such as geophysical studies may be used to plan and augment direct field methods, but should

not be used as a substitute for them.

4.2.1

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

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