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Former 76 Station 4409 S E C O R <br /> July 28, 2005 <br /> I i Page 3 <br /> are monitored and sampled quarterly, with MW-16 and MW-17 to be added to the quarterly <br /> monitoring program starting with the third quarter 2005 sampling event. Samples are <br /> analyzed for total petroleum hydrocarbons as gasoline (TPHg); benzene, toluene, <br /> ethylbenzene, total xylenes (BTEX); and fuel oxygenates by EPA Method 8260B. Selected <br /> Lsamples were also analyzed for lead by EPA Method 60106. <br /> DISCUSSION <br /> The 15 existing groundwater monitoring wells at the site were gauged and sampled by TRC <br /> on April 12, 2005. Water levels within groundwater monitoring wells that are part of the <br /> L monitoring and sampling program at the ARCO Service Station (16 East Harding Way) <br /> were cooperatively measured by SECOR on April 13, 2005, and sampled on April 13 <br /> through 15, 2005. Water level measurements were collected from the two new C <br /> monitoring zone wells MW-16 and MW-17, as well as MW-15 on..April 20, 2005. .Regional <br /> groundwater flow direction within the A monitoring zone was to the northeast :at <br /> approximately 0.005 feet per foot (ft/ft) beneath the ConocoPhillips site and radially outward <br /> k beneath the ARCO site. Regional groundwater flow direction within the B monitoring zone <br /> k was to the northeast at'an approximate gradient of 0.007 ft/ft, and groundwater flow in the C <br /> monitoring zone was to the northwest'ot an approximate gradient of 0.07 ft/ft. A, B, and C <br /> Lmonitoring zone groundwater contour maps are included as Figures 1, 2, and 3. <br /> Based on the second quarter 2005 monitoring and sampling event (April 12 2005 Table 2 <br /> Land the sampling event for the new monitoring wells on April 20, 2005, the maximum TPHg, <br /> ` benzene, methyl tertiary butyl ether (MtBE), and 1,2-dichloroethane (1,2-DCA) <br /> concentrations were 30,000 parts per billion (ppb) (MW-17), 1,100 ppb (MW-17), 2,300 ppb <br /> (MW-10), and 26 ppb (MW-12), respectively. <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, likely due to the upgradient ARCO <br /> gasoline service station site. 1,2-DCA is also increasing slightly in MW-3 and MW-4. <br /> Concentrations of TPHg and benzene in on-site B monitoring zone wells MW-6, MW-8, <br /> L MW-9, and MW-10 are decreasing. Concentrations of TPHG in on-site well MW-5 are <br /> fluctuating, but benzene has decreased to less than the laboratory reporting limit. <br /> Concentrations of MtBE in on-site B monitoring zone wells MW-6, MW-7, and MW-8 are <br /> € � reported at low levels periodically. Concentrations of MtBE are increasing in groundwater <br /> at MW-5, MW-9, and MW-10. Concentrations of 1,2-DCA in groundwater at MW-7 were <br /> consistent (2.2 to 2.4 ppb) for the first four quarters of 1,2-DCA analysis but increased <br /> during the subsequent two quarters. The concentration decreased during the second <br /> quarter 2005 event. Concentrations of 1,2-DCA are increasing in groundwater at MW-8 and <br /> a <br /> MW-9. <br /> Concentrations of TPHg'!in off-site wells MW-11 through MW-14 have fluctuated from less <br /> than the reporting limit during initial sampling to concentrations ranging from 56 ppb to 120 <br /> ppb and back to less than the reporting limit, with the exception of MW-13, which had a <br /> second quarter 2005 TPHg concentration of 71 ppb. Benzene was reported in off-site wells <br /> MW-11 and MW-14 during one sampling event but is currently less than the reporting limit <br /> for both wells. MtBE has been .fluctuating in off-site well MW-12 at concentrations ranging <br /> L <br /> L <br />