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Mr. Doug Stidham - 2 - 23 August 2001 <br /> not exist on site." Intrawell comparison procedures are considered appropriate when samples can be <br /> collected prior to application of wastewater. Because the wastewater treatment plant has been in <br /> operation since at least 1962, adequate historical data is not available. Intrawell comparisons are <br /> properly used when a background well cannot be installed in the same aquifer or site heterogeneity <br /> prevents installation of groundwater monitoring wells in similar geologic materials; this is not the <br /> situation at the site. <br /> Because the intrawell data comparison method is not appropriate, the City shall proceed with an <br /> interwell data comparison method. Interwell comparison relies on reliable upgradient characterization of <br /> groundwater quality. Well MW-2R might be an appropriate well for upgradient comparison; the City <br /> shall evaluate the well and determine if it will provide reliable data on upgradient conditions. If not, the <br /> City shall provide a workplan for installation of additional wells which can be used to characterize <br /> upgradient conditions. <br /> Conclusions <br /> Because of the uncertainty of the status of Well MW-2R as upgradient or not, and the possibility of local <br /> influences of groundwater quality due to the nearby storm water pond, the City may elect to install <br /> additional upgradient groundwater monitoring wells. If this option is selected,please submit a workplan <br /> with the new monitoring well location(s)by 19 October 2001. The workplan shall contain the <br /> information listed in the fist section of the enclosed guidance document. Alternatively, the City may <br /> proceed with interwell comparison of the analytical data and resubmit the Groundwater Impact <br /> Evaluation Report by 24 September 2001. <br /> If you have any questions on this report review, please telephone me at (916) 255-3116. <br /> TIMOT R. O'BRIEN <br /> Waste Discharge to Land Unit <br /> San Joaquin River Watershed <br /> cc: Mr. Mike Huggins, San Joaquin County Environmental Health Department, Stockton <br /> Mr. Raynold Kablanow,Ph.D., Geological Technics, Modesto <br /> C:\Projxt Fila\Sitm\Exalon�Stidham061501a.dm <br />