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Jim Tjosvold <br /> Page 4 <br /> July 3 , 1987 <br /> sample, should provide information on how much of a <br /> contaminant might be associated with the soil matrix in <br /> the aquifer. <br /> 17 . If any total chromium is detected in groundwater, testing <br /> for hexavalent chromium must be added to the general and <br /> regular scan. Information on the analytical method, <br /> preservation, and analysis time must be provided for <br /> hexavalent chromium at that time. <br /> 18 . Detection limits for nitrosoamines are not acceptable. <br /> The laboratory must be able to achieve limits stated in <br /> EPA method 607 . <br /> Detection limit for benzene should be lowered to 0. 5 ug/l <br /> for drinking water protection. <br /> 19 . Water quality monitoring must be frequent enough to build <br /> a reliable data base for plume definition from monitoring <br /> wells and to protect public health in private wells. <br /> Frequency of testing should be increased to quarterly for <br /> wells DSW-2A and 2B, DSW-4C and 4E, A6, OS-1E, and OS-3E. <br /> Additionally, private wells at Gold Bond and Stockton <br /> City Police Department are to be monitored on a quarterly <br /> basis. All other wells proposed for annual testing are <br /> to be analyzed semi-annually for a minimum of eighteen <br /> months. <br /> Aquifer Testing <br /> 20. At this point we are unconvinced that the pump tests as <br /> proposed will produce substantial new information. <br /> Accordingly, to ensure that pump tests are valid, <br /> McCormick and Baxter should reconsider the proposed pump <br /> test configurations and analysis method. <br /> In our opinion, one method which appears appropriate for <br /> performing pump tests is according to procedures <br /> described by Neuman and Witherspoon (1969) . Based on <br /> preliminary calculations, it appears that leakage in the <br /> hydraulic system underlying the site is so high that it <br /> will obscure the interpretation of transmissivity values <br /> during pump tests as now proposed by McCormick and <br /> Baxter. It is also noted that observation wells appear <br /> not to be properly placed to determine the vertical <br /> permeability of confining beds. To mitigate these <br /> issues, observation wells in permeable horizons should be <br /> located no farther than 50 feet from pumping wells. <br /> Also, piezometers (maximum well screen length of 5 feet) <br /> used as observation wells should be installed in <br /> confining beds above and below the primary observation <br /> well (s) . Pumping should be performed during short <br /> duration periods comprising only a few hours. In <br />