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

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