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OARGIS - ASSOCIATES. Mr. Dave Bates May 6, 1988 Page 2 hydrogeologic settings, they are not appropriate in all hydrogeologic settings. The portion of the aquifer that has been impacted by chromium may be as much as 150 to 200 feet thick. Three wells screened over 10-foot intervals in this 150- to 200-foot thickness will probably not provide adequate definition of the distribution of chromium in the aquifer. The length verticalfsection for heicollecti noofdbe representative id to sampde an adequate the screened les� DHS has argued that 40-foot and 60-foot screened intervals contribute to the dilution of the highest concentration within that screened interval . Samples collected from wells with 40 or 60 feet of screen do represent an integration of variations in chromium concentration over that interval . However, well CWS-20 is screened from approximately 140 to 460 feet below land surface. Approximately 0.07 milligrams per liter of chromium were detected in water samples obtained from well CWS-20 despite dilution over the approximately 320-foot screened interval . In the context of well CWS- 20, a 40-foot screened interval in the proposed monitor wells should provide adequate definition of the vertical distribution of chromium in the aquifer. DHS has also apparently mandated that water samples from all monitor wells at the site be analyzed routinely for copper, arsenic, and general minerals, as well as hexavalent chromium and total chromium. During the previous phases of the groundwater assessment, the initial water samples from each new monitor well have been analyzed for copper, _ arsenic, and general minerals. The low mobility of copper and arsenic in groundwater is well documented. No plume of these trace metals emanating from the source areas at the site has been detected in more than three years of monitoring. It is not necessary to monitor every monitor well every quarter for these trace metals. General minerals are often used to characterize water type. With the exception of nitrate, no health standards have been set for general minerals in groundwater. General minerals are not contaminants of concern at this site, and quarterly analysis for them is unnecessary. The contaminant of concern in the groundwater at this site is chromium. BC Labs in Bakersfield, California currently charges 127.00 to analyze a single water sample for hexavalent chromium and total chromium. The cost of analyzing a single water sample for all the analytes mandated by DHS would be 1136.00. The addition of these analytes constitutes an unnecessary quadrupling of monitoring costs.