Monitoring wells can provide several types of information to the consultant.
A monitoring well is installed by first drilling a borehole with a diameter somewhat greater than the intended outer diameter of the well. While this may seem obvious, note that the increase in radius must also be somewhat significant for several reasons. There is always a chance of cave in, therefore a wider whole is preferred to acquire the desired depth for the monitoring well. The sand pack may not be deposited properly for the installation of the well due to limited space. Geotechnical personnel tend to install piezometers, which I feel does not serve the purpose required for Environmental investigations.
The purpose of a monitoring well is to provide access to the groundwater table. This is provided by means of slotted Poly Vinyl Chloride (PVC) piping (or other material) in combination of a sand pack that permits the groundwater to flow. The groundwater will flow through the well if the well was properly installed and developed (this concept will be explained elsewhere).
The general structures of the wells can be depicted as follows:
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| Drawing No. 1 |
Monitoring wells can be either flush with the ground surface or stick up. Both serve different purposes, such as an operational commercial property may not want something sticking out of their parking lot. At the same time, you may have difficulty finding a well in the middle of a farmer's field if you install flush mount wells.
Monitoring wells are typically 50 mm (2 inches) in diameter (the consultant can always change that). The slotted PVC pipe comes in 5 ft and 10 ft lengths (the lengths can be customized, however, you will need to invert the monitoring wells to do so). Typically, I have installed Schedule 40 50 mm PVC as well as High Density PolyEthylene (HDPE) pipes.
The sand pack is paced to approximately 1 ft above the slotted pipes. Then the rest of the void is filled with a Bentonite seal to mitigate infiltration of undesired fluids from certain depths as well as to mitigate cross contamination from sources from the surface.
The flush mount and stick up casings are usually made of metal and have a locking mechanism to prevent vandalism and introduction of impacted material to these wells.
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| Drawing No. 2 |
Drawing No. 2 depicts a monitoring well that has been installed properly for sampling of Light Non-Aqueous Phase Liquid (LNAPL) such as gasoline. The slotted sections have been isolated to the clay layer without allowing the fill layer to cross contaminate the sampling section. This is more of an ideal installation if we were not concern about impacts to the fill layer.
The groundwater sample collected here would represent the quality of the water within this layer.
The final depth of the monitoring well is not shown, however, is important as well. You do not want to penetrate another layer and potential provide a new pathway for contaminants to be transferred.
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