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.,
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
7 8






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