Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p34
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 34/117)
Character Range: 441939–444835

within the mesh can be different from that present in the aquifer, which can produce misleading sample results. Where LNAPLs are present, a greater thickness of NAPL would be required to overcome the increased surface tension forces present in the fine mesh compared with the well screen.

8.2.3.3         Sealing and backfilling of boreholes
The annular space from the top of the filter pack to ground level (or next monitoring screen in multi-level monitoring wells) should be backfilled with bentonite or a non-shrinking bentonite-based grout (cement grout or a cement/bentonite mix). A seal should always be placed on top of the filter pack to prevent these materials from entering the well screen.

Incorrect installation of wells can result in costly cross-contamination of aquifers. In installations above the water table, the use of bentonite pellets in isolation to form a seal is discouraged as the bentonite pellets can set dry and crack, resulting in an ineffective seal between the aquifer and contamination near the ground surface. In these circumstances it is preferable to hydrate the bentonite at surface and then install as a slurry. For guidance on appropriate installation procedures see EA (2006) and ASTM D5092-04.

Drill cuttings should be collected in suitable containers and disposed of appropriately. It is not acceptable practice in site assessment to use drill cuttings as backfill in boreholes.

8.2.3.4         Headworks
It is essential to correctly finish all monitoring wells at the surface such as with a suitable bentonite plug and cement seal, to ensure that surface water runoff does not collect at the wellhead and leak down the outside of the casing. The borehole headworks form the interface between the borehole and the surface environment.

In designing headworks a number of issues should be considered (EA, 2006):
    * security—to prevent vandalism or malicious actions and to prevent access by animals
    * protection—from entry of surface water or other foreign material and from activities at surface (such as vehicle movements)
    * accommodation of equipment—storage of equipment such as data loggers and dedicated sampling devices
    * visibility—designed to be clearly visible or non-obtrusive depending on location.
In general, an above-ground completion is preferred as this type of design is less likely to suffer from inundation, is easier to find in the field and is more easily secured.

Casing materials such as PVC, ABS, Teflon, etc. which project from the ground can easily be damaged and should be protected by a steel or similar outer protective collar.

8.2.3.5         Well development
All bores intended for monitoring water quality should be developed after drilling to remove fine sand, silt, clay and any drilling mud residues from around the well screen to ensure the hydraulic functioning of the well. Development should be