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

met.

5.1.2.2

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 4.3.3.4. 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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