subsurface conduits are the primary pathways that contaminant releases follow. Identifying
and intercepting these conduits with wells is an extremely formidable task. Identifying
contaminant transport pathways requires detailed site characterization beyond what is
currently performed at most RCRA facilities. In addition, the wide fluctuations in the water
table that are characteristic of aquifers dominated by conduit flow make identification and
satisfactory monitoring of the uppermost aquifer particularly difficult.
It may be possible for some facilities that are sited above conduit flow aquifers to
have ground water monitoring systems that meet the performance standards of 40 CFR
 264.97. The Regional Administrator may require the facility owner or operator to monitor
seeps, springs, and caves that are hydraulically connected to the uppermost aquifer and that
are within the facility boundary to supplement the monitoring well network. These
supplemental monitoring sites can be used in conjunction with point of compliance wells to
detect releases from the facility ( 264.97(a)(3)). However, the Agency expects that these
cases will be rare, and that most facilities sited in karst settings will be unable both to meet
the performance standards of  264.97 and to receive an operating permit. Therefore, prior to
locating a waste management facility in an area where any conduit flow exists, an owner or
operator should consider the inherent difficulties in meeting the ground water monitoring
requirements of Subpart F. The owner/operator should select a different location if it appears
as though a release to a conduit flow aquifer could not be detected or controlled during a high
precipitation event. The following sections of requirements and guidance are for owners and
operators of facilities located above aquifers dominated by conduit flow that meet the
performance standards of  264.97. These sections provide additional information on
designing a supplemental monitoring well network for the seeps, springs, and conduits that
are hydraulically connected to the uppermost aquifer and that are located on the facility
Using Springs as Monitoring Sites in Aquifers Dominated by Conduit
In certain circumstances, the Regional Administrator may request that a strategy of
monitoring seeps, springs, and cave streams be applied to supplement monitoring well
systems in all aquifers dominated by conduits that drain to springs and that discharge on land
or along the shores of streams, rivers, lakes, or seas. In terranes where conduit flow
predominates, springs and cave streams (if they have been shown by tracer studies to drain
from the facility being evaluated) are the easiest and most reliable sites at which to monitor
ground water quality (Field, 1988; Quinlan, 1989; Quinlan, 1990).
Most springs that should be sampled regularly during tracer tests and ground water
monitoring are not shown on USGS topographic quadrangle maps. Furthermore, the inclusion
of a spring on a topographic map is not necessarily an indicator of the significance of its
discharge, as many minor springs are included on maps because of their cultural associations.
In certain cases, the owner/operator will need to conduct detailed field work to locate all
November 1992
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