lubricants on the physical and chemical characteristics of the subsurface and on

ground water quality; and

The volume of drilling fluids, drilling fluid additives, and lubricants used

during the drilling of a monitoring well should be recorded.

The following sections summarize the most commonly used methods for drilling

ground water monitoring wells. These methods also are summarized briefly in Table 7.

Table 8 summarizes the limitations and applications of each drilling method. Aller et al.

(1989) should be consulted for additional information on the selection of drilling methods.

6.1.1

Hollow Stem Augers

The hollow stem, continuous flight auger is the most frequently employed tool for

drilling monitoring wells in unconsolidated materials. Augers are likened to giant screws, and

continuous flighting refers to a design in which the flights ("threads") of the auger extend the

entire length of the auger core or stem. Individual auger sections, typically 5 feet in length,

are also called flights.

When drilling, a cutting head is attached to the first auger flight, and as the auger is

rotated downward, additional auger flights are attached, one at a time, to the upper end of the

previous auger flight. As the augers are advanced downward, the cuttings move upward

along the continuous flighting. The hollow stem or core of the auger allows drill rods and

samplers to be inserted through the center of the augers. The hollow stem of the augers also

acts to temporarily case the borehole, so that the well screen and casing may be inserted

down through the center of the augers once the desired depth is reached, minimizing the risk

of possible collapse of the borehole that might occur if it is necessary to withdraw the augers

completely before installing the well casing and screen.

The hollow stem auger drilling technique is not without problems. These are more

completely described in Aller et al. (1989) but generally include:

Cross contamination of subsurface materials    Because drill cuttings are in

contact with the entire length of the borehole as they are transported up the

auger flights, hollow stem augers may cause cross contamination of subsurface

materials;

Heaving    Sand and gravel heaving into the hollow stem may be difficult to

control and may necessitate adding water to the borehole;

Smearing of silts and clays along the borehole wall    In geologic settings

characterized by alternating sequences of sands, silts, and clays, the action of

the augers during drilling may cause smearing of clays and silts into the sand

November 1992

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