The Calfornia Deparment of Toxic Substance Control Describes:
When a monitoring well installation is planned, sufficient thought should be given to the quality of the borehole that will contain the well. The following factors should be considered in the borehole construction:
The diameter of a monitoring well borehole should be sufficiently large to contain the well casing and provide an adequate annular space (as measured from the outside of the casing to the borehole wall). Additional allowances should be made as needed for other pipes that may be installed in the annular space, such as sand fill pipes or sounding tubes.
The annular space is the gap between the outside of the casing and the borehole wall. The annular space should be large enough to allow clearance of a 1.5-inch I.D. tremie pipe and for a sufficient width of filter pack and annular seal material. Recommended annular space widths are as follows:
Annular space widths larger than 5 inches may reduce the ability to develop a well, or may contribute to casing damage from heating during grout curing.
In situations where precise lithologic data are needed (e.g., dipping or folded strata), or the location of target zones is critical, borehole alignment becomes an important criterion for monitoring well screen placement. Borehole alignment can be assessed through a borehole deviation survey, using a borehole dipmeter or similar downhole tool. Fortunately, misalignment is usually not significant for shallow monitoring well boreholes (less than 200 feet deep, based on Cal EPA experience); therefore, the additional cost for borehole deviation surveys is usually not justified. However, where precise geologic or hydrogeologic information is needed from deep boreholes (significantly greater than 200 feet), borehole deviation surveys are recommended.
The depth of each monitoring well is determined by site-specific hydrogeologic conditions and monitoring objectives. For example, wells may be designed to monitor the water table, within a water-bearing zone or at the base of an aquifer. Regardless of monitoring depth, the depth of completion of the monitoring well borehole should generally be within one foot of the bottom of the screened interval.
Sometimes boreholes are drilled to a depth greater than the final design depth of the monitoring well, either for exploratory purposes or by error. Boreholes that are not sealed below the final design depth (whether collapsed or left open) may create a vertical conduit for preferential flow. Purging and sampling of the completed well may bring up a non-representative volume of water from below the screen. Therefore, boreholes should be backfilled with a low-permeability material (e.g., a cement-bentonite grout mixture) to the design depth. In highly permeable formations, where vertical preferential flow is less critical, sand may be used in place of the grout seal to stabilize the hole to the design depth.