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r Beacon Station No. 474 July 28,'1999 Stockton California Groundwater Monitoring Report (EPA) Method 8260B; and the seven oxygenated fuel additive compounds methyl tertiary butyl ether (MTBE), di-isopropyl ether (DIPS), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), tertiary butyl alcohol (TBA), methanol, and ethanol, as well as the lead scavenger compounds 1,2-dichl oro ethane (ethylene dichloride or EDC) and 1,2- dibromoethane (ethylene dibromide or EDB) by modified EPA Method 8260. Laboratory analytical results are also summarized in Tables 1 and 1 a. Copies of the laboratory reports and chain-of-custody are included as Attachment C. Conclusions Water-level data collected by Doulos on June 3, 1999 was used to construct the Groundwater Contour Map (Figure 2). Deeper monitoring wells with submerged well screens (wells MW-1, MW-2 and MW-3) were not utilized for creating the groundwater contour map. The average depth-to-water for the wells was approximately 54.5 feet below surface grade (bsg). Water levels have risen approximately 0.5-foot since the last monitoring in March 1999. The flow direction at the time of this sampling was generally toward the south beneath the site, with an average gradient of 0.002 foot/foot. The flow direction beneath East Main Street at the time of sampling was toward the north from a groundwater"high" along the northern site boundary. Data collected from this sampling .event was used to construct a Benzene Isoconcentration Map (Figure 3). Benzene was detected in the water samples collected from four (MW-2, MW-6, VW-1, and VW-2) of the eleven.wells sampled. Dissolved benzene concentrations have decreased in wells MW-6, VW-1, and VW-4 since the March 1999 sampling event, while benzene concentrations have increased only in well VW_I. The fuel oxygenates MTBE, DIPE, ETBE, TAME, TBA, methanol and ethanol were not detected in any of the samples collected this quarter. The lead scavenger compounds EDC and EDB were detected in wells MW-1, MW-5, and MW-8. EDC was detected in wells MW-2, MW-3, MW-7, MW-9, VW-1, and VW-4. EDB was detected in wells MW-2, MW-6, VW-I, and VW-4. Recommendations Horizon recommends continued quarterly monitoring and to proceed with the proposed assessment work referenced in Horizon's June 5, 1998 Work Plan For A Remedial Investigation and December 28, I998 Letter Addendum. At present, Horizon is awaiting processing of an encroachment permit with the City of Stockton for work to be..performed - - ,within the-Main Street right-0f--way. - - QMR99.Q2 Job No. 1474.42 2 HORIZON ENVIRONMENTAL, INC.