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

characteristics.

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

migration.

4.3.3

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

4 39