Water wells that are no longer in use (abandoned) pose a significant threat to groundwater quality. If an abandoned well is not properly filled with impermeable material it can act as a direct conduit for contaminated surface water to enter the groundwater. Surface water that enters an abandoned well bypasses the purifying action that normally takes place in the upper layers of the soil.
In many cases, a new well will be drilled in the vicinity of an old well and under certain conditions the contaminants that entered the old well can move to the new active well. In addition large diameter wells pose a physical threat since humans and animals can fall into a well that has inadequate surface protection.
Undesirable surface water, rodents or insects can enter a well directly through the top of the casing if the well has not been capped and if the casing pipe does not extend high enough above the ground surface. Open wells offer tempting disposal sites for liquid or solid wastes. Wells located in low areas may be subject to contamination by bacteria-laden flood waters.
Not all hazards are visible from the surface. Depending on the age and construction of the well, there are several situations that may take place:
The landowner is generally responsible for plugging a well that is no longer being used as a water supply. In some provinces, during the drilling of a new well, the drilling contractor is responsible for immediately plugging the well if the well cannot be completed due to construction problems or inadequate yield.
A well is generally considered "abandoned" when: 1) it is currently not in use, and 2) is not intended to be used in the future for water supply purposes. After wells are taken out of service they are seldom used again. If cut off and buried, they can be very difficult to find at a later date for proper decommissioning, especially in the case of a property transfer. Large diameter open wells are a significant physical safety hazard. Wells should be properly filled when they are removed from service or at the time a replacement well is drilled.
The main objective of proper well decommissioning is to restore the geology at the well site to its original condition. When this is done successfully, significant vertical movement of water within the former well bore will be prevented. A water well is said to be "reclaimed" or "decommissioned" if it is properly sealed. It is generally recommended that a licensed water well driller be hired to complete this well plugging process. A licensed water well driller has the expertise and proper equipment and will know the requirements for well decommissioning, which vary from province to province.
The first step in decommissioning a well is to obtain information on the construction and condition of the well. Pertinent information includes: diameter of casing, year drilled, well depth, and land location. This information is useful in matching up a Water Well Drilling Report for the well which may be on file in the provincial agency . Any information on why the well is no longer in use, whether or not the pump has been removed or if a pump jack had ever been installed, knowledge of previous attempts to plug the well and a recent static water level reading are helpful to the decommissioning process.
In some cases a landowner is aware that a well exists on a property but does not know the location of the well. It's important to note that for proper decommissioning to occur, the well casing will need to be accessible.
In the past, wells were commonly constructed in concrete vaults or well pits to provide protection against freezing. Old wells are also likely to be located below out-of-use windmills. A well that has been cut off, capped and buried can be exposed with a backhoe or shovel. A metal detector is helpful, as is any historical information that can be used to pinpoint the location of the well such as old photographs or the knowledge of previous landowners.
A typical on-site procedure for decommissioning an unused water well involves setting up a drill rig over the well, removing the pump, liner/casing and any obstructions from within the well casing, sounding the well depth to ensure that it is clear to bottom, thoroughly flushing and cleaning the well of all foreign materials and disinfecting the well with a 200 mg/L chlorine solution. Ideally, the surface casing should be removed from the well before the plugging process begins but often it is too difficult to remove. A downhole camera can be used to investigate the condition of the inside of the casing or the type of obstruction, if present. Obstructions can be retrieved by hooking on/pulling or by drilling them out. If the surface casing is left in place, the inside of the casing should be ripped or mechanically perforated at several locations to allow the subsequent grouting process to squeeze out and fill any gaps that may be present along the outside of the casing.
Materials that are used to seal a well must be impervious to water. Bentonite grout is one of the most common materials used to seal wells. It can be mixed as a slurry and pumped into the well. Cement (no aggregate) is another material that can be used particularly if natural gas is present in the well. Cement grout sets up structurally stiff and is hard whereas bentonite grout remains in a semi-solid or plastic state. Large diameter bored wells (typically 600 - 750 mm diameter) are usually filled with clean, uncontaminated clay due to the large volume of material required. Other acceptable materials that prevent vertical movement of water include manufactured bentonite pellets or chip.
The proper grouting process involves lowering a pipe to the bottom of the well and pumping the grout slurry into the well from the bottom up, displacing the water in the well. Improper methods (e.g. pouring the filling material into the well from the top) could cause the plugging material to become diluted and still permeable or bridge off in the well, not completely filling it.
An alternative to grouting is to use coated bentonite pellets (chips) designed to be introduced into the well from the top. These pellets have a weight material added to ensure they sink to the bottom of the hole and are coated to prevent immediate swelling on contact with water. When poured slowly into the well, they reach the bottom before swelling and closing off the hole. However, care must be taken to avoid bridging.
Finally, if the casing was not already removed, it should be cut off a minimum of 0.5 m below the ground surface and covered with compacted clay.
Costs of water well decommissioning vary widely for many reasons:
Water well drillers may be reluctant to quote an exact price for the work beforehand because of the unpredictable nature of the work, however the rates for drill rig, labour and material can all be established prior to well plugging. The time spent with the drill rig is usually the largest component of the cost.