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. recover prior to collecting a sample If the Advantages <br />' recovery rate is fairly rapid and time allows, <br /> evacuation of more than one volume of water C Only practical limitations on size and <br /> is preferred If recovery is slow, sample the materials <br />' well upon recovery after one evacuation <br /> C No power source needed <br /> 6 A non-representative sample can also result <br /> from excessive pre-pumping of the C Portable <br /> monitonng well Stratification of the <br /> leachate concentration in the ground water C Inexpensive,so it can be dedicated and hung <br /> formation may occur, or heavier-than-water in a well, thereby reducing the chances of <br /> compounds may sink to the lower portions of cross contamination <br />' the aquifer Excessive pumping can dilute or <br /> increase the contaminant concentrations from C Minimal outgassing of volatile organics <br /> what is representative of the sampling point while sample is in bailer <br />' of interest <br /> C Readily available <br /> 43 Materials <br /> C Removes stagnant water first <br />' Materials of construction for samplers and evacuation <br /> equipment (bladders, pump, bailers, tubing, etc) C Rapid, simple method for removing small <br /> should be limited to stainless steel, Teflon', and glass volumes of purge water <br /> in areas where concentrations are expected to be at or <br /> near the detection limit The tendency of organics to Disadvantages <br /> leach into and out of many materials make the <br /> selection of materials cntical for trace analyses The C Time-consuming to flush a large well of <br /> use of plastics, such as PVC or polyethylene, should stagnant water <br /> be avoided when analyzing for organics However, <br /> PVC may be used for evacuation equipment as it will C Transfer of sample may cause aeration <br />' not come in contact with the sample, and in highly <br /> contaminated wells, disposable equipment (i e , C Stoppers at the bottom of the bailer usually <br /> polypropylene bailers) may be appropriate to avoid leak thus the bailer must be brought to the <br /> cross-contamination surface rapidly <br />' Materials of construction (bladders/pumps, bailers, C If the bailer is allowed to hit the bottom of <br /> tubing, etc) suitable for collecting and handling the well boring,gravel can displace the ball <br /> Volatile Organic Samples should be limited to valve not allowing the bailer to hold water <br />' stainless steel, Teflon and glass in areas which <br /> detection limit range concentrations are expected 4 4 2 Submersible Pumps <br /> The tendency of organics to leach into and out of <br />' many materials, make the selection of matenals Advantages <br /> critical for these trace analyses The use of plastics, <br /> e g , PVC etc , should be avoided There are C Portable and can be transported to several <br /> numerous ways of introducing foreign contaminants wells <br />' into a sample,and these must be avoided by following <br /> strict sampling procedures and utlization of trained C Depending upon the size of the pump and the <br /> personnel pumping depths, relatively high pumping <br />' rates are possible <br /> 4.4 Advantages/Disadvantages of <br /> Certain Equipment C Generally very reliable and does not require <br /> priming <br /> 4 4 1 Bailers <br /> 3 <br />