Effective Groundwater Monitoring Well Installation and Design

Groundwater monitoring wells are critical tools used to track water quality, measure water levels, and collect data for hydrogeological studies. Their installation requires precision, quality materials, and expert techniques to ensure reliability over time. This guide outlines the key components and processes involved in the design and installation of monitoring wells, along with maintenance considerations to ensure long-term performance.

Key Components of Monitoring Wells

1. Casing:
The casing is a crucial part of the well that prevents contamination from surface water and provides structural integrity to the well. It is typically made of materials like PVC, stainless steel, or fiberglass. The casing’s diameter and material selection depend on the environment and project requirements.

2. Screen:
The well screen allows water to flow into the well while preventing sediment from entering. It is placed at the desired sampling depth and is made of slotted material, usually PVC or stainless steel. The slot size depends on the groundwater type being monitored.

3. Filter Pack:
A filter pack, often composed of gravel or sand, is placed around the well screen to support proper water flow. It also helps to filter out fine sediment, ensuring that only clean groundwater enters the well.

4. Sealing Material:
To prevent contamination from surrounding soils and surface water, sealing materials like bentonite or cement grout are used to seal the well. These materials ensure that water from surrounding areas does not interfere with the monitoring process.

5. Headworks and Wellhead:
The wellhead is the uppermost part of the well, which is protected by a cover or seal. Proper sealing prevents contamination and ensures that the well is accessible for sample collection or measurement.

The Installation Process

Site Assessment and Planning:
Before installation begins, a site assessment is conducted to determine the location and type of well required. This includes understanding the geological conditions, water table levels, and proximity to potential contamination sources. Proper site selection is crucial for accurate monitoring.

Borehole Drilling:
The installation begins with drilling a borehole that will house the well casing and screen. The diameter and depth of the borehole depend on the required monitoring depth and the type of groundwater to be sampled.

Casing and Screen Installation:
Once the borehole is drilled, the casing and screen are installed. The casing is placed in the borehole, and the screen is lowered to the desired depth. A clean sand or gravel pack is added around the screen to facilitate filtration and provide stability. The well is then sealed with bentonite, which creates a low-permeability seal above the screen to prevent contamination from surface water. The annular space around the well is grouted to further prevent the intrusion of surface water and contaminants.

Wellheads Installation: 
Protecting the well from damage or contamination is essential. This is achieved by installing appropriate headworks, which also help secure the well and make it easily accessible for sampling.

Development and Testing

After installation, the well undergoes development to remove any fine materials that may hinder accurate sampling. This is typically achieved by flushing and purging using high-pressure air or mechanical tools like bailers. Flow rates and water quality are tested to ensure that the well functions properly and delivers accurate results.

Maintenance

Routine maintenance is crucial to ensure the well's longevity and performance:

• Cleaning: Regular cleaning prevents clogging and ensures consistent water flow.
• Inspection: Checking for structural integrity and confirming that data is reliable ensures accurate monitoring.
• Well Head Maintenance: Ensuring that the well head is in good condition protects the well and guarantees a long lifespan.

Products Used in Monitoring Well Construction

Casing and Screens:

• PVC (Polyvinyl Chloride): Lightweight, corrosion-resistant, and cost-effective.
• Stainless Steel: High durability and chemical resistance, ideal for harsher environments.

Gravel Pack or Sand Pack:
Used to surround the monitoring well screen and prevent sediment from infiltrating the well while still allowing water to flow.

Sealing Materials:

• Bentonite: Expanding clay that seals gaps to prevent contamination.
• Grout: Cement-based material used for additional sealing and stabilization.

End Caps and Centralizers:
End caps prevent debris from entering the well, and centralizers ensure that the well remains centered in the borehole.

Sampling Equipment:

• Bailers, pumps, or passive samplers are used to extract groundwater for analysis.

Monitoring Well Design and Installation Techniques

Proper monitoring well construction techniques are critical to achieving accurate results. For example, the borehole should be as vertical as possible to ensure proper alignment. Any deviation from vertical should be noted, and centralizers should be used to ensure proper well placement.

Hollow Auger Installation and Filter Pack Placement:
During installation, it is important to carefully control the withdrawal of the augers and monitor sand placement to prevent bridging. The filter pack is essential for maintaining groundwater flow and preventing sediment infiltration.

Bentonite Seal and Grouting:
After the filter pack is in place, a bentonite seal is added to prevent surface contamination. Grout is then used to fill the annular space around the casing and create a permanent seal.

Surface Pad and Monument Covers:
Once the well is installed, a surface pad is placed to protect the well. The well should also be marked with a permanent ID and covered with a protective casing to prevent damage from traffic or other external factors.

Potential Issues with Monitoring Well Design and Installation

Several factors can impact the success of monitoring well installation, including:

• Improper Site Selection: Poor placement can lead to inaccurate data or failure to intercept contamination plumes.
• Faulty Design: Issues like incorrect screen aperture size or gravel pack placement can hinder water flow or allow sediment infiltration.
• Material Compatibility: Incompatible materials can cause degradation, contamination, or structural failure.
• Inadequate Sealing: Poor sealing can allow surface water to enter and cross-contaminate different aquifers.
• Construction Errors: Damage to surrounding formations during drilling can disrupt natural groundwater flow.
• Neglecting Well Development: Insufficient flushing can leave debris in the well, leading to unreliable data.

If a monitoring well is not installed or developed properly, it may yield groundwater samples that are not only of poor quality, but do not truly represent groundwater conditions beneath a Site. Therefore, data collected from an improperly installed or developed well may lead to incorrect conclusions about the environmental quality of the Site and, ultimately, the implementation of improper remedial actions which may pose financial and/or health and safety risks to property owners, operators and the general public

Conclusion

Monitoring well installation is a complex process that requires careful planning, skilled labor, and the use of high-quality materials. Whether you're monitoring groundwater quality, conducting hydrogeological studies, or tracking remediation efforts, Site Investigation Supply is here to provide the products and expertise you need to ensure the success of your project. Proper installation and maintenance are key to ensuring the long-term reliability and accuracy of your groundwater monitoring systems.