Owners and operators should obtain approval from the Regional Administrator prior to

implementing a plan to drill through a possible confining layer.

There are at least two approaches for drilling through confining layers. Based on site 

specific conditions, one or both of these approaches may be appropriate:

Install the first boreholes on the perimeter of the site (in less contaminated

areas or uncontaminated areas). The initial boreholes could penetrate the

confining zone to allow characterization of the lower units. This approach is

essentially to monitor from the "outside in." At a minimum, boreholes

upgradient of the source (and upgradient of a DNAPL and/or dissolved phase

plume) could be drilled through the possible confining layer to characterize the

geology of the site. The appropriateness of this approach should be evaluated

on a site specific basis (e.g., DNAPLs may migrate in directions different from

ground water flow).

Drill the boreholes using techniques that minimize the danger of cross 

contamination between water bearing zones. Such techniques typically involve

drilling an initial borehole partially into the possible confining layer, installing

(grouting in) an exterior casing, emplacing grout in the cased portion of the

borehole, and drilling a smaller diameter hole through the cased off/grouted

portion of the borehole (i.e., telescoping casing) through the confining layer.

Millison et al. (1989) provide an example of the use of telescoping casing to

prevent cross contamination of aquifers. The appropriateness and actual design

of telescoping borings and casings should be determined on a site specific

basis. Telescoping boreholes may be completed as wells or piezometers.

A subsurface boring program usually requires more than one round of borehole

installation. The number, placement, and depth of initial borings should be planned to

provide sufficient information upon which to plan a more detailed site characterization. An

example of a simple boring program is illustrated in Figure 2. If characterization is largely

achieved with the initial placement, fewer additional boreholes and fewer additional indirect

investigations will be necessary. In most cases, however, the Agency believes that additional

boreholes will be necessary to complete the characterization because most hydrogeologic

settings are relatively complex, even to experienced ground water scientists. Figure 3

illustrates how subsequent borings and supplementary indirect techniques can be added to an

initial boring configuration to characterize the site specific geology.

Drilling logs and field records should be prepared detailing the following information:

The lithology or pedology (i.e., geologic or soil classification) of each geologic

and soil unit in the unsaturated and saturated zones, including the confining

layer. The classification system used for lithologic and pedologic descriptions

November 1992

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