such as geophysical studies may be used to plan and augment direct field methods, but should
not be used as a substitute for them.
Subsurface Boring Program
All hydrogeological site investigations should include a subsurface boring program to
identify the lithology, stratigraphy, and structural characteristics of the subsurface.
Information obtained from boreholes is necessary to characterize the subsurface at a site and
to identify potential contaminant migration pathways.
A subsurface boring program should be designed as follows:
The initial number of boreholes and their spacing should be based on
information obtained during the preliminary investigation and on the spatial
orientation of the waste management units. Initial boreholes should be drilled
to provide sufficient information to determine the scope of a more detailed
evaluation of geology and to identify potential contaminant migration pathways.
Boreholes should be spaced closely enough so that accurate cross section(s) can
be constructed. Factors that influence the initial number of borings are listed
in Table 2.
Additional boreholes should be drilled as needed to provide more information
about the site and to refine the conceptual model. The number and placement
of additional boreholes should be based on a preliminary conceptual model that
has been refined with data obtained from initial boreholes and other site
investigatory techniques (e.g., geophysical investigations).
Samples should be collected from boreholes at all suspected changes in
lithology. The deepest borehole drilled at the site should be continuously
sampled. For boreholes that will be completed as monitoring wells, at least
one sample should be collected from the interval that will be the monitoring
well intake interval (i.e., screened interval or open (uncased) interval). EPA
recommends that all borings be continuously sampled to obtain good
stratigraphic control.
All borehole samples should be collected with a Shelby tube, split barrel
sampler, rock corer, or other appropriate device.
Borehole samples should be classified according to their lithology or pedology
by an experienced geologist. Owner/operators should ensure that samples of
every geologic formation, especially all confining layers, are collected and
described, and that the nature of stratigraphic contacts is determined. EPA
recommends that owners/operators take color photographs (with scale) of
November 1992
4 8






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