At certain existing units, physical obstacles may prevent the installation of monitoring
wells at the point (or points) of compliance. In these cases, the Regional Administrator may
specify an alternate point (or points) of compliance that is as close to the waste management
area as practical, provided the performance standard of early detection of contamination is
Lateral Placement of Point of Compliance Monitoring Wells
Point of compliance monitoring wells should be placed laterally along the
downgradient edge of hazardous waste management units to intercept potential pathways for
contaminant migration. The local ground water flow direction and gradient are the major
factors in determining the lateral placement of point of compliance wells. In a homogenous,
isotropic hydrogeologic setting, well placement can be based on general aquifer characteristics
(e.g., direction and rate of ground water flow), and potential contaminant fate and transport
characteristics (e.g., advection, dispersion). More commonly, however, geology is variable
and preferential pathways exist that control the migration of contaminants. These types of
heterogeneous, anisotropic geologic settings can have numerous, discrete zones within which
contaminants may migrate.
Potential migration pathways include zones of relatively high intrinsic (matrix)
hydraulic conductivities, fractured/faulted zones, and subsurface material that may increase in
hydraulic conductivity if the material is exposed to waste(s) managed at the site (e.g., a
limestone layer that underlies an acidic waste). In addition to natural hydrogeologic features,
human made features may influence the ground water flow direction and thus, the lateral
placement of point of compliance wells. Such human made features include ditches, areas
where fill material has been placed, buried piping, buildings, leachate collection systems, or
adjacent disposal units. These considerations are discussed further in Section The
lateral placement of monitoring wells should be based on the number and spatial distribution
of potential contaminant migration pathways and on the depths and thicknesses of
stratigraphic horizons that can serve as contaminant migration pathways.
In some settings, the ground water flow direction may reverse seasonally (depending
on precipitation), change as a result of tidal influences or river and lake stage fluctuations, or
change temporally as a result of well pumping or changing land use patterns. In other
settings, ground water may flow away from the waste management area in all directions. In
such cases, EPA recommends that to comply with the requirements of  264.97(a)(3),
monitoring wells should be installed on all sides (or in a circular pattern) around the waste
management area to allow for the detection of contamination. In these cases, certain wells
may be downgradient only part of the time, but such a configuration should ensure that
releases from the unit will be detected. In these hydrogeologic settings, ground water
sampling and water level elevation measurements must be performed more frequently than
semi annually, which is the required minimum frequency specified in 40 CFR Part 264,
Subpart F.
November 1992
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