ARCHIVED REPORTS_SECOND 2008 SEMI-ANNUAL GROUNDWATER MONITORING AND SAMPLING REPORT

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' M. Scott Mansholt—CEMC Second Semi-Annual 2008 Groundwater Monitoring and Sampling Report September 26,2008 Vernalis Pump Station Page 5 of 7 usable as qualified for the purpose of determining chemical concentrations in groundwater at the site. Qualified data with applicable reason codes are provided in Table B-1 of Attachment B. Analytical data collected in support of this investigation are considered precise, accurate,representative, complete,and comparable, and may be used for their intended purposes unless noted otherwise. CONCLUSIONS AND RECOMMENDATIONS The second quarter groundwater monitoring and sampling event, which included MW-6 and MW-7 only, was completed on April 9,2008. The groundwater elevations for those wells were 70.28 (MW-6) and 70.09 (MW-7)feet above MSL. Well MW-7 had concentrations of TPHd at 150 gg/L (120 gg/L after silica-gel cleanup), naphthalene at 0.018 µg/L, and acenaphthalene at 0.018µ9/L. Well MW-6 had no detectable TPHd and had a naphthalene concentration of 0.031 µg/L. Detected PAH concentrations were below ESLs and WQOs during the second quarter sampling of MW-6 and MW-7. The third quarter 2008 groundwater monitoring and sampling event included wells MW-1 through MW-7 ' and was completed on July 16 and 17, 2008. The groundwater elevation at the site ranged from 67.72 feet (MW-1)to 70.93 feet(MW-5) above MSL. The average groundwater elevation at the site is 70.93 feet above MSL based on all groundwater elevation measurements collected to date from wells MW-1 through ' MW-7. MW-1 was not sampled during the third quarter 2008 event because there was a sheen on the water in the well. Replacement of the oleophilic sock in MW-1 during this event was not necessary. Well MW-4 was not sampled due to a damaged casing. TPHd concentrations in excess of the WQO of 100 gg/L were detected in samples from wells MW-2 (110 gg/L; <50 gg/L after silica gel cleanup) and MW-7 (320 gg/L; 170 gg/L after silica-gel cleanup). The MW-7 duplicate indicated TPHd at 230 gg/L; the concentration was 68 gg/L after silica-gel cleanup. 1 There were no other TPHd concentrations detected above the WQO or ESL. Recent and historic site groundwater monitoring data indicate that the TPHd groundwater plume appears ' to be localized in a small area of the site with wells MW-1 through MW-4 defining the lateral extent of the TPHd groundwater plume. Specific well status is as follows: a Minor amounts of SPO have been observed in MW-1 since monitoring and sampling began in 1998; therefore, this well has never been sampled. A total of approximately 0.02 gallon of SPO has been removed from MW-1 since July 1998. • Well MW-2 has had sporadic TPHd exceedances at concentrations close to the WQO (120 gg/L; 67 gg/L after silica-gel cleanup) in the first quarter of 2006, and 110 gg/L (<50 gg/L after silica gel cleanup) in the third quarter of 2008. A sheen is periodically observed in this well. • Well MW-3 has had only one TPHd exceedance(980 µg/L in the first quarter of 2006) since quarterly sampling began. Subsequent sampling events have not reported TPHd exceedances from this well. ' • Well MW-4 has not been sampled since January 2006 due to insufficient water volume recovery and casing damage. MW-4 had a TPHd detection of 290 gg/L(150 gg/L after silica-gel cleanup) when last sampled.