This chapter discusses the design of detection monitoring systems. Section 5.1
addresses the design of detection monitoring systems in environments where ground water
flow occurs through porous media. As discussed in Section 4.3.1, the term "porous media"
generally encompasses both unconsolidated granular deposits and rock (Freeze and Cherry,
1979). In some areas underlain by fractured rock or karst terrane, ground water flow does not
conform to the principles that describe ground water flow through porous media. In these
settings, ground water flow may occur predominantly through conduits and fractures.
Appropriate supplemental monitoring strategies for these settings are discussed in detail in
Section 5.2.
Ground Water Monitoring in Aquifers Dominated by Ground Water
Flow Through Porous Media
This section provides guidance for determining the number and location of detection
monitoring wells in aquifers dominated by ground water flow through porous media. The
correct placement of monitoring wells relative to hazardous waste management units is an
obvious goal of a detection monitoring program.
The location of both background and point of compliance (i.e., downgradient)
monitoring wells at permitted facilities must comply with the requirements of  264.97. Point
of compliance monitoring wells should be located so that they intercept potential pathways of
contaminant migration. Background wells should be located so that they provide ground 
water samples that are representative of the quality of ground water that has not been affected
by leakage from the waste management unit. The number and location of monitoring wells
must allow for the detection of contamination when hazardous waste or hazardous
constituents have migrated from the waste management area to the uppermost aquifer
( 264.97(a)(3)).
There is no required minimum number of wells at permitted facilities; the
owner/operator is simply required to install a "sufficient" number of wells to allow for
determination of background water quality and the water quality at the point of compliance.
Typically, the minimum number of wells specified for interim status facilities in 40 CFR
 265.91(a) will not be sufficient for achieving the performance objectives of a detection
monitoring system because site hydrogeology is too complex or the hazardous waste unit is
too large. Supplemental monitoring wells may be required in conjunction with point of
compliance wells to ensure early detection of contamination. In addition, unsaturated zone
November 1992
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