enough, a double check valve (point source) bailer or a submersible pump (see Section 7.3)
can be used. If a bailer is used, the bailer should be lowered slowly until contact is made
with the surface of the LNAPL. The bailer should then be lowered to a depth less than the
depth of the LNAPL/water interface, as determined beforehand using the interface probe. A
double check valve bailer also may be used to sample a DNAPL. A submersible pump also
may be used to sample a DNAPL if the DNAPL layer is of sufficient thickness.
When the thickness of the LNAPL layer in the well casing is too small to be sampled
with a double check valve bailer or pump, the bailer should be modified to allow filling only
from the top. If a top filling bailer is not available, sampling personnel can disassemble the
bottom check valve of a bailer and insert a piece of fluorocarbon resin sheet between the ball
and ball seat. This will seal off the bottom valve. The ball from the top check valve should
be removed to allow the sample to enter from the top. The buoyancy that occurs when the
bailer is lowered into the LNAPL can be overcome either by using a stainless steel bailer or
by securing a length of 1 inch stainless steel pipe (Type 304, Type 316) below the bailer.
The bailer should be lowered carefully into the well, measuring the depth to the surface of the
LNAPL layer, until the top of the bailer is level with the top of the LNAPL layer. The bailer
should be lowered an additional one half thickness of the LNAPL layer and the sample
should then be collected. This technique is the most effective method of sample collection if
the LNAPL is only a few inches thick.
When the LNAPL layer in the well casing is less than approximately 2 inches thick,
an alternative method is necessary. In this situation, a sample should be collected from the
top of the water column using a bailer. The two phase water/LNAPL sample should be
appropriately containerized and submitted for laboratory analysis. The laboratory should be
instructed to analyze the non aqueous phase of the two phase sample.
Because the water standing in a well prior to sampling may not represent in situ
ground water quality, stagnant water should be purged from the well and filter pack prior to
sampling. The QAPjP should include detailed, step by step procedures for purging wells,
including the parameters that will be monitored during purging and the equipment that will be
used for well purging.
The purging procedure should ensure that samples collected from the well are
representative of the ground water to be monitored. Over the years, investigator opinions
have varied widely regarding the most appropriate procedure for purging wells. Many
investigators believe that a specified number of well volumes should be purged from a well,
some investigators believe that purging procedures should be based on hydraulic performance
of the well, others believe that wells should be purged until certain geochemical parameters
have stabilized, and yet others believe that wells should not be purged at all. The Agency's
guidance regarding well purging is based on information based on research and studies