Additional casing diameter selection criteria include: 1) drilling or well installation method
used, 2) anticipated depth of the well and associated strength requirements, 3) anticipated
method of well development, 4) volume of water required to be purged prior to sampling, 5)
rate of recovery of the well after purging, and 6) anticipated aquifer testing.
6.2.5
Casing Cleaning Requirements
Well casing and screen materials should be cleaned prior to installation to remove any
coatings or manufacturing residues. Prior to use, all casing and screen materials should be
washed with a mild non phosphate detergent/potable water solution and rinsed with potable
water. Hot pressurized water, such as in steam cleaning, should be used to remove organic
solvents, oils, or lubricants from casing and screens composed of materials other than plastic.
At sites where volatile organic contaminants may be monitored, the cleaning of well casing
and screen materials should include a final rinse with deionized water or potable water that
has not been chlorinated. Once cleaned, casings and screens should be stored in an area that
is free of potential contaminants. Plastic sheeting can generally be used to cover the ground
in the decontamination area to provide protection from contamination. Aller et al. (1989)
describe the procedures that should be used to clean casing and screen materials.
6.3
Well Intake Design
The owner/operator should design and construct the intakes of monitoring wells to (1)
accurately sample the aquifer zone that the well is intended to sample, (2) minimize the
passage of formation materials (turbidity) into the well, and (3) ensure sufficient structural
integrity to prevent the collapse of the intake structure.
6.3.1
Well Screen
The goal of a properly completed monitoring well is to provide low turbidity water
that is representative of ground water quality in the vicinity of the well. Although wells
completed in rock often do not require screens, the majority of monitoring wells installed for
RCRA purposes are completed in unconsolidated sediments.
6.3.1.1
Screen Length
The selection of screen length usually depends on the objective of the well.
Piezometers and wells where only a discrete flow path is monitored (such as thin gravel
interbedded with clays) are generally completed using short screens (2 feet or less). To avoid
dilution, the Agency prefers that well screens be kept to the minimum length appropriate for
intercepting a contaminant plume, especially in a high yielding aquifer. The screen length
should generally not exceed 10 feet. If construction of a water table well is the objective,
either for defining gradient or detecting floating phases, then a longer screen is acceptable
because the owner/operator will need to provide a margin of safety that will guarantee that at
November 1992
6 39






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