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Former 76 Station 4409 S E C O R July 28, 2005 Page 4 from 1.1 ppb to 7.3 ppb.. 1,2-DCA has been reported in each of the off-site B monitoring. zone wells. 1,2-DCA concentrations within well MW-11, MW-12, and MW-13 have = consistently ranged between 7.3 and 35 ppb. The 1,2-DCA concentration in well MW-14 i has varied between 3.2 ppb and 4.8 ppb. No trends are apparent in the C monitoring zone wells because adequate data for determining trends have not been collected. Review of the A monitoring zone isoconcentration maps indicates that ConocoPhillips has performed adequate delineation within the A monitoring zone (TPHg and MTBE concentrations are likely due to upgradient ARCO station). B monitoring zone isoconcentration maps indicate the following: TPHg is delineated to less that 50 ppb with the exception of 71 ppb in well MW-13; MtBE and benzene are delineated to below their r respective-maximum-contaminant.levels (MCLs);.and 1,2-DCA is not currently delineated,. TPHg, benzene, and 1,2-DCA are not delineated in the C monitoring zone.. MtBE is i delineated to less than the reporting limit in the C monitoring zone. A, B, and C monitoring zone isoconcentration maps are included as Figures 4 through 15. CHARACTERIZATION STATUS Soil contamination delineated?., Yes.,4, --. (Dissolved groundwater delineated? No Free product delineated? NIA Current phase of project? _ Assessment/Monitoring Frequency of sampling? Quarterly { Soil remediation in progress? No Anticipated start date? N/A Anticipated completion date? N/A A monitoring zone groundwater gradient? 0.005 ft/ft northeast B monitoring zone groundwater gradient? 0.007 ft/ft northeast C monitoring zone groundwater gradient? 0.07 ft/ft northwest NEIGHBORING SITE ACTIVITIES The upgradient ARCO Service Station, located at 16 East Harding Way, appears to be- affecting eaffecting groundwater concentrations within wells at the site the A zone wells at the ConocoPhillips site. Remediation technology at the ARCO site includes soil vapor extraction, groundwater extraction, and ozone sparging. An off-site ozone sparge system was installed during the second quarter of 2005 and become operational during third quarter 2005. Certified laboratory analytical reports, chain-of-custody documentation, and field data sheets for the ARCO station are included as Attachment D. WORK COMPLETED IN SECOND QUARTER 2005 • TRC performed quarterly groundwater monitoring and-sampling at the site. • SECOR submitted the Quarterly Summary Report — First Quarter 2005 on May 26, f 2005. r