described in Puls and Powell (1992), Puls and Barcelona (1989a), Puls et al. (1991),

Barcelona, et al. (1990), Kearl et al. (1992), Puls et al. (1990), Puls and Barcelona (1989b),

Barcelona et al. (1985b), Robin and Gillham (1987), Barcelona (1985b), Keeley and Boateng

(1987), Puls and Eychaner (1990), and USEPA (1991).

Purging should be accomplished by removing ground water from the well at low flow

rates using a pump. The use of bailers to purge monitoring wells generally should be

avoided. Research has shown that the "plunger" effect created by continually raising and

lowering the bailer into the well can result in continual development or overdevelopment of

the well. Moreover, the velocities at which ground water enters a bailer can actually

correspond to unacceptably high purging rates (Puls and Powell, 1992; Barcelona et al.,

1990).

The rate at which ground water is removed from the well during purging ideally

should be less than approximately 0.2 to 0.3 L/min (Puls and Powell, 1992; Puls et al., 1991;

Puls and Barcelona, 1989a; Barcelona, et al., 1990). Wells should be purged at rates below

those used to develop the well to prevent further development of the well, to prevent damage

to the well, and to avoid disturbing accumulated corrosion or reaction products in the well

(Kearl et al., 1992; Puls et al., 1990; Puls and Barcelona, 1989a; Puls and Barcelona, 1989b;

Barcelona, 1985b). Wells also should be purged at or below their recovery rate so that

migration of water in the formation above the well screen does not occur. A low purge rate

also will reduce the possibility of stripping VOCs from the water, and will reduce the

likelihood of mobilizing colloids in the subsurface that are immobile under natural flow

conditions. The owner/operator should ensure that purging does not cause formation water to

cascade down the sides of the well screen. At no time should a well be purged to dryness if

recharge causes the formation water to cascade down the sides of the screen, as this will

cause an accelerated loss of volatiles. This problem should be anticipated; water should be

purged from the well at a rate that does not cause recharge water to be excessively agitated.

Laboratory experiments have shown that unless cascading is prevented, up to 70 percent of

the volatiles present could be lost before sampling.

To eliminate the need to dispose of large volumes of purge water, and to reduce the

amount of time required for purging, wells may be purged with the pump intake just above or

just within the screened interval. This procedure eliminates the need to purge the column of

stagnant water located above the well screen (Barcelona et al., 1985b; Robin and Gillham,

1987; Barcelona, 1985b; Kearl et al., 1992). Purging the well at the top of the well screen

should ensure that fresh water from the aquifer moves through the well screen and upward

within the screened interval. Pumping rates below the recharge capability of the aquifer must

be maintained if purging is performed with the pump placed at the top of the well screen,

below the stagnant water column above the top of the well screen (Kearl et al., 1992). The

Agency suggests that a packer be placed above the screened interval to ensure that "stagnant"

casing water is not drawn into the pump. The packer should be kept inflated in the well until

after ground water samples are collected.

November 1992

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