areas, such as the karst terrane described in a study by Shuster and White (1971). Quinlan
(1990) discusses the differences between conduit and diffuse flow and provides a relatively
simple method for distinguishing between a conduit flow spring and a diffuse flow spring.
Karst ground water systems developed in both younger limestones, such as those in
Puerto Rico and Florida, and in older limestones, such as those in the Appalachians, the
Ozarks, and the Kentucky Indiana karst region, may be either conduit flow or diffuse flow.
Younger limestones, however, may have significant primary porosity, so that they can be
likened to a gigantic sponge in which flow occurs throughout the entire aquifer through huge
pores rather than being constrained in conduits. Consequently, the type of flow found in
some younger, highly porous limestones may be rapid and turbulent    not the slow, linear
flow described by Darcy's law.
In the United States, lava tubes and caves occur in areas of great thicknesses of
basaltic lava flows (Hawaii and the Columbia Plateau and Snake River Plain of the Pacific
Northwest), but conduit flow rarely is present.
Definition of the "Uppermost Aquifer"
The owner/operator is required under 40 CFR  264.97 to install a ground water
monitoring system that yields representative samples from the uppermost aquifer beneath the
facility. The ground water monitoring system should allow for the detection of contamination
when hazardous waste or hazardous constituents have migrated from the waste management
area to the uppermost aquifer. Owners and operators should properly identify the uppermost
aquifer when establishing a ground water monitoring system that meets the requirements of
 264.97. EPA has defined the uppermost aquifer as the geologic formation nearest the
ground surface that is an aquifer, as well as lower aquifers that are hydraulically connected
within the facility's property boundary. "Aquifer" is defined as the geologic formation, group
of formations, or part of a formation that is capable of yielding a significant amount of
ground water to wells or springs (40 CFR  260.10). The identification of the confining layer
or lower boundary is an essential facet of the definition of uppermost aquifer. Interconnected
zones of saturation below an aquifer that are capable of yielding significant amounts of water
also comprise the uppermost aquifer. Quality and use of ground water are not factors in the
definition. Even though a saturated zone may not be presently in use, or may contain water
not suitable for human consumption, it should be monitored if it is part of the uppermost
aquifer to ensure that the performance standard of  264.97(a)(3) is met. Identification of
formations capable of "significant yield" is made on a case by case basis.
There are saturated zones, such as low permeability clays, that do not yield a
significant amount of water, yet act as pathways for contamination that can migrate
horizontally for some distance before reaching a zone that yields a significant amount of
water. If there are hydrogeologic data supporting the belief that potential exists for
contamination to migrate along such pathways, the Regional Administrator may invoke the
November 1992
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