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A r E Beacon Station No.474 January 22, 1999 Stockton California Groundwater Monito-ing Repo w Method 8015, and for the volatile aromatics benzene, toluene, ethylbenzene, total xylenes (BTEX) and the fuel oxygenate methyl tertiary-butyl ether (MTBE) by EPA Method 8020. Laboratory results are also summarized in Table 1. 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 wells was approximately 57.2 feet below surface grade (bsg). Water levels have risen approximately 2 feet since the last monitoring in September 1998. The flow direction is generally toward the southeast, with an average magnitude of 0.001 foot/foot. Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3). Detectable concentrations of benzene were reported in the water samples collected from four of the seven wells sampled. No aetectable concentrations of benzene were reported in wells MW-5, MW-7 and MW-9. Dissolved benzene concentrations have decreased in wells MW-6 and VW-1, and have increased in wells MW-8 and VW-4 since the September 1998 sampling event. No concentrations of MtBE were reported above laboratory detection limits. Recommendations Horizon recommends continued quarterly monitoringand proceed with to P the proposed assessment work referenced in Horizon's June, 5, 1998 Work Plan For A Remedial Investigation and December 28, 1998 Letter Addendum, Report Distribution We recommend a copy of this report be forwarded to: Mr. Michael J. Infurna Environmental Health Division San Joaquin County Public Health Services P.O. Box 2009 Stockton, California 95201 Mr. Gordon Boggs CRWQCB - Central Valley Region 3443 Routier Road, Suite A Sacramento, California 95827-3098 i � � 474QMR.Q4 Job No. 1474.41 2 HORIZON ENVIRONMENTAL INC. l �