5.1.3
Placement of Background (Upgradient) Monitoring Wells
The ground water monitoring well system must allow for the detection of
contamination when hazardous waste or hazardous constituents have migrated from the waste
management area to the uppermost aquifer. A sufficient number of background wells must be
installed at appropriate locations and depths to yield ground water samples from the
uppermost aquifer that represent the quality of background water that has not been affected by
leakage from a regulated unit ( 264.97(a)). In most cases, background wells should be
located hydraulically upgradient of the waste management unit(s); however, in certain
circumstances a determination of background ground water quality may include sampling of
wells that are not hydraulically upgradient of the waste management area. Specifically,
 264.97(a)(1)(i) provides that the determination of background ground water quality may
include sampling wells that are not hydraulically upgradient of the waste management area
where:
Hydrogeologic conditions do not allow the owner or operator to determine
what wells are hydraulically upgradient; or
Sampling at other wells will provide an indication of background ground water
quality that is representative or more representative than that provided by the
upgradient wells.
A sufficient number of background monitoring wells should be installed to allow for
stratified (depth discrete) comparisons of water quality and to account for spatial variability in
ground water quality. Background monitoring wells should not be screened over the entire
thickness of any saturated zone that can act as a contaminant transport pathway. Screening
the entire thickness of such zones will not allow depth discrete water quality data to be
obtained. Instead, shorter well screens should be placed at depths comparable to those used
for detection monitoring wells as shown in Figure 10. Background and point of compliance
wells must be screened in the same hydrostratigraphic position to allow collection of
comparable ground water quality data. Stiff and Piper diagrams can aid in this determination.
Hem (1989) is a good reference on ground water chemistry.
To establish background ground water quality, it is necessary to establish ground water
flow direction(s) and to place wells hydraulically upgradient of the waste management area.
Certain geologic and hydrologic situations make the determination of hydraulically upgradient
locations difficult. These cases require careful site characterization to position or place
background wells properly. Examples of such cases include the following:
Waste management areas above naturally occurring or human made
ground water mounds;
November 1992
5 10






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