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