Bentonite muds may adsorb metals, potentially reducing contaminant

concentrations and affecting the reliability of sampling results.

Direct mud rotary drilling is recommended by some investigators for use at heavily

contaminated sites or at sites where the contaminants of concern are highly toxic and where

proper containerization of drill cuttings and fluids is important. The technique requires

creating a leak proof seal in a portable mud pit, so that returned drilling fluids and cuttings

will be contained within the pit. The cuttings may be transferred from the pit to drums as

necessary. Heavy gauge plastic sheeting may be used to cover the exclusion zone and to

prevent equipment from contaminating surface soils. Obviously, owners/operators should

ensure that this application of direct mud rotary drilling does not cause cross contamination of

subsurface materials.

6.1.6

Dual Wall Reverse Circulation

The dual wall reverse circulation rotary method utilizes a double wall drill pipe, and

has the reverse circulation of other conventional rotary drilling methods. Air or water is

forced down the outer casing and is circulated up the inner drill pipe. Cuttings are lifted up

to the surface through the inner drill pipe. Either a hammer or tricone bit can be used to cut

the formation. A triple wall design, involving the placement of an additional single wall

casing around the dual wall drill string, may be useful in situations where it is necessary to

case a contaminated upper formation to install a well in an underlying formation.

The greatest advantage of dual wall reverse circulation drilling is that it allows

continuous sampling of the subsurface, and largely eliminates or reduces problems associated

with lost circulation and borehole stability. The disadvantages of dual wall reverse circulation

drilling include the necessity of using larger drilling equipment and a large borehole to

accommodate the dual wall pipe.

6.1.7

Driven Wells

Driven wells consist of a steel well screen that is either welded or attached with drive

couplings to a steel casing. The well screen and attached casing are forced into the ground

by hand using a weighted drive sleeve, or with a heavy drive head mounted on a hoist. As

the well is driven, new sections of casing are attached to the well in 4  or 5 foot sections.

Several problems are commonly associated with the installation of driven wells. First,

it is difficult or impossible to drive a well through dense silts, clays or materials containing

boulders. If penetration in these materials is accomplished, the well screen may be destroyed

in the process. In addition, silts and/or clays can clog the well screen to the point where the

well cannot be satisfactorily developed. Two techniques, described in Aller et al. (1989) have

been employed in an attempt to alleviate these problems. Driven wells may be helpful as a

tool for preliminary field studies requiring installation of shallow piezometers. However, in

November 1992

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