6.1.4

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

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