TM 5-814-7
ing hydrogeologic conditions are detailed in chapter 3 of
than 20 feet thick), the well should be perforated
this manual.
throughout the zone. In thicker aquifers, multiple wells
b. A minimum of four ground-water monitoring
(see figure 8-2) should be used to define water quality
wells will be installed, one hydraulically upgradient of the
stratification within the aquifer. Care should be taken to
waste disposal unit, to provide background groundwater
ensure that the perforated interval does not provide
quality data, and three downgradient of the facility to
hydraulic connection between isolated aquifers.
detect contaminant discharge. Small indoor waste piles
(3) Also important is the sizing of the
are the only waste facilities at which fewer wells will be
perforations or screen.
A properly sized screen,
considered.
generally one designed to exclude up to 60 percent of
c. Upgradient wells should be installed in the
formation materials, will prevent passage of fines from
the formation, while allowing passage of sufficient water
uppermost aquifer at a location not likely to be affected
for sampling. In most cases a commercially fabricated
by the waste facility. Downgradient wells should also be
screen is recommended, although a factory-slotted
installed in the uppermost aquifer, but along pathways
casing may be adequate for some applications. Field
likely to transport contaminants, should any be released
perforation of well casings is not recommended.
from the facility. Care must be taken in locating and
(4) Materials selected for the well casing
constructing monitoring wells to ensure that they not
should be compatible with the expected contaminants to
serve as conduits for contaminants to enter the ground
minimize the potential for interaction between the casing
water, or allow contaminated ground water to migrate to
material and the sample. Steel casings may contribute
an uncontaminated aquifer.
d. Well depth should be determined on a
iron and other ions to the sample. Furthermore, the
metallic oxides which form on a steel casing influence
sitespecific basis. Factors which influence well depth, as
concentrations of cautions and some organic molecules.
well as the depth of the sampling (or intake) interval of
PVC pipe, unlike steel well casing, is resistant to most
the well casing, include ground-water levels and the
chemicals, nonconductive, and chemically inert;
behavior of specific contaminants in the aquifer. These
however, PVC is not recommended for sampling certain
determinations are dependent on a detailed log of
reactive organic constituents such as ketones or
borings and on the subsurface geologic conditions. e.
aromatic compounds, which can better be accomplished
The principal components of the monitoring well are the
using stainless steel or teflon. However, the final
well casing and the perforated or screened sampling
selection of well materials should be determined by a
interval. A typical ground-water monitoring well is shown
person knowledgeable about the probable chemical
on figure 8-1. Details on well design and sampling
reactions (e.g., a chemist or chemical engineer).
methods appear in SW-611 and in the RCRA guidance
Needed joints in PVC casings should be fashioned using
manual on ground-water monitoring. It must be stressed
threaded couplings instead of glue to avoid
that well design must always be based on a clear and
contamination.
detailed understanding of site hydrogeologic conditions.
(5) Locking caps and concrete pads
(1) One of the considerations in design of
should be installed on all monitoring wells. Pads should
the well is selection of the proper well diameter, which
be designed to divert drainage from the casing, thereby
depends on a number of factors, including state and
preventing precipitation or extraneous substances from
federal requirements, drilling method and subsurface
entering the well.
conditions, as well as the diameter of the sampler.
f.
Well drilling methods, filter packing, sealing
Monitoring wells generally have casing diameters of
and development are the components of concern in well
either 2 or 4 inches. The larger casing size permits
construction, both to maintain the integrity of the
greater flexibility in sampling methods, since an inner
borehole and to prevent contamination of samples.
diameter of 4 inches is generally required to
(1) The drilling method selected should
accommodate submersible pumps and other equipment
avoid spreading any ground-water contamination and/or
used for evacuation and sampling. Two-inch casings
interfering with the sample to be collected. Both dry and
may be necessary or favorable in some instances,
wet drilling methods are commonly used to construct
however, since they can be installed by the dry hollow-
monitoring wells.
Conventional auger drilling is
stem continuous flight auger drilling method. Some drill
advantageous, since the potential for introducing
rigs can install 4-inch casings but such rigs are not
extraneous fluids is less than with rotary drilling methods.
always readily available.
Auger drilling is best suited to fine-grained,
(2) Proper location of the intake, or
nonconsolidated materials; rotary (air or water) drilling is
sampling, interval of the monitoring well is extremely
required for wells in cemented or consolidated materials
important to ensure that it is in the path of likely
such as bedrock. The maximum casing diameter in
contaminants and therefore likely to yield representative
wells drilled by the standard continuous flight auger
samples. Where aquifer zones are relatively thin (i.e., no
more
8-2
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