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3 <br />N <br />b. Organic lead: Tetraethyl/tetramethyl-lead are volatile; <br />therefore, soil samples should be collected in cylinders and. <br />frozen as described for volatile hydrocarbons above. <br />C . Shipping Samples: Where commercial shippers are involved, <br />dry ice may present Department of Transportation (DOT) <br />shipping problems and "blue ice" may have to be substituted. <br />Water Samples <br />G <br />a. Free floating product (from a well): Sampling of free <br />floating product on the surface of ground water should not <br />be performed until the well has been allowed to stabilize <br />for at least 24 hours after development or other withdrawal <br />procedure. A sample should be collected that is indicative <br />of the thickness of floating product within the monitoring <br />well. This may be accomplished by the use of a clear, <br />acrylic bailer designed to collect a liquid sample where <br />free product and ground water meet. A graduated scale on <br />the bailer is helpful for determining the thickness of free <br />product. Samples should be field -inspected for the presence <br />of odor and/or sheen in addition to the above evaluation. <br />Electronic measuring devices also are available for deter- <br />mining the thickness of the hydrocarbon layer floating on <br />ground water. <br />b. Dissolved product (from a well): If free product is <br />detected, analysis of water for dissolved product should be <br />conducted after the free product has been substantially <br />removed from the well. Before collecting a water sample, a <br />well should be purged until temperature, conductivity and pH <br />stabilize. Often, this will require removal of four or more <br />well volumes by bailing or pumping. Once well volumes are <br />removed and well water is stabilized, a sample can be taken <br />after the water level approaches 80 percent of its initial <br />level. Where water level recovery is slow, the sample can <br />be collected after stabilization is achieved. <br />Ground water samples should be collected in a manner which <br />reduces or eliminates the possibility of loss of volatile <br />constituents from the sample. For collecting samples, a <br />gas -actuated positive displacement pump or a submersible <br />pump is preferred. A Teflon or stainless steel bailer is <br />acceptable. Peristaltic pumps or airlift pumps should not <br />be used. <br />Cross -contamination from transferring pumps (or bailers) <br />from well to well can occur and should be avoided by thor- <br />ough cleaning between sampling episodes. Dedicated (i.e., <br />permanent installation) well pumps, while expensive, are <br />-61- <br />