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' Ground Water Sampling <br /> • sampler Single check valve bailers should not be used if the well has <br />' been purged by withdrawing water from the top or center of the well <br /> screen because the bailer may sample stagnant water located above the <br /> screened interval as it is raised through the water column <br /> Double Check Valve Bailers <br /> The double check valve bailer (also known as the point source bailer) is <br /> designed for sampling a prescribed depth within a water column In this <br /> design, water flows through the sample chamber as the bailer is lowered <br /> A tapered inlet and outlet ensures that water passes freely through the <br /> bailer with minimum disturbance to the water column When the desired <br /> depth is reached, the bailer is retrieved Because the tolerance between <br /> each ball and check valve seat is maintained by a pin that blocks the <br /> vertical movement of the check ball, both check valves close <br /> simultaneously upon retrieval A bottom-emptying device is placed into <br /> the bottom of the bailer to discharge the sample The device minimizes <br />' agitation and allows the sample to flow slowly into the sample container <br /> Inert tubing may also be attached to the bottom emptying device and <br /> connected to a two way stoppered bottle into which the nonaerated sample <br /> flows Double check valve bailers provide a means for collecting a <br /> relatively undisturbed water sample within a water column <br /> • Syringe Bailer <br /> A syringe bailer is distinguished from other bailers by the means of water <br /> entry (Morrison, 1983) The syringe is lowered into a well and water is <br /> drawn into the chamber by activating a plunger via suction To recover <br /> the sample, the syringe is withdrawn from the well and the sample is <br /> transferred into a collection bottle or infected directly into an appropriate <br />' instrument for water quality analysis The syringe bailer is a good <br /> sampling method when used as both a sampler and a sample container <br /> The advantages of this approach include the ability to collect point source <br /> samples and the ability to collect samples for volatile organic analysis <br /> The small syringe size is a limitation when large sample volumes are <br /> required <br /> 23.2 Positive Displacement (Submersible)Mechanism <br /> Positive displacement mechanisms for ground-water sampling include gear drive <br />' electric submersible pumps, bladder pumps, helical rotor electric submersible <br /> pumps, gas-drive piston pumps, and centrifugal pumps The following sections <br /> describe each of these types of pumps and their applications and limitations with <br /> regard to collecting ground-water samples <br /> Bladder Pumps <br />' Bladder pumps (also referred to as gas squeeze pumps) consist of a flexible <br /> membrane often enclosed in a rigid stainless steel housing A strainer or screen <br /> attaches below the bladder to filter any material that could clog either of the check <br /> 12 <br />