7.3.2.6

Peristaltic Pumps

A peristaltic pump (also called rotary peristaltic) is a low volume pump that operates

by suction lift. Plastic tubing is inserted around the pump rotor. Rotating rollers compress

the tubing as the rollers revolve around the rotor, forcing fluid movement ahead and inducing

suction behind each roller. As the rotor revolves, water is drawn into a sampling tube that

has been inserted into the well, and discharged into the sample container. Peristaltic pumps

often require the use of flexible silicone tubing, which is unsuitable for ground water

sampling purposes. The withdrawal rate of peristaltic pumps can be carefully regulated by

adjusting the rotor head revolution. The use of a peristaltic pump is limited by the depth of

sampling; the depth of sample collection is limited to situations where the potentiometric level

is less than 25 feet below land surface (Herzog et al., 1991).

The Agency does not recommend the use of peristaltic pumps to sample ground water,

particularly for volatile organic analytes. The method can cause sample mixing and oxidation

resulting in degassing and loss of volatiles. Although Tai et al. (1991) indicated that

peristaltic pumps may provide adequate recovery of volatile organic compounds, Imbrigiotta

et al. (1988) concluded that for sampling volatile organic compounds, peristaltic pumps were

inferior in comparison to other sampling devices. Imbrigiotta et al. attributed the poor

performance of the peristaltic pump to degassing of volatile contaminants into the vacuum

created by the pump. Puls and Barcelona (1989a) and Puls and Barcelona (1989b) indicated

that vacuum pumps such as peristaltic pumps may significantly alter ground water chemistry

leading to colloid formation in the monitoring well.

7.3.2.7

Gas Lift Pumps

An air  or gas lift pump allows collection of ground water samples by bubbling air or

gas at depth in the well. Sample transport occurs primarily as a result of the reduced specific

gravity of the water being lifted to the surface. Water is forced up a discharge pipe, which

may be the outer casing or a smaller diameter pipe inserted into the well. The considerable

pressures required for deep sampling can result in significant redox and pH changes. Gas lift

pumps should not be used for any purpose in ground water investigations.

7.3.2.8

Gas Drive Pumps

Gas drive (gas displacement) pumps are distinguished from gas lift pumps by their

method of sample transport. Gas drive pumps force a column of water under linear flow

conditions to the surface without extensive mixing of the pressurized gas and water. A

vacuum also can be used to assist the gas. The disadvantages of a gas drive pump are that

the drive gas comes into contact with the water and therefore, can be a source of

contamination; also, the pump can be difficult to clean. Gas drive pumps are not

recommended for sampling monitoring wells.

November 1992

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