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February 14, 1997 Page 9 . State Water Board December 8, 1995 Interim Guidance on Requited Cleanup at Low-Risk Fuel Sites, RWQCB, January 5, 1996) Groundwater monitoring well completions between November 1988 and January 1989 (Wells MW-I through MW-8) were performed by screening across groundwater at approximately 36 feet bgs The wells varied in total depth of screen from approximately 39 to 48 feet bgs Groundwater elevations then decreased resulting in all wells becoming 'dry by October 1991 through approximately October 1994 Groundwater monitoring well completions between July 1991 and January 1994 (MW-9 through MW-14, and MW-2R) were constructed by screening between approximately 40 and 65 (to 75) feet bgs reflecting the decreased groundwater conditions With the + exception of Well MW-12 (completed between 30 and 65 feet bgs), rising groundwater 'resulted in the screen intervals of these wells becoming submerged, but also resulted in gauging and sampling previously dry Wells MWA, and MW-3 through MW-8 I The SJCEHD commented that wells with gr eater than 20 feet of scl eel) through the g7oundwate7 have prohlenis with drhitlon as do wells with screened nitervals below fn st + wate7 For example, [Well]MW-11 which 1 s scl eened between 43 and 73 feet bgs currently has a gl oundwate7 level c,,f 37 74 feet hgs, thelefole this well not only has 30 feet of sc7 eened interval but its screened interval i s approximately 6 feet below the current groundwater level, representing ilgf7ificant dihition prohlen7s It is true that with the protocol of purging a well prior to sampling dilution occurs with increasing screen length No well standard has been maintained which stipulates the length or diameter of the screen interval Therefore with increased screen lengths, purge volumes increase, resulting in increased mixing and diluting of contaminant concentrations during purging If the source of dissolved contaminants is soil contamination at or above the water table, then a downward decrease in dissolved concentration is expected in the formation When a well screened across such a concentration gradient is purged, ground water with varying contaminant concentrations will flow into the well throughout the screen interval Some of the groundwater entering the well may contain high concentrations of contaminants, some may contain non-detectable concentrations As a result of mixing within the well during the recovery period following purging, the concentration at the top of the water column will consequently be lower than it was before purging (Ground Water Sarnpling�a Pilot Study of the Effects of Well Purging, Williams et at , 1995) 5 1 JI 1 320133781CLOSCIRE3 1