998""Response to Delay No Further Action Required Decision and Closure Request", on behalf of Moore Petroleum. The report provides a determination of the mass of contamination remaining on-site, a fate and transport study and estimate of the contaminant concentrations that may impact the on-site domestic well and nearby off-site domestic well, and explains why conditions remaining at the site will not adversely impact water quality, health or other beneficial uses. Estimates of the mass of contamination remaining!in the subsurface soil and groundwater are as follows: Contaminant: Mass: ti MTBE ' `'0.068 lbs. i_ i dissolved in groundwater 1,2-DCA 0.032 lbs. dissolved in groundwater 0.01 lbs. t� remaining in soil TPHdiesel 0.56 lbs. dissolved in groundwater The estimates for residual soil contamination are based on the analytical results for the 29 August 1997 sampling event. 1,2-DCA was the only contaminant detected above the laboratory reporting limits during this sampling event, therefore, based on the assumptions, this was the only contaminant used in the mass balance calculation for soil. �'II The residual contaminant mass calculation for groundwater, is based on the analytical results of groundwater sampling performed on 21 November 1997. MTBE, 1,2-DCA and TPHd, were the only contaminants detected above the laboratory reporting limits during the indicated sampling event. Based on the assumptions, these were the only contaminants used in the mass balance calculation. The driller's log for the new on-site domestic welfwas provided. The log suggests thatinterbedded sand and clay is present below the site to a depth of approximately 175 feet bgs. Between 175 feet and 278 . feet bgs, the-log-documents-a.clay-unit that is,appr`oximately 100 feet_thick, with one.2:�foot_unit of.sand ._ between 229 and 231 feet bgs:Below 278 feet bgs;the boring log reports interbedded sand and clay. The 6-inch diameter domestic well is screened from 340 to 3. 60 feet bgs. A bentonite sanitary seal is present from 220 feet bgs to the surface. I The new on-site domestic well was drilled between 8 and 13 October 1997. On 13 October 1997, the well was developed, static water level was measured, and the well was sampled. Analytical results of this sample detected 1200 ppb TPHg and 3.4 ppb total Xylenes. The well was resampled on 21 November 1997. No contaminants were detected above their respective reporting limits during the November sampling event. On 1 September 1998, I called Tom Landwehr of APEX, requesting an explanation for the contaminant concentrations in the new well during the 13 October 1998 sampling event. During the telephone conversation, 'Mr. -Landwehr attributed the contamination to the well developer's pump and/or drilling fluids, and indicated that the 21 November 1997 sampling results support this conclusion.