When it comes to groundwater monitoring, testing the water often requires the creation of a borehole at the test site to reach the water below.
The process of creating a safe and secure site for everyone involved can sometimes be a lengthy process. The size of the borehole, which drill to use and the response zone are just a few of the considerations that will need to be taken into account during its installation.
With so much to consider, it can help to have a plan to follow throughout the whole process. Follow our guide below to ensure your groundwater monitoring test site can be installed safely and securely.
The first thing you need to decide before beginning the installation itself is what your key objectives are. While each site will have its own unique needs, they can generally fall into one of three predetermined categories.
Strategic monitoring involves obtaining groundwater samples over a short period of time, typically to detect trace concentrations of diffuse contaminants. This form of monitoring can also be incorporated when building a test site in a sensitive location, such as a site of special scientific interest or in a special area of conservation.
For test sites intending to be in use for longer periods of time, defensive monitoring is a more suitable solution. It is also recommended for tests intending to give early warning of any adverse impacts on the activity of the groundwater below.
The final type of monitoring category is that of investigative monitoring. This type of monitoring should be used when trying to determine the quality and characteristics of groundwater in a potentially contaminated area, or within a plume.
Naturally, your objectives may change throughout the process, but it’s important to have a clear set of objectives before you begin any physical installation of the site.
To gain access to the groundwater below, the use of a drill will be needed to create the borehole itself. There are many different techniques available, and selecting the best one for your test site can help ensure the best conditions for analysis.
One of the main things you will need to consider is the depth in which you will need to drill down to. Cable tool methods can be used for depths up to 50m, but anything deeper than this will typically require rotary techniques.
The type of location may also influence the type of drilling technique used. Some places may have a number of volatile organic compounds in the area, meaning the air flush method of drilling would not be recommended due to the risk of explosive and inhalation.
If you are unsure of the best technique, be sure to contact the relevant bodies or organisations, such as the Environment Agency, for clarification on what is best for your specific site.
The response zone
During installation, the response zone will be the area that connects to the aquifer and will include the filter pack and the well screen.
These components are vital for effective groundwater monitoring as they help filter out any solid residue within the water that could potentially impact sample results.
The depth of the aquifer will help determine the location and size of both the filter pack and well screen, but various factors can affect the recommended size of the well screen itself.
While most will be between 1-2m, a screen over 3m long may be suitable if you are working with a thick aquifer or if you are monitoring light non-aqueous phase liquid.
It is also important to consider which materials will be used in the casing for the well screen, as well as the walls of the site itself.
Metal can be effective from a strength perspective, but using any other than stainless steel leads to a high probability of corrosion. While this is less of an issue for short term sites, it could greatly damage the effectiveness of a site that will be used for a long period of time.
Aside from stainless steel, various plastics can also be suitable alternatives, such as High-density polyethylene (HDPE), Polytetrafluoroethylene (PTFE), Acrylonitrile butadiene styrene (ABS) and unplasticized polyvinyl chloride (uPVC).
Your choice of material may also depend on the presence of dissolved contaminants in the water. For example, stainless steel should not be used if there are any metallic contaminants in the water, and ABS and uPVC should not be used if there are any chlorinated solvents.
With the borehole and materials in place, the last set of steps is to apply any necessary seals to the test site.
Most sites will use a bentonite seal above the filter pack. This is typically done through the use of bentonite pellets which help slow the rate of hydration and prevent bridging.
Once the bentonite seal has fully hydrated and expanded, a grout seal can also be added. It is important to ensure the bentonite has completely hydrated to reduce the risk of contamination in the filter pack.
With the grout seal in place, all that remains is to install the headworks to complete the installation of your own groundwater monitoring test site.
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