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FROM 1997- •14 17:46 #888 P.10/24 <br /> February 14, 190 <br /> Paee 9 <br /> State Water Board December SJ 995 Interim Guidance on Required Cleanup at <br /> Low-Risk Fuel Sites, RWQCB, January >. 1996). <br /> Groundwater monitoring well completions between November 1989 and January 1959 <br /> (Wells MI1r-1 through MW-8) were performed by screening across groundwater at <br /> approximately 36 feet bus_ The wells varied in total depth of screen from approximately <br /> 39 to 48 feet bgs. Groundwater' elevations then decreased resulting in all wells becoming <br /> dry by October 1991 through approximately October 1994. <br /> Groundwater monitoring well completions between July 1991 and January 1994 (N%1-9 <br /> through MW-14, and NIW-2R) were constructed by screening between approximately 40 <br /> and 65 (to 7.5)feet bgs reflecting the decreased groundwater conditions. With the <br /> exception of Well MW-12 (completed between 30 and 65 feet bgs), rising groundwater <br /> resulted in the screen intervals of these wells becoming submerged, but also resulted in <br /> gauging and sampling previously dry Wells MW-1, and MW-3 through MW-8- <br /> The SJCEHD commented that wells with greater than 20,feet of screen through the <br /> ,w-oundvater heme problems with dihrtion as do wells with screened hiternols below ice! <br /> Nater. For example, [Well]MW--11 which is screened between 43 and 73 fey!bgs <br /> currently has a groundwater Ierel Uf�7.7�feet bgs; drer�fore this well not only has 30 <br /> feet of screened internal but its screened internal is approximately 6,feet below the <br /> current groundwater level, representing sil;lrificwa dihrtion problems_ <br /> It is true that with the protocol of purging a well prior to sampling dilution occurs with <br /> increasing screen length. No well standard has been maintained which stipulates the <br /> length or diameter of the screen interval. Therefore with increased screen lengths, purge <br /> volumes increase, resulting in increased mixing and diluting of contaminant <br /> concentrations during purging. <br /> if the source of dissolved contaminants is soil contamination at or <br /> above the water table,then a downward decrease in dissolved . <br /> concentration is expected in the formation. When a well screened <br /> across such a concentration gradient is purged, ground water with. <br /> varying contaminant concentrations will flow into the weU throughout <br /> the screen interval. Some of the groundwater entering the well may <br /> contain high concentrations of contaminants, some may contain <br /> non-detectable concentrations. As a result of mixing within the well <br /> during the recovery period following purging,the concentration at the <br /> top of the water column will consequently be lower than it was before <br /> purging(Ground Water Samplings a Pilot Study of the Effects of <br /> Well Purging, Williams et al., 1995) <br /> 32013378 CLORWX3 <br />