TM 5-814-7
ter of the screened interval; in wells drilled in rock, well
essary to detect seasonal changes or other fluctuations
diameters may be as little as 2 inches larger than the
in the water table which could affect flow direction and
screened interval.
the well's ability to yield a representative sample.
(4) Regulations in 40 CFR 264:F require
(2) Before a sample is withdrawn,
that the annular space between the well casing and the
standing water should be purged from the well. This is
borehole be sealed to prevent contamination of the
an important procedure, since such water can have
ground water and/or sample. Both cement grout and
substantially different chemical characteristics from the
bentonite are effective agents that are commonly used
ground water to be sampled, due to dissolution of gases;
for sealing monitoring wells. If portland cement is used,
leaching or adsorption of casing, screen or grout
special care should be taken to minimize shrinkage, as
materials; and/or biological activity within the well. It is
well as to prevent migration of the grout into adjacent
generally recommended that wells be completely
formations. Alternatively, a grout mixture of portland
evacuated before sampling. High-yield wells should, if
cement, sand, bentonite and water can be used. If
possible, be pumped dry twice and allowed to recover
bentonite is used, a 3to 5-foot seal of bentonite pellets
before sampling; one complete evacuation is sufficient
must be placed between the well casing and the
for lowyield wells. If complete evacuation is not possible,
borehole. A base of sand may also be necessary around
a volume of water equal to 4 to 10 times the amount of
and above the screen. Installation of sealing agents is
standing water should be withdrawn. The exact volume
best accomplished with a tremie pipe; pouring and
to be withdrawn will depend on site-specific conditions.
tamping may, however, be adequate for shallow wells of
(3) A variety of sampling devices are
small diameter.
available, including bailers, portable pumps, air-lift
(5) Well development is necessary to
sampler and suction pumps. Care should be taken to
ensure the free flow of water into the sampling interval,
choose equipment that will not contaminate the sample,
to purge drilling fluids and other contaminants, and to
particularly when trace elements are to be analyzed. All
eliminate clay, silt and other fines which could contribute
equipment should be thoroughly cleaned before
to water turbidity and interfere with chemical analysis. In
introduction into a monitoring well. Once a sampling
developing the well, ground water within the casing is
device has been chosen, the same equipment and
repeatedly forced in and out of the sampling interval by
sampling procedure should be used in subsequent
flow reversal or surge. The well is then pumped or
sampling, if values are to be compared.
bailed until a volume of clear water equal to that required
(4) Accepted procedures for preserving
for operation of the sampling program is obtained. If the
and protecting ground-water samples during shipping
well cannot be adequately developed, it should be
and while awaiting laboratory analysis should be
replaced with a new well.
followed. All samples should be firmly sealed, clearly
g. Federal regulations for both existing
labelled and packed in compatible containers that will
facilities and new facilities require that a ground-water
prevent breakage, spills and contamination.
The
sampling and analysis plan be prepared which details
sampling schedule and methods of analysis should be
procedures and techniques to be followed in collecting,
according to the regulations in 40 CFR 264:F and the
preserving, shipping and analyzing samples.
guidelines presented in the RCRA ground-water
(1) Water level measurements are
monitoring guidance manual.
required each time a sample is collected.
Such
measurements are nec-
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