(e.g., a 20 foot screen) because a slightly longer screen would represent a fairly discrete
interval in a very thick formation. Formations with very low hydraulic conductivities also
may require the use of longer well screens to allow sufficient amounts of formation water to
enter the well for sampling. The importance of accurately identifying such conditions
highlights the need for a complete hydrogeologic site investigation prior to the design and
placement of detection wells.
Multiple monitoring wells (well clusters or multilevel sampling devices) should be
installed at a single location when: (1) a single well cannot adequately intercept and monitor
the vertical extent of a potential pathway of contaminant migration, or (2) there is more than
one potential pathway of contaminant migration in the subsurface at a single location, or (3)
there is a thick saturated zone and immiscible contaminants are present, or are determined to
potentially occur after considering waste types managed at the facility. Conversely, at sites
where ground water is contaminated by a single contaminant, where there is a thin saturated
zone, and where the site is hydrogeologically homogeneous, the need for multiple wells at
each sampling location is reduced. Table 6 summarizes factors affecting the decision to
install multiple or single wells at a single location. The number of wells that should be
installed at each sampling location increases with site complexity.
Vadose Zone Monitoring
At some sites where the potentiometric surface or water table is considerably below
the ground surface, contaminants may migrate in the vadose zone for long distances or for
long periods of time before they reach ground water. At other sites, the potential may exist
for contaminants to migrate laterally beyond the downgradient extent the monitoring well
network along low hydraulic conductivity layers within the vadose zone. A vadose zone
monitoring system may be necessary in these and other cases to detect any release(s) from the
hazardous waste management area before significant environmental contamination has
occurred. Leachate released to the vadose zone, for example, may be detected and sampled
using tensiometers. The use of vadose zone monitoring equipment can potentially save the
owner/operator considerable expense by alerting him or her to the need for corrective action
before large volumes of the subsurface have been contaminated.
The Agency recommends unsaturated zone monitoring where it would aid in detecting
early migration of contaminants into ground water. The Regional Administrator also can
require this monitoring on a case by case basis as necessary to protect human health and the
environment under 3004(u) and 3005(c). The elements, applications, and limitations of a
vadose zone monitoring program are provided by Wilson (1980) and USEPA (1986b).
Moreover, the Agency is currently updating its existing guidance on vadose zone monitoring.