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Former Beacon Station No 419 January 27, 1999 Stockton California Groundwater M nitoring Report ` 8020 and 8260B Laboratory results are summarized in Tables 1 and la Copies of the laboratory reports and chain-of-custody are included as Attachment C Conclusions Water-level data collected by Doulos on December 30, 1998 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the 10 wells was approximately 45 5 feet below surface grade (bsg) Water levels have risen an average of approximately 1 5 feet since September 1998 The flow direction is generally toward the south (ranges from southwest to southeast), with a slight mounding beneath the tank pit and dispenser islands at the site The groundwater gradient magnitude ranges from 0 01 to 0 001 foot/ foot beneath the site Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3) Detectable concentrations of benzene were reported in seven of the ten water samples collected No detectable concentrations of benzene were detected in wells MW-1, MW-2 and MW-10 Dissolved benzene concentrations have decreased in wells MW-1, MW-3 and MW-4 since the September 1998 sampling event, while dissolved benzene concentrations have increased in wells MW-2, MW-4, MW-5 and MW-6 since the September 1998 sampling event No detectable concentrations of MtBE were reported in water samples collected during this sampling event No analyses were performed for other fuel oxygenates during this sampling event Recommendations Horizon recommends continued quarterly monitoring Horizon is currently preparing the final draft of Report No 1419 12,Additional Subsurface Investigation Report Distribution We recommend a copy of this report be forwarded to Mr Michael J Infuma Mr Gordon Boggs Environmental Health Division CRWQCB - Central Valley Region San Joaquin County Public Health Services 3443 Routier Road, Suite A P O Box 2009 Sacramento, California 95827-3098 Stockton, California 95201 98gmr q4 Job No 1419 41 2 HORIZON ENVIRONMENTAL INC.