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Former 76 Station 4409 S E C ® R
<br /> October 27, 2005
<br /> Page 4
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<br /> flow in the C monitoring zone was to the southeast at an approximate gradient of 0 003 ftlft 3
<br /> Shallow, intermediate, and deep monitoring zone groundwater contour maps (A, B and C ,
<br /> zones respectively) are included as Figures 1, 2, and 3 k
<br /> Based on the third quarter 2005 monitoring and sampling event (July 12, 2005) and the
<br /> sampling event for the new monitoring wells on August 19, 2005, the maximum TPHg,
<br /> benzene, MtBE, and 1,2-DCA concentrations were 13,000 pg/L (MW-17), 700 pg/L (MW-
<br /> 17), 5,000 pg/L (MW-1), and 19 pg/L (MW-12), respectively (Table 1)
<br /> TPHg and MtBE concentrations in groundwater in the A monitoring zone have exhibited an
<br /> increasing trend in wells MW-1, MW-3 and MW-4, which is likely due to migration of °4
<br /> dissolved concentrations of TPHg and MtBE from the upgradient ARCO gasoline service
<br /> station site However, the third quarter 2005 TPHg concentrations for MW-3 and MW-4 "v
<br /> were less than they were in second quarter 2005 1,2-DCA is also increasing slightly in `
<br /> MW-3 and MW-4, although concentrations of 1,2-DCA also decreased in third quarter 2005 '
<br /> Concentrations of TPHg and benzene in on-site B monitoring zone wells MW-6, MW-8, and
<br /> MW-10 are decreasing However, concentrations of TPHg have increased over the last two
<br /> quarters in groundwater at MW-9 Concentrations of TPHG in on-site well MW-5 are
<br /> fluctuating but have decreased over the last two quarters, and benzene has decreased to
<br /> less than the laboratory reporting limit Concentrations of MtBE in the last year in on-site B ;d
<br /> monitoring zone wells MW-6, MW-7, and MW-8 ranged from < 0 50 pg/L to 5 1 pg/L
<br /> Concentrations of MtBE are increasing in groundwater at MW-5, MW-9, and MW-10 ,tl
<br /> Concentrations of 1,2-DCA in groundwater at MW-7 were consistent (2 2 to 2 4 pg/L) for
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<br /> the first four quarters of 1,2-DCA analysis but have generally increased during the past
<br /> year Concentrations of 1,2-DCA are fluctuating but generally increasing in groundwater at
<br /> MW-8 and MW-9
<br /> Concentrations of TPHg in off-site wells MW-11, MW-12, and MW-14 have ranged from
<br /> <0 50 to 63 pg/L TPHg has been reported in groundwater at MW-13 since first quarter
<br /> 2004 when the historical maximum concentration was reported at 110 pg/L Benzene was
<br /> reported in off-site wells MW-11 and MW-14 during one sampling event but is currently less
<br /> than the reporting limit for both wells Benzene has not been detected in groundwater at
<br /> MW-12 or MW-13 MtBE concentrations have been fluctuating in off-site well MW-12
<br /> between 0 57 pg/L and 7 3 pg/L but have decreased over the last two quarters
<br /> Concentrations of MtBE have also fluctuated in MW-14 and increased from <0 50 pg/L
<br /> during first and second quarter 2005 to 32 pg/L during third quarter 2005 MtBE has not
<br /> been detected in groundwater at MW-11 or MW-13 1,2-DCA has been reported in each of
<br /> the off-site B monitoring zone wells 1,2-DCA concentrations within well MW-11, MW-12,
<br /> and MW-13 have consistently ranged between 73 and 35 pg/L The 1,2-DCA
<br /> concentrations in well MW-14 had varied between 3 2 pg/L and 4 8 pg/L until third quarter
<br /> 2005 when a concentration of 18 pg/L was reported
<br /> No concentration trends are apparent in the C monitoring zone wells because adequate
<br /> data for determining trends have not been collected However, during third quarter 2005,
<br /> concentrations were generally low to non-detectable in the groundwater samples collected
<br /> from MW-15 and MW-16 At MW-17, concentrations of TPHg and BTEX in groundwater
<br /> IN
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<br /> 4409 3Q2005 QSR doc SECOR International Incorporated
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