authorities of  264.97 to require such zones to be monitored. In addition, the Regional
Administrator may require the use of supplemental monitoring wells in conjunction with point
of compliance wells to monitor sites where hydrogeologic conditions or contaminant
characteristics allow contaminants to move past or away from the point of compliance without
being detected ( 264.97(a)(3)). The Agency recommends the use of unsaturated zone
monitoring where it would aid in detecting early migration of contaminants into ground water.
In determining the necessity for and scope of unsaturated zone monitoring, the Regional
Administrator will consider site specific factors that include geologic and hydrogeologic
Other authorities that can be used to require monitoring include  3004(u) for
corrective action for permitting; the "omnibus" permitting authority under  3005(c)(3) of
RCRA and 40 CFR  270.32(b) that mandates permit conditions to protect human health and
the environment; and  3013 authority that authorizes the Agency to require monitoring,
testing, analyses, and reporting in certain circumstances upon a finding of a substantial
hazard. If a release to ground water is detected, the release should be characterized in all
saturated zones regardless of yield.
The owner/operator should assess hydraulic connection between zones of saturation
yielding significant amounts of water, and properly define potential zones of contaminant
migration. The owner/operator also should be able to demonstrate to the satisfaction of the
EPA Regional Administrator (e.g., through the use of aquifer testing and/or modeling) that the
units identified as the confining units below the uppermost aquifer are of sufficiently low
permeability to minimize the passage of contaminants to saturated, stratigraphically lower
units. Owners and operators should be aware that true confining layers rarely exist. Facies
changes are the rule, and not the exception at most sites, and may preclude the existence of a
confining layer. Furthermore, particularly with regard to DNAPLs, a confining layer may not
inhibit flow laterally downdip of the layer. Solvents also have been shown to interact with
clays, causing dessication and the formation of fractures. Consequently, even if the confining
layer is continuous (it usually is not), the confining layer may not prevent contaminant
Determining Ground Water Flow Direction and Hydraulic Gradient
Installing monitoring wells that will provide representative background and
downgradient water samples requires a thorough understanding of how ground water flows
beneath a site. Developing such an understanding requires obtaining information regarding
both ground water flow direction(s) and hydraulic gradient. Ground water flow direction can
be thought of as the idealized path that particles of ground water follow as they pass through
the subsurface. Hydraulic gradient (i) is the change in static head per unit of distance in a
given direction. The static head is defined as the height above a standard datum of the
surface of a column of water (or other liquid) that can be supported by the static pressure at a
given point (i.e., the sum of the elevation head and pressure head).
November 1992
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