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SESC0R �LS�QLf\� zone to the northeast (downgradient) is delineated by the ConocoPhillips A monitoring zone wells 5 2 2 2 B Monitoring Zone Historically benzene has been reported in on-site B monitoring zone wells MW-5 through MW- 10 at concentrations ranging from 0 64 ppb (MW-7) to 600 ppb (MW-8) During the second ' quarter 2005, benzene was detected in groundwater at only MW-6 (100 ppb) and MW-9 (3 9 ppb) Benzene was not detected in the off-site wells (MW-11 through MW-14) during the second quarter 2005 Based on the isoconcentration map included as Figure 12, benzene is ' delineated to nondetect levels in the B monitoring zone in all directions : 5223 C Monitoring Zone N ' During the initial sampling event and the second quarter 2005 sampling event, benzene was not detected in the sample collected from MW-15 However, the sample collected from MW-15 on April 20, 2005 had a benzene concentration of 3 6 ppb Benzene was also detected in the a; r samples collected from MW-16 and MW-17 at concentrations of 11 ppb and 1,100 ppb, respectively Benzene is not delineated in any direction in the C monitoring zone (Figure 13) ;A ' 523 Distribution of MtBE in Groundwater 5 2 3 1 A Monitoring Zone i { r y I Historically MtBE had not been reported in A monitoring zone well MW-1 prior to second quarter of 2004 and MW-2 prior to the first quarter of 2003 MtBE has been reported in A monitorings zone wells MW-3 and MW-4 since their installation in the first quarter of 2000 During the ' second quarter 2005, MtBE was reported in MW-1, MW-3, and MW-4 at concentrations ranging ° from 92 ppb to 2,000 ppb MW-2 could not be sampled during the second quarter 2005, but MtBE was reported at a concentration of 2 6 ppb during the first quarter of 2005 Based on the 1� ' isoconcentration map included as Figure 14, MtBE detections in wells MW-1 through MW-4 are , likely being affected by the upgradient source from ARCO station 4932 Currently, the MtBE plume in the A monitoring zone to the northeast (downgradient) is delineated to less than the k3` MCL by ConocoPhillips well MW-2 (2 6 ppb, 1Q05) F� 5 2 3 2 B Monitoring Zone ' MtBE has periodically been reported in on-site B monitoring zone wells MW-5 through MW-10 at , concentrations ranging from 1 3 ppb (MW-8) to 3,200 ppb (MW-10) Only B monitoring zone wells MW-5, MW-9, and MW-10 exceeded the MCL for MtBE during the second quarter 2005 „y with concentrations of 300 ppb, 28 ppb, and 2,300 ppb, respectively Off-site wells MW-11 and MW-13 have had no reported detections of MtBE since their installation in 2003 Off-site well MW-12 has had MtBE concentrations ranging from 1 1 ppb to 7 1 ppb, and MW-14 has had '$g MtBE concentrations ranging from 0 78 ppb to 1 4 ppb Based on the isoconcentration map included as Figure 15, MtBE is delineated to nondetect levels in the B monitoring zone v� downgradient of the Former 76 Station MtBE is delineated to less than the MCL in the B monitoring zone upgradient and crossgradient by ARCO well DMW-3 and Former 76 Station well MW-12, respectively (Figure 15) Updated SCM 2005 doc 13 77CP 64409 05 0005 a k