stainless steel cable. The tubes convey the gases to and from the pump; the electric cable
powers the water level indicator, and a steel cable supports the downhole assembly. Flow
rate can be controlled by adjusting the driving pressure to the pump. The piston pump
provides continuous sample withdrawal at depths that are greater than most other devices.
The pump can be constructed of materials that minimize the possibility of chemical alteration
of the sample.
The bulk of associated equipment may reduce the portability of the pump. The
valving mechanism may cause a series of pressure drops in the sample that could cause
sample degassing and pH changes. The tubing bundles may be difficult to decontaminate
between wells. The pump intake should be filtered so that particulate matter does not damage
the pump's valving. A study by Yeskis et al. (1988) indicates that gas drive piston pumps
perform similarly to bladder pumps when collecting samples for volatile organics analysis.
Gear Drive Electric Submersible Pumps
Gear drive submersible pumps are designed to be portable and easily serviceable in the
field. A gear drive pump operates using a small high efficiency electric motor that is located
within the pump housing. The electric motor rotates a set of PTFE gears from an intake
screen at the top of the pump. The water is drawn through the gears and driven to a
discharge line that transports the water to the surface. The pumps have self contained power
sources, however, external sources may be used. Flow rates cannot be controlled on
conventional gear drive submersible pumps. Wells that have high levels of suspended solids
may cause the gears to require frequent replacement.
Centrifugal Pumps
Centrifugal (also called impeller) pumps transport fluid by accelerating it radially
outward. Specifically, a motor shaft rotates an impeller that is contained within a casing.
Water that is directed into the center of the rotating impeller is picked up by the impeller
vanes, accelerated by the rotation of the impeller, and discharged by centrifugal force into the
casing. A collection chamber within the casing converts much of the kinetic energy into head
or pressure. Certain submersible centrifugal pumps are constructed for ground water
monitoring purposes. These pumps are fabricated of stainless steel and PTFE, and can be
adjusted to achieve flow rates as low as 0.1 L/min. Studies conducted by Gass et al. (1991)
concluded that low flow rate submersible centrifugal pumps can deliver "representative"
ground water samples. A study conducted by Paul and Puls (1992) comparing a low flow 
rate submersible centrifugal pump, a bladder pump, and a peristaltic pump concluded that the
low flow rate submersible centrifugal pump produced the least negative impacts when trying
to obtain representative and reproducible ground water samples at the particular site and wells
investigated. Research performed by Yeskis et al. (1988) indicates that submersible impeller
pumps perform similarly to bladder pumps when collecting samples for volatile organics
November 1992
7 15






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