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.
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
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