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1 <br /> Ground Water sampling <br /> 3 Determine the static ground-water level using a water level sounder or <br />' other device listed in Section 2 2 1 <br /> 4 Two possible methods to determine the presence of DNAPL are <br />' 1)lower an interface probe (conductivity or resistivity sensor)to the well <br /> bottom to determine if an organic liquid is present, or 2) lower a <br /> transparent, double check-valve bailer to the bottom of the well and <br /> twithdraw a sample to visually check for the presence of DNAPL <br /> The best method for collecting DNAPLs is to use a double-check valve bailer <br /> DNAPLs should be sampled prior to well purging The key to sample collection <br />' is controlled, slow lowering(and raising)of the bailer within the well <br /> 2.2.3 Well Purging <br />' Because the water standing in a well prior to sampling may not be representative <br /> of in-situ ground-water quality,the standing water should be purged from the well <br />' and filter pack prior to sampling The SAP should include detailed, step-by-step <br /> procedures for purging wells, including the number of well volumes (casing and <br /> filter pack) to be removed prior to each sampling event, and the parameters that <br />' will be monitored during purging The equipment to be used for well purging <br /> should also be described in the SAP. Purging should generally be performed by <br /> pumping water from the well,but in some cases it may be necessary to purge the <br /> well by bailing <br />' The purging procedure should ensure that samples collected from the well are <br /> representative of the ground water to be monitored Two primary criteria are <br />' typically used to determine if wells have been purged sufficiently to produce <br /> representative ground-water samples Some investigators believe that the removal <br /> of a set number of casing and filter pack volumes,usually between 3 and 10,will <br />' yield representative ground-water samples Other investigators argue that <br /> representative samples are obtained(particularly when VOCs are the analytes of <br /> interest)when values for certain parameters, usually temperature, pH, dissolved <br />' oxygen, turbidity,and specific conductance,have stabilized(e g ,are reproducible <br /> within 10percent) At some sites, these parameters may stabilize at slow rates, <br /> which would likely result in the withdrawal of too much water from the well The <br /> opposite result, of removing too little water, is of concern when using the fixed <br />' casing volume criterion Both criteria for indicating sufficient purging are <br /> irrelevant when applied to low yielding formations that are incapable of yielding <br /> three casing and filter pack volumes before the well goes dry Most reports on <br />' low-yielding wells recommend purging them to dryness and obtaining the sample <br /> after water levels have recovered sufficiently to allow collection of the required <br /> sample volume, a recommendation with which Cal/EPA agrees <br />' For most wells, Cal/EPA recommends that a minimum of 3 casing volumes of <br /> water(determined from the total column of water in the well)be removed during <br />' purging An acceptable alternate method for purging wells is to pack off the well <br /> screen and purge only from the screened interval(in this case, casing volumes are <br /> calculated from the pumped interval only) Cal/EPA does not recommend a <br /> maximum number of casing volumes for purging Purging should continue until <br /> 1 <br /> 6 <br />