What is a Groundwater Monitoring Well?
Posted on Thursday July 4, 2019
Groundwater monitoring wells are installed by Garrison Environmental Services throughout Garrison Petawawa in order to ensure DND’s compliance with federal water quality guidelines and enables a better understanding of the groundwater flow directions. Between the Garrison and the range and training area there are over two hundred groundwater monitoring wells. Annual sampling is completed by the Garrison’s environmental technician and then sent to a certified lab for analysis. Typically the sampling program is divided into a summer and fall campaign.
There are two basic types of monitoring wells that are installed throughout Garrison Petawawa. The first type is an individual monitoring well that consists of a single PVC casing within a borehole. These are the most common type of monitoring well found throughout the base. The second type is a nested monitoring well that consists of two or more PVC casings within the same borehole. These types of monitoring wells are designed to obtain water from different aquifers or water bearing zones. The well is then protected with either a flush mount casing or a steel protective casing which allows for the well to be locked to prevent damage or tampering. Damage or tampering of the wells can become costly due to replacement costs and adverse effects it may cause to the aquifer.
In Garrison Petawawa monitoring wells are constructed using environmental quality threaded PVC pipe, which is typically 3 metres in length. [Ontario Water Resources Act, Regulation 903]. There are many factors that determine the length of the screen that is used, such as the thickness of the hydrogeological unit and the specific requirements of the monitoring program. Most wells use a 3.0 meter screen as long as the screen does not extend within two metres of the ground surface. The screen of the monitoring well is the area in which the water enters the well from the surrounding soil or bedrock. The size of the borehole must be at least 102 millimetres or approximately four inches larger than the outside diameter of the PVC casing and screen.
Most wells installed in Garrison Petawawa are a 52 millimetre (two inch) well casing which means the drilled borehole is approximately 154 millimetres (six inches) in diameter [Ontario Water Resources Act, Regulation 903]. This space between the casing and borehole wall is necessary to insure the placement of materials to properly seal the well. The borehole is backfilled with washed gravel or sand to at least the top of the well screen. Only properly cleaned sand or gravel should be used in order to ensure the integrity of the water sampled. The well is then sealed with a water tight clay bentonite seal that is at least 0.3 metres thick above the sand or gravel pack [Ontario Water Resources Act, Regulation 903]. Bentonite is an inert absorbent clay like material that is used to effectively seal off the well at that depth. A weighted tape measure is used during this process to ensure that each material is reaching the appropriate depth from surface. The monitoring well is then capped with a snug fitting plug or PVC cap. A small vent hole or slot is necessary to allow air movement in and out of the monitoring well to allow the water levels to fluctuate properly.
In conclusion the monitoring wells that are installed throughout Garrison Petawawa allow the environmental staff to monitor the quality of the groundwater and compare it to the standards set by the Federal government. This monitoring program will ensure the use of the Garrison Petawawa training area for soldiers into the future. Watch for monitoring wells on your Garrison travels now that you know what to look for, you will be surprised where you will see them.