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Mr. James L. Barton
<br /> April 8, 2009
<br /> ' Page 2
<br /> are included in Table 1 of Attachment A, and well construction details are included as Table A in
<br /> ' Attachment A. The groundwater elevations for first quarter 2009 are consistent with those observed
<br /> historically.
<br /> ' Groundwater Sampling and Analytical Results
<br /> Groundwater samples were collected from the following monitoring wells: Unit A/B- MW-1, MW-2, MW-3,
<br /> ' MW-4, MW-6, MW-9R, MW-10A, MW-11, MW-12, MW-16, MW-25S, MW-25D, MW-28, MW-29, and
<br /> MW-30; Unit C-MW-24S, MW-32; and Unit D - MW-8D, MW-23D, MW-24D, and MW-31 on
<br /> January 7, 2009 by G-R. Unit A/B monitoring well MW-18D was mistakenly skipped on January 7, 2009
<br /> and was sampled on January 17, 2009 by G-R. As identified in the G-R report(Attachment A), the
<br /> monitoring wells were purged as noted on each Well Monitoring/Sampling Field Data Sheet. Laboratory
<br /> analytical results with chain-of-custody(COC) documentation are included as part of the G-R report.
<br /> ' Current and historical analytical results are summarized in Tables 1 through 3 of the G-R report
<br /> (Attachment A) for total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene,
<br /> and total xylenes, the(BTEX)constituents, in Table 1; the fuel oxygenates in Table 2; and the additional
<br /> ' volatile organic compounds (VOCs)detected in the groundwater monitoring samples since 2003 in Table 3.
<br /> Data validation was performed by AECOM, and the checklist is included as Attachment B. The data
<br /> validation found no discrepancies resulting in questionable data. One field duplicate was collected at MW-
<br /> 3 and labeled as"DUP" on the COC.
<br /> The three primary constituents of concern in groundwater beneath the Site are TPHg, benzene, and
<br /> 1,2-dicholorethane (1,2-DCA). Maps showing the dissolved concentrations of TPHg in groundwater from
<br /> hydrostratigraphic units A/B, C, and D are presented as Figures 3, 5, and 7, respectively. Maps showing
<br /> the dissolved concentrations of benzene in groundwater from hydrostratigraphic units A/B, C, and D are
<br /> ' presented as Figures 4, 6, and 8, respectively. Dissolved concentrations of 1,2-DCA were reported above
<br /> the laboratory detection limit in one monitoring well for the first quarter 2009 monitoring event. The
<br /> dissolved concentration of 1,2-DCA for MW-11 was reported as 0.6 micrograms per liter(µg/L) and was
<br /> ' flagged by the laboratory as an estimated value. Charts depicting dissolved concentrations of TPHg and
<br /> benzene versus time for groundwater monitoring wells MW-2, MW-3, MW-4, MW-6, MW-8D, MW-11, and
<br /> MW-25D are included as Attachment C. Updated with first quarter 2009 data, the concentration trends
<br /> indicated in these charts show that:
<br /> • MW-2-The concentration of TPHg in groundwater historically has not shown a decline, except for
<br /> seasonal fluctuations associated with groundwater elevation changes. TPHg in the third quarter
<br /> ' 2008 decreased significantly from the second quarter sampling from 15,000 pg/L to 780 pg/L in
<br /> the third quarter, possibly due to the remediation system upgrade. In the fourth quarter 2008, the
<br /> TPHg concentration rebounded to 6,400 pg/L, but decreased to 5,100 µg/L in the first quarter
<br /> 2009. The first quarter 2009 TPHg concentration is approximately one order of magnitude lower
<br /> than pre-remediation upgrade concentrations. Since the remedial system was not in operation for
<br /> ' approximately three weeks prior to the first quarter 2009 sampling event(system shut off on
<br /> December 15, 2008 due to carbon loading, which is discussed under the Soil Vapor Extraction
<br /> System Status), AECOM expects TPHg concentrations to continue their downward trend in the
<br /> ' second quarter 2009. Benzene concentrations have decreased since 2000, although other than
<br /> seasonal fluctuations, the benzene concentration had not demonstrated measureable decrease
<br /> AECOM Environment AECOM
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