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I S E C 0 R i Mr Mike Infurna August 10, 2004 Page 4 I MONITORING AND SAMPLING 1 TRC's Groundwater Monitoring Report dated June 15, 2004 is included as Attachment B, including Tables 1- 3, field data sheets, and groundwater analytical reports The site has been monitored and sampled since lst quarter, 2000 Currently, 14 wells are monitored and sampled quarterly Samples 1 are analyzed for TPHg, BTEX, and fuel oxygenates Well construction details are included as SECOR's Table 1 Each of the 14 groundwater monitoring wells at the site was gauged and sampled on April 26, 2004 Groundwater flow direction within the shallow zone was to the northeast at a gradient of 0 006 fYft Groundwater flow direction within the deep zone was to the northeast at a gradient of 0 004 ft/ft Shallow and deep groundwater contour maps are included as Figures 1 and 2 Based on the second quarter 2004 monitoring and sampling event (April 26, 2004) (Attachment B) the maximum TPHg, MtBE, and 1,2-DCA concentrations were 5,100 ppb (MW-5), 1,300 ppb(MW- 10), and 24 ppb (MW-11 and MW-12), respectively TPHg and MtBE concentrations in shallow groundwater have exhibited an increasing trend in wells MW-1 and MW-4, likely due to upgradient gasoline service stations such as the ARCO site TPHg, benzene, MtBE, and 1,2-DCA generally show a decreasing trend in deep wells with the exception of MtBE in well MW-10 which reported its highest concentration to date of 1,300 ppb Based upon data collected during ongoing quarterly groundwater monitoring and sampling, TPHg, benzene, and MtBE are effectively delineated both laterally and vertically in the vicinity of the site 1,2-DCA is not currently delineated Review of shallow isoconcentration maps indicates that ConocoPhillips has performed adequate delineation within the shallow zone Deep isoconcentration maps indicate the following TPHg is delineated to 100 ppb or below with the exception of well MW-12 which is cross-gradient, MtBE and benzene are delineated in all directions to below their respective maximum contaminant levels (MCI_'s), and 1,2-DCA is not currently delineated, however concentration are higher offsite than onsite, indicating that other sources of 1,2—DCA may be affecting groundwater Shallow and deep isoconcentration maps are included as Figures 4 through 10 ` RECENT QUARTER ACTIVITIES (Second Quarter 2004) • Quarterly groundwater monitoring and sampling performed by TRC on April 26, 2004 • Prepared and submitted the quarterly summary report • Conducted additional vertical assessment of soil and groundwater NEXT QUARTER ACTIVITIES (Third Quarter 2004) • Perform quarterly groundwater monitoring and sampling (TRC) • Prepare and submit a quarterly summary report • Submit report documenting vertical assessment and updated site conceptual model Submit work plan for the installation of deep monitoring wells i 1ConocoPhi1hpslTosco Marketing144091Qssrs1200412Q2004—QSR doc SECOR Intemational Incorporated s