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' E C 0 R i�.enrr■nwrr i rr..r__-_. - 5 2 3 3 C Monitoring Zone MtBE has not been reported in groundwater in the C monitoring zone wells (MW-15 through ' MW-17) MtBE is delineated in the C monitoring zone (Figure 16) 524 Distribution of 1,2-DCA in Groundwater 5241 A Monitoring Zone Analysis for 1,2-DCA in groundwater samples began in the fourth quarter of 2003 1,2-DCA has been reported in A monitoring zone wells MW-3 and MW-4 at concentrations ranging from 0 87 1 ppb (MW-3) to 7 2 ppb (MW-4) Based on the isoconcentration map included as Figure 17, 1,2- DCA detections in wells MW-3 and MW-4 are likely being affected by the upgradient source wk L ' from ARCO station 4932 Currently the 1,2-DCA plume in the A monitoring zone to the northeast (downgradient) is delineated by Former 76 Station well MW-2 (less than the reporting �esti limit, 1Q05) ' 5242 B Monitoring Zone n 1,2-DCA was reported in seven of the nine B monitoring zone wells sampled during the second ' quarter 2005 sampling event (MW-8 could not be sampled) The maximum concentration ' reported was 26 ppb (MW-12) Concentrations of 1,2-DCA were greater in cross-gradient wells v MW-12 (26 ppb) and MW-13 (16 ppb) and downgradient well MW-11 (7 3 ppb) than in any of � the on-site wells Based on the isoconcentration map included as Figure 18, 1,2-DCA is not 1 delineated in the B monitoring zone The source of 1,2-DCA in the B monitoring zone, particularly wells MW-11 through MW-14 is not clear , 5.2 4 3 C Monitoring Zone �� During the initial sampling event and the second quarter 2005 sampling event, 1,2-DCA was not detected in the sample collected from MW-15 However, the sample collected from MW-15 on ' April 20, 2005 had a 1,2-DCA concentration of 1 0 ppb 1,2-DCA was also detected in the sample collected from MW-16 at a concentration of 0 62 ppb 1,2-DCA was not reported in the sample collected from MW-17 1,2-DCA is not delineated in the downgradient direction to the C �" monitoring zone (Figure 19) it 53 Dissolved Concentration Trends 5 3 1 A Monitoring Zone Trends As previously discussed, TPHg, benzene, and MtBE were not routinely detected in A monitoring zone wells MW-1 through MW-4 prior to the first quarter of 2003 During recent groundwater ' monitoring and sampling events, TPHg and MtBE concentrations in wells MW-1, MW-3, and MW-4 have shown an increasing trend (likely due to impacts from upgradient ARCO 4932) from nondetect levels of TPHg to a maximum of 1,600 ppb and nondetect levels of MtBE to a 1 maximum of 2,000 ppb 1,2-DCA concentrations in wells MW-3 and MW-4 are also showing an increasing trend Groundwater elevations have decreased approximately seven to nine feet Y since 2000 in the A monitoring zone wells Graphs of concentrations vs time and groundwater Z y ' elevation vs time are included in Appendix G i Updated SCM 2005 doc 14 77CP 64409 05 0005 1 w; a "a 'ke-