Air Rotary
Rotary drilling involves the use of circulating fluids (i.e., mud, water, or air) to
remove the drill cuttings and to maintain an open hole as drilling progresses. Air rotary
drilling forces air down the drill pipe and back up the borehole to remove the drill cuttings.
The air rotary drilling technique is best suited for use in hard rock (versus unconsolidated or
poorly consolidated materials).
Accurate detection of ground water contamination at hazardous waste disposal sites is
dependent on the generation of high quality chemical data from the analysis of representative
soil, unconsolidated material, rock, and ground water samples. One of the most important
goals of any method used to obtain samples is to create minimal effects on the media and
contaminants of concern. The air rotary drilling method may jeopardize the collection of
representative and accurate chemical data. For this reason, and for others listed below, the air
rotary drilling method should be used with caution during environmental investigations:
Air rotary does not allow collection of representative samples, therefore, the
boring cannot be logged with accuracy. Moreover, air/ground water losses into
fractures or other highly permeable zones cannot be measured.
The injection of air into the borehole during air rotary drilling may alter the
natural properties of the subsurface. Specifically, the following chemical and
physical processes may occur:
Air stripping of volatile organic constituents can occur during
drilling, leading to erroneous chemical data for these compounds
for both soil and ground water samples;
Injection of air into the subsurface can significantly alter aquifer
geochemistry. Alteration of such properties as pH and redox
potential can often be irreversible, thus preventing the well from
yielding ground water samples that are representative of in situ
conditions. Changes in pH can affect the solubility of metallic
compounds; changes in oxidation state can result in the
precipitation of metallic and organo metallic compounds; and
The introduction of oxygen into the aquifer can initiate or
greatly increase biodegradation of organic compounds in the
aquifer near the vicinity of the borehole. Monitoring wells
installed under these circumstances would be unable to yield
representative ground water samples.
November 1992
6 10






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