Air rotary drilling requires that care be taken both to prevent cross contamination of

subsurface materials and to prevent contamination or chemical alteration of ground water or

subsurface materials.

6.1.5

Mud Rotary and Water Rotary

The mud rotary and water rotary drilling methods involve the introduction of drilling

fluids (various drilling muds or water) into the borehole through the drill pipe to maintain an

open hole, provide lubrication to the drill bit, and remove drill cuttings.

Water rotary drilling is a rapid and effective drilling method for most geologic

materials. However, the water used as a drilling fluid tends to react with the surrounding

formation and ground water. For this reason, the utility of water rotary drilling is limited. In

addition, there are other problems associated with water rotary drilling. The identification of

water bearing zones is hampered by the addition of water into the borehole. In clay rich

sediments, the water may form a slurry that can rapidly cause plugging of the formation,

resulting in a well that is difficult to develop. In poorly consolidated sediments, drillers may

have a problem with caving of the borehole prior to installation of the well screen and casing.

In highly fractured rock, it may be difficult to maintain effective water circulation because of

water losses to the subsurface. The drilling fluids used in rotary drilling can grossly

contaminate upper or lower uncontaminated zones if a contaminated zone is penetrated.

Driving casing as the borehole is advanced can help resolve this problem.

While there are hydrogeologic conditions where mud rotary drilling is the best option

(e.g., where it is extremely difficult to maintain a stable borehole), mud rotary creates a high

potential for affecting aquifer characteristics and ground water quality. If the mud rotary

method is used, the drilling mud(s) should not affect the chemistry of ground water samples

or samples from the borehole, or adversely impact the operation of the well. To minimize the

influence to the surrounding formation and ground water, drilling muds should be limited to

water based, locally occurring clays. The following describes the type of adverse affects that

can occur to the aquifer, ground water quality, and/or well performance as a result of using

certain drilling muds. A more comprehensive review of the properties, applications, and

impacts of drilling fluids is given in Aller et al. (1989):

Bentonite muds form a filter cake on the sides of the borehole, thus reducing

the effective porosity of formations in the borehole, and compromising the

design of the well. Bentonite may also affect local ground water pH.

Additives to modulate viscosity and density may also introduce contaminants to

the system or force large, unrecoverable quantities of mud into the formation.

Some organic polymers and compounds provide an environment for bacterial

growth, which reduces the reliability of sampling results.

November 1992

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