Unplugged or improperly plugged abandoned wells pose a serious threat to ground water. These wells serve as a pathway for surface pollutants to infiltrate into the subsurface, and present an opportunity for various qualities of water to mix. The objectives of an abandonment procedure are to eliminate physical hazards, prevent ground water contamination, conserve aquifer yield and hydrostatic head, and prevent intermixing of subsurface waters.
The purpose of sealing an abandoned well is to prevent further disturbance to the hydrogeologic conditions within the subsurface. The seal should prevent vertical movement within the borehole, and confine the water to the original zone of occurrence.
Abandonment ConsiderationsSelection of the appropriate method for abandonment is based on the information compiled for each well. Factors that must be considered:
- casing material
- casing condition
- diameter of the casing
- quality of the original seal
- depth of the well
- well plumbness
- hydrogeologic setting
- presence of contamination and the zone(s) where contamination occurs.
Abandonment ProceduresWell abandonment procedures involve filling the well with grout. The well may be filled completely, or seals placed in appropriate zones and the well only partially filled with grout. Completely filling the well reduces the possibility of borehole collapse and shifting of seals.
Wells can be abandoned either by removing the casing or by leaving all or part of the casing in place and cutting off the casing below ground level. Because the primary purpose of well abandonment is to eliminate vertical fluid migration along the borehole, the preferred method of abandonment involves casing removal. If the casing is removed and the borehole is unstable, grout must simultaneously be emplaced as the casing is removed to prevent borehole collapse and an inadequate seal. When the casing is removed, the borehole can be sealed completely, and there is less concern about channeling in the annular space or inadequate casing/grout seals. However, if the casing is left in place, the casing should be perforated and completely pressure-grouted to reduce the possibility of annular channeling.
If the casing is in poor condition, the interval adjacent to the water-bearing zone can be ripped or perforated with casing rippers, and the casing filled and pressure-grouted. The top portion of the casing should be pulled so a watertight seal is attained in the upper 15 feet to 20 feet of the well. Fill the uppermost 5 feet of the well with clay or an impermeable material appropriate for the intended land use.
Grouting ProceduresAll materials used for grouting should be clean and stable. Potable water should be used, and it should be free from oil and other contaminants. Grout should be applied in one continuous grouting procedure from bottom to top to prevent segregation, dilution and bridging of the sealant. The end of the tremie pipe should always remain immersed in the slurry of grout throughout the emplacement procedure.
To ensure the borehole was properly grouted, records should be kept of the calculated volume of the borehole and the volume of grout used. Any discrepancies should be noted and explained. This information should be kept on file with other well documentation records. A concrete cap should be placed on top of the sealed well.
After abandonment is completed, a proper site cleanup should be performed. For example, any pits should be backfilled and the area should be left clean. Accurate documentation of all procedures, conditions and materials used should be recorded for future reference.
This article is provided through the courtesy of the Kentucky Division of Water. Its Web site, www.water.ky.gov, offers a variety of water-related resources.