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Peter MacNicholl <br /> February 23, 2005 <br /> Page 2 of 4 <br /> PROJECT SUMMARY <br /> The purposes of the Draft 2004 Annual Monitoring Report were to (1) summarize <br /> groundwater analytical data and depth-to-groundwater measurements collected during <br /> the 2004 monthly and quarterly sampling events (conducted from October 2003 through <br /> September 2004), (2) present conclusions drawn from those results, and (3) provide <br /> recommendations for sampling, analysis, or remediation. <br /> GENERAL COMMENTS AND RECOMMENDATIONS <br /> A) Page 5-4, Section 5.3.2 (Contaminant Capture): This section of the Draft 2004 <br /> Annual Monitoring Report presents the results of the contaminant capture analysis for <br /> the groundwater treatment systems at the Site. <br /> Based on its attendance at a recent United States Environmental Protection Agency <br /> (USEPA) seminar (Capture Zone Analysis for Pump-and-Treat Systems), the GSU <br /> understands that the USEPA will soon be issuing guidance (expected to be released in <br /> May 2005) for a systematic approach to capture zone analysis which will include the <br /> development of target capture zones, use of multiple lines of evidence to interpret actual <br /> capture and comparison to target capture zones, and systematic reporting of the <br /> capture zone analysis effort. As presented in the seminar, lines of evidence used to <br /> analyze plume capture should include potentiometric contour maps, numerical <br /> groundwater model particle tracking, comparing water levels between well pairs along <br /> capture zone margins (horizontal and vertical), reviewing analytical concentration <br /> trends, and simple horizontal capture zone calculations. <br /> A specific concern when developing potentiometric contour maps is to minimize the use <br /> of water levels from active groundwater extraction wells. The use of water levels from <br /> active groundwater extraction wells will result in the production of misrepresentative <br /> contour maps (exaggerated cones of depression) due to issues associated with well <br /> inefficiencies and well losses at pumping wells. To circumvent this issue, care should <br /> be taken to use water levels from monitoring wells or piezometers located closely to <br /> adjacent pumping wells, or to correct the water levels taken from pumping wells to <br /> account for well effects. <br /> Based on a review of Figure 5.3-1A (Extraction Well Capture Zones for the Upper <br /> Hydrologic Zone Third Quarter 2004), it appears that water levels measured from <br /> pumping groundwater extraction wells may have been used to develop groundwater <br /> contour lines presented on this figure. <br /> Tracy DDD_URS121704_DraftwellMonitodngProg2004AnnualRpt.doc <br />