well screen, as illustrated in Figure 13. In deep wells, the filter pack may not compress when

initially installed, consequently, when the annular and surface seals are placed on the filter

pack, the filter pack compresses sufficiently to allow grout into, or very close to, the screen.

Consequently, filter packs may need to be installed as high as five feet above the screened

interval in monitoring wells that are deep (i.e., greater than 200 feet). The precise volume of

filter pack material required should be calculated and recorded before placement, and the

actual volume used should be determined and recorded during well construction. Any

significant discrepancy between the calculated volume and the actual volume should be

explained.

Prior to installing the annular seal, a one  to two foot layer of chemically inert fine

sand may be placed over the filter pack to prevent the intrusion of annular or surface sealants

into the filter pack. When designing monitoring wells, owners and operators should

remember that the entire length of the annular space filled with filter pack material or sand is

effectively the monitored zone. Moreover, if the filter pack/sand extends from the screened

zone into an overlying zone, a conduit for hydraulic connection is created between the two

zones.

6.4

Annular Sealants

Proper sealing of the annular space between the well casing and the borehole wall is

required ( 264.97(c)) to prevent contamination of samples and the ground water. Adequate

sealing will prevent hydraulic connection within the well annulus. The materials used for

annular sealants should be chemically inert with the highest anticipated concentration of

chemical constituents expected in the ground water at the facility. In general, the

permeability of the sealing material should be one to two orders of magnitude lower than the

least permeable part of the formation in contact with the well. The precise volume of annular

sealants required should be calculated and recorded before placement, and the actual volume

used should be determined and recorded during well construction. Any significant

discrepancy between the calculated volume and the actual volume should be explained. Aller

et al. (1989) provide detailed discussions of the proper placement of sealants into the annular

space.

When the screened interval is within the saturated zone, a minimum of two feet of

sealant material such as raw (>10% solids) bentonite should be placed immediately over the

protective sand layer overlying the filter pack. Granular bentonite, bentonite pellets, and

bentonite chips may be placed around the casing by means of a tremie pipe in deep wells

(greater than approximately 30 feet deep), or by dropping them directly down the annulus in

shallow wells (less than approximately 30 feet deep). Dropping the bentonite pellets down

the annulus presents a potential for bridging (from premature hydration of the bentonite),

leading to gaps in the seal below the bridge. In shallow monitoring wells, a tamping device

should be used to prevent bridging from occurring.

November 1992

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