In certain situations, purging must be performed with the pump placed at, or

immediately below, the air/water interface. If a bailer must be used to sample the well, the

well should be purged by placing the pump intake immediately below the air/water interface.

This will ensure that all of the water in the casing and filter pack is purged, and it will

minimize the possibility of mixing and/or sampling stagnant water when the bailer is lowered

down into the well and subsequently retrieved (Keeley and Boateng, 1987). Similarly,

purging should be performed at the air/water interface if sampling is not performed

immediately after the well is purged without removing the pump. Pumping at the air/water

interface will prevent the mixing of stagnant and fresh water when the pump used to purge

the well is removed and then lowered back down into the well for the purpose of sampling.

In cases where an LNAPL has been detected in the monitoring well, a stilling tube

should be inserted into the well prior to well purging. The stilling tube should be composed

of a material that meets the performance guidelines outlined in Section 7.3 for sampling

devices. The stilling tube should be inserted into the well to a depth that allows ground water

from the screened interval to be purged and sampled, but that is below the upper portion of

the screened interval where the LNAPL is entering the well screen. The goal is to sample the

aqueous phase (ground water) while preventing the LNAPL from entering the sampling

device. To achieve this goal, the stilling tube must be inserted into the well in a manner that

prevents the LNAPL from entering the stilling tube. One method of doing this is to cover the

end of the stilling tube with a membrane or material that will be ruptured by the weight of

the pump. Some investigators place a piece of aluminum foil over the end of the stilling

tube. The stilling tube is lowered slowly into the well to the appropriate depth and then

attached firmly to the top of the well casing. When the pump is inserted, the weight of the

pump breaks the foil covering the end of the tube, and the well can be purged and sampled

from below the LNAPL layer. The membrane or material that is used to cover the end of the

stilling tube must be fastened firmly so that it remains attached to the stilling tube when

ruptured. Moreover, the membrane or material must retain its integrity after it is ruptured.

Pieces of the membrane or material must not fall off of the stilling tube into the well.

Although aluminum foil is mentioned in this discussion as an example of a material that can

be used to cover the end of the tube, a more chemically inert material may be required, based

on the site specific situation. Stilling tubes should be decontaminated prior to each use

according to the procedures outlined for sampling equipment in Section 7.3.

For most wells, the Agency recommends that purging continue until measurements of

turbidity, redox potential, and dissolved oxygen in in line or downhole analyses of ground

water have stabilized within approximately 10% over at least two measurements    for

example, over two successive measurements made three minutes apart (Puls and Powell,

1992; Puls and Eychaner, 1990; Puls et al., 1990; Puls and Barcelona, 1989a; Puls and

Barcelona, 1989b; USEPA, 1991; Barcelona et al., 1988b). If a well is purged to dryness or

is purged such that full recovery exceeds two hours, the well should be sampled as soon as a

sufficient volume of ground water has entered the well to enable the collection of the

necessary ground water samples.

November 1992

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