ARCHIVED REPORTS_QUARTERLY GROUNDWATER MONITORING AND REMEDIATION STATUS REPORT AND UPDATED SITE CONCEPTUAL MODEL 2009

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1 Mr. James L. Barton April 8, 2009 Page 3 from 2003 to second quarter 2008. Benzene concentrations were below the trend over the last three years in third quarter 2008 at 31 pg/L compared to 350 pg/L in second quarter 2008. In the fourth quarter 2008, the benzene concentration remained stable, compared to the third quarter, at 37 pg/L. The benzene concentration continued a downward trend in the first quarter 2009 with an estimated concentration of 3 pg/L reported by the laboratory. The recent trend in MW-2 concentrations confirms that the past remediation system was not addressing this area, and that ' the upgraded remediation system appears to be having an immediate effect on dissolved benzene concentrations in MW-2. • MW-3— Historically, the concentrations of TPHg and benzene in groundwater have remained fairly ' stable, although with seasonal fluctuations. After the startup of the upgraded remediation system in third quarter 2008, the concentration of benzene decreased significantly from 4,500 pg/L to 44 pg/L, and the concentration of TPHg decreased an order of magnitude from 32,000 pg/L to 2,500 ' pg/L. In the first quarter 2009, the benzene concentration increased to 360 pg/L; however, that is still an order of magnitude lower than the benzene concentration when compared to second quarter 2008 (4,500 pg/L). In the first quarter of 2009, the TPHg concentration increased to ' 19,000 pg/L, which is slightly higher than the fourth quarter 2008 (15,000 pg/L) concentration. The increase in TPHg concentration may be attributed to the remediation system being turned off on December 15, 2008 due to carbon loading. AECOM expects the downward concentration ' trends to continue in the second quarter 2009. • MW-4—The concentration of TPHg in groundwater has decreased since 2002, although the TPHg concentration increased from non-detect up to 410 pg/L in April 2008. Benzene has shown a ' similar trend, including being non-detect for three quarterly monitoring events before increasing to 73 pg/L during April of 2008. The concentrations in August 2008 returned to non-detect for TPHg and benzene, and have remained non-detect for three consecutive quarters, including first quarter 2009. The slight increase of dissolved concentrations in April 2008 appears to be a short-term fluctuation due to the remediation system not being operated or a slight rise in the groundwater elevation during that time. ' • MW-6 -The concentrations of TPHg and benzene in groundwater have been declining over time. After showing a slight increase following system startup in July 2008, both TPHg and benzene have decreased to non-detect at detection limits of 50 pg/L and 0.5 µg/L for TPHg and benzene, ' respectively in the first quarter 2009. • MW-8D -The concentrations of TPHg and benzene in groundwater have historically been increasing, as has the groundwater elevation. In August 2008 following the remediation system ' upgrade, TPHg was observed at the lowest concentrations ever observed in MW-8D (52 pg/L). Following an increase in the fourth quarter to 14,000 gg/L, TPHg has decreased to non-detect at 50 µg/L in the first quarter 2009. Benzene has not been detected since system startup in July 2008, and was non-detect at 0.5 pg/L in January 2009. The upgraded remediation system appears to be addressing this deeper area of the plume effectively. ' • MW-11 - In August 2008, the concentrations of TPHg and benzene (200 pg/L and 55 pg/L, respectively) were the lowest observed since February 2004. In October 2008, the decreasing trend continued in MW-11, with both TPHg and benzene decreasing to non-detect levels. Both ' TPHg and benzene remained non-detect in the first quarter 2009 at detection limits of 50 µg/L and AECOM Environment AECOM