representative samples from the boring. Where boreholes are drilled or cored

through fractured rock, the boreholes, cores, or samples should be used to

determine the orientation of the fractures. Keys and MacCary (1971) and Keys

(1988) discuss the application of borehole geophysics to fracture

characterization.

Geophysical techniques can be used to plan and supplement the subsurface

boring program. For example, surface geophysical surveys may be used to

verify and modify the initial conceptual model prior to drilling boreholes.

Based upon the results of the geophysical surveys, boreholes can be effectively

located to obtain necessary hydrogeologic information. Information obtained

from initial boreholes can be used to evaluate the geophysical data and resolve

any ambiguities associated with the preliminary interpretation of the

geophysical survey results. When continuous sampling is not performed,

borehole geophysical methods should be used to correlate unsampled with

sampled core sections. The use of surface to borehole geophysical methods

may allow better resolution of geophysical and borehole data, and may help

delineate the subsurface geology between boreholes.

Any borehole that will not be completed as a monitoring well should be

properly decommissioned. When considering the installation of ground water

monitoring wells in the vicinity of decommissioned boreholes, owners and

operators should ensure that borehole sealant materials (e.g., cement) will not

alter the chemistry of the ground water to be monitored.

The objective of a subsurface boring program is to begin to refine the broad,

conceptual model derived during the preliminary investigation to better reflect the true

site specific hydrogeologic conditions. In other words, the boring program is necessary to

directly investigate and to describe the geology of the area beneath the facility, and place it in

the context of the regional geologic setting.

In some situations, it may be necessary to drill through actual or possible confining

layers at a site. Special precautions should be taken when investigators believe they may

encounter a confining layer during drilling. Moreover, if field personnel suspect they may

have encountered a possible confining layer while drilling a borehole, drilling should be

stopped immediately and the borehole should be decommissioned. Investigators, in

conjunction with the appropriate regulatory authority, may then develop an appropriate

method for drilling through the confining layer. Extreme care should be taken when drilling

into confining units so that the borehole does not create a pathway for the migration of

contaminants, particularly dense non aqueous phase liquids (DNAPLs), between upper and

lower hydraulically separated saturated zones. In all cases, owners and operators should

prevent DNAPL mobilization (e.g., through gravity driven transport) when drilling boreholes.

November 1992

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