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Ground Water Sampling <br /> • Gas-dnve Piston bangs <br /> A piston pump uses compressed air to force a piston to raise the sample to the <br /> surface A typical design consists of a stainless steel chamber between two <br />' pistons The alternating chamber pressurization activates the piston, which allows <br /> water entry during the suction stroke of the piston, and forces the sample to the <br /> surface during the pressure stroke Pumping rates of 500 mllmm have been <br /> reported from 30 5 meters, sampling depths of 150 meters are possible The <br /> piston pump provides continuous sample withdrawal at depths greater than is <br /> possible with most other approaches Nevertheless,contribution of trace elements <br /> from the stainless steel and brass fittings is a potential problem Pumping rates at <br /> depths less than 150 meters are generally slower than with other pumps <br /> Centrifugal Pump <br />' A centrifugal(sometimes called impeller)pump is similar to the direct line pump <br /> except that a centrifugal pump is connected to the tubing at the surface rather than <br /> a vacuum pump A foot valve is usually attached to the end of the well tubing to <br /> assist in priming the extraction tube A centrifugal pump is capable of delivering <br /> large quantities of water, against high as well as low head conditions, with good <br /> efficiency Under field conditions a centrifugal pump has an average suction lift <br /> capability of 20-25 feet (6.1-7.6 meters) (Driscoll, 1986). Although relatively <br /> high pumping rates can be attained, centrifugal pumps cause sample agitation <br />�. 2.3.3 Suction Lift Pumps <br /> Suction lift pumps can be categorized as direct line and peristaltic The direct line <br /> pump requires lowering one end of a plastic tube into a well or piezometer The <br /> surface end of the tube is connected to a two-way stoppered bottle, and a manually <br /> or auxiliary powered vacuum pump is attached to a second tube that leads from <br /> the bottle A check valve is attached between the second tube and the vacuum <br /> pump to maintain a constant vacuum control <br />' A peristaltic pump (also called rotary peristaltic) is a self-priming, low-volume <br /> suction pump consisting of a rotor and three ball bearing rollers Plastic tubing <br /> inserted around the pump rotor is squeezed by the rollers as they revolve in a <br /> circle around the rotor One end of the tubing is placed into the well while the <br /> other end is connected directly to a two-way stoppered flask As the rotor <br /> revolves, water is drawn into the sampling tube and discharged into the collection <br /> vessel A drive shaft connected to the rotor head can be extended so that multiple <br /> rotor heads can be attached to a single drive shaft The withdrawal rate of <br /> peristaltic pumps can be carefully regulated by adjusting the rotor head revolution <br /> The system can be arranged so that the sample contacts only fluorocarbon resin <br /> tubing prior to entering the sample container A limiting factor is the depth of ' <br /> sampling, the depth of sample collection is limited to situations where the <br /> potentiometric level is less than 25 feet below land surface (Nielsen, 1991). <br />' The suction lift approach offers a simple retrieval method for shallow monitoring <br /> wells However, the method can result in sample mixing and oxidation <br /> Degassing and loss of volatiles also occur to some extent A peristaltic pump <br /> 1 <br /> 14 <br /> I <br />