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r Y ATC Associates Inc. " 1117 Lone Palm Avenue, Suite 201 Modesto,California 95351 209-579-2221 Fax:209-579-2222 r A S S O C I A T E S I N C . The analyte 1,2-DCA was detected in the groundwater samples collected from MW1, MW2, `r MW3, MW4, MW6, MW7, MW8, and MW10 at concentrations ranging from 0.6 to 98 pg/L, exceeding the MCL of 0.5 pg/L for drinking water standards. An isoconcentration map illustrating the distribution of 1,2-DCA in groundwater during the March 2011 sampling event is provided as Figure 6. GEOTRACKER DATA UPLOAD V The depth to water data was submitted electronically to the State Water Resources Control Board (SWRCB) Geotracker database (confirmation number 2967112243). The laboratory data were submitted electronically to the SWRCB Geotracker database (confirmation numbers `" 2065556347, 9765187084, and 5734430968). Documentation of the data submittal is contained in Attachment 3. r ADDITIONAL ACTIVITIES The sampling event on March 24 and 25, 2011 also served as the baseline sampling event for an ozone sparge pilot test begun on April 1, 2011. Additional sample analyses were conducted for this purpose, and are included in the attached laboratory data report and GeoTracker upload. The pilot study is still in operation and upon completion a report will be sent under separate cover. CONCLUSIONS The groundwater flow direction on March 24, 2011, was generally consistent with historical data. Groundwater has historically flowed in a northeasterly direction. Groundwater elevations increased since the previous groundwater monitoring event conducted on October 7, 2010. The laboratory analytical results of the groundwater samples collected from MW1 through MW10 are generally similar to recent results of the groundwater samples collected from these wells. The shallow groundwater plume is not defined toward the southwest regarding TPHg. The plume seems to have traversed across Union Street. The groundwater plume is not defined toward the east with respect to MTBE. The MTBE and 1,2-DCA impacted groundwater plume is not defined toward the northeast. The western extent of the 1,2-DCA plume lies in the vicinity of MW9, as low concentrations are periodically detected in the groundwater samples collected from this well. RECOMMENDATIONS Based on the results of thf urth quarter 2010 oundwater monitoring and sampling event, ATC recommends the follows r • Continue to sample MW1 through MW10 semi-annually and MW11 annually. Analyze samples for TPHg by EPA Method 8015M and for BTEX, MTBE, TBA, DIPE, ETBE, TAME, 1,2-DCA, and EDB by EPA Method 8260B. ATC recommends removing TPHd and fuel oil from the analyses performed. r.r s:\environmental\22493\Reports\QMR 2011 SA1.doc 5 r