A neat cement or shrinkage compensated neat cement grout seal should be installed on

top of the bentonite seal and extend vertically up the annular space between the well casing

and the borehole wall to within a few feet of land surface. Annular sealants in slurry form

(e.g., cement grout, bentonite slurry) should be placed by the tremie/pump (from the bottom

up) method. The bottom of the placement pipe should be equipped with a side discharge

deflector to prevent the slurry from jetting a hole through the protective sand layer, filter

pack, or bentonite seal. The bentonite seal should be allowed to completely hydrate, set, or

cure in conformance with the manufacturer's specifications prior to installing the grout seal in

the annular space. The time required for the bentonite seal to completely hydrate, set, or cure

will differ with the materials used and the specific conditions encountered, but is generally a

minimum of four to twenty four hours. Allowing the bentonite seal to hydrate, set, or cure

prevents the invasion of the more viscous and more chemically reactive grout seal into the

screened area.

When using bentonite as an annular sealant, the appropriate clay should be selected on

the basis of the environment in which it is to be used, such as the ion exchange potential of

the sediments, sediment permeability, and compatibility with expected contaminants. Sodium

bentonite is usually acceptable. Other industrial grade clays without chemical additives that

may affect ground water quality can be used if sodium bentonite is incompatible with either

the natural formation or the analytes of concern (e.g., calcium bentonite may be more

appropriate in calcareous sediments and soils because of its reduced cation exchange

capacity). The sealing properties of clays may be adversely affected by chlorine salts, acids,

alcohols, ketones, and other polar compounds. If these materials are expected at the facility,

alternative sealants should be considered.

When the annular sealant must be installed in the unsaturated zone, EPA recommends

that neat cement or shrinkage compensated neat cement mixtures be used for the annular

sealant. Bentonite is not recommended as an annular sealant in the unsaturated zone because

the moisture available is insufficient to fully hydrate bentonite. Adding calcium bentonite to

cement should be avoided. Ca

++

and OH

ions in the cement cause flocculation of the clay,

reducing its ability to swell. The bentonite also weakens the cement, reducing its

compressive strength. A better solution for shrinkage control is to use

shrinkage compensating additives components: K, M, and S (ASTM C845). However, the

high heat of hydration should be taken into account when these materials are used.

November 1992

6 43