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