Introduction
Installation of monitoring wells is one of the most critical steps in a Phase II Environmental Site Assessment (ESA), particularly when groundwater contamination is suspected or confirmed. These wells provide direct, repeatable access to subsurface conditions, allowing environmental professionals to evaluate groundwater quality, determine contaminant distribution, and assess hydrogeological conditions over time.
Unlike soil sampling—which provides a snapshot in time—monitoring wells enable long-term observation. They serve as permanent infrastructure that supports ongoing sampling, remediation verification, and regulatory compliance.
In Ontario, the installation process is governed by strict regulatory frameworks, including Ontario Regulation 153/04, which outlines the requirements for conducting environmental site assessments and filing Records of Site Condition (RSCs). Compliance ensures that monitoring wells are constructed in a way that produces reliable data while preventing cross-contamination between subsurface zones.
This article provides a comprehensive overview of monitoring well installation, including design considerations, materials, installation procedures, and documentation requirements.
Purpose of Monitoring Wells
Monitoring wells are installed to achieve several key objectives within a Phase II ESA program:
Groundwater Sampling
The primary purpose of a monitoring well is to collect representative groundwater samples for laboratory analysis. These samples are used to identify contaminants such as petroleum hydrocarbons, volatile organic compounds (VOCs), metals, and other site-specific parameters.
Properly installed wells ensure that samples are not influenced by surface infiltration or drilling disturbances, thereby providing accurate chemical data.
Hydraulic Characterization
Monitoring wells are also used to measure static groundwater levels. By surveying the elevation of multiple wells across a site, environmental professionals can determine groundwater flow direction and hydraulic gradient.
Understanding groundwater flow is essential for:
- Predicting contaminant migration pathways
- Designing remediation systems
- Assessing potential risks to receptors (e.g., nearby properties or water bodies)
Long-Term Monitoring
Monitoring wells provide permanent access points for ongoing environmental monitoring programs. These programs may be required:
- During remediation activities
- As part of regulatory compliance
- To confirm site closure conditions
Long-term datasets collected from wells are invaluable for evaluating trends in contaminant concentrations and groundwater conditions.
Materials and Design
Monitoring wells must be constructed using materials that are chemically inert and structurally durable. Improper material selection can compromise sample integrity and lead to inaccurate results.
Casing and Screen
The well casing and screen are typically constructed from Schedule 40 PVC. PVC is widely used because it is:
- Chemically inert for most contaminants
- Cost-effective
- Durable and easy to install
The well screen is a slotted section of pipe that allows groundwater to enter the well while preventing sediment intrusion. Screen slot size is selected based on the grain size distribution of the surrounding soil.
Screen placement is critical and depends on the monitoring objective:
- Across the water table for general groundwater monitoring
- At a specific depth to target a contaminant plume
- At the base of an aquifer to assess deeper conditions
Filter Pack (Sand Pack)
A filter pack consisting of clean, washed silica sand is placed around the well screen within the annular space. Its functions include:
- Preventing fine soil particles from entering the well
- Enhancing hydraulic connectivity between the formation and the well
- Stabilizing the well screen
The filter pack must be carefully selected to match the surrounding soil conditions and installed to the correct height above the screen.
Bentonite Annular Seal
Above the filter pack, a bentonite seal is installed. Bentonite is a swelling clay that expands when hydrated, forming a low-permeability barrier.
This seal is essential to:
- Prevent vertical migration of water along the borehole
- Eliminate cross-contamination between subsurface layers
- Protect the integrity of groundwater samples
Bentonite may be installed as chips, pellets, or slurry, depending on site conditions.
Riser Pipe
The riser pipe extends from the top of the bentonite seal to the ground surface. It provides a conduit for accessing groundwater and measuring water levels.
The riser must be properly aligned and secured to prevent damage or misalignment over time.
Protective Casing
At the surface, a steel protective casing is installed to safeguard the well. Two common completion types include:
- Flush Mount (Road Box): Installed level with the ground surface, typically in paved or high-traffic areas
- Stick-Up Completion: Extends above ground and is used in secure or low-traffic environments
Protective casings are typically lockable to prevent tampering or vandalism.
Installation Process
Monitoring well installation is performed immediately after borehole drilling and soil sampling to ensure that the borehole remains open and stable.
1. Lowering Materials
The pre-assembled well screen and riser pipe are carefully lowered into the borehole to the target depth. Care must be taken to:
- Avoid damaging the screen
- Maintain vertical alignment
- Ensure correct placement relative to the target zone
Centralizers may be used in deeper wells to maintain proper positioning.
2. Placing the Filter Pack
Silica sand is introduced into the annular space around the well screen. This is typically done using a tremie pipe or by carefully pouring the material from the surface.
Field staff measure the depth of the sand pack to confirm it extends several inches above the top of the screen.
3. Installing the Seal
Bentonite is placed above the filter pack to form the annular seal. Once installed, it is hydrated using clean water (if necessary) to activate its swelling properties.
Proper seal installation is critical to prevent vertical leakage pathways.
4. Backfilling the Riser
The remaining annular space above the bentonite seal is backfilled to ground surface. Materials may include:
- Native soil cuttings (if suitable)
- Granular bentonite
- Cement-bentonite grout
The choice of material depends on regulatory requirements and site conditions.
5. Surface Completion
The protective casing is installed and secured in a concrete pad. This ensures:
- Long-term durability
- Safety for site users
- Ease of access for sampling
Surface completion must meet site-specific requirements, especially in areas with vehicle traffic.
6. Well Development
After installation, the well is allowed to equilibrate—typically for 12 to 24 hours—so that the bentonite seal can fully hydrate.
The well is then developed using methods such as:
- Surging
- Pumping
- Bailing
Development removes fine sediments introduced during drilling and improves hydraulic connection with the surrounding formation. Properly developed wells yield clear, sediment-free groundwater samples.
Quality Assurance and Best Practices
Successful monitoring well installation depends on strict adherence to quality control procedures and best practices.
Accurate Depth Measurements
All depths must be measured and recorded precisely, including:
- Borehole depth
- Screen interval
- Filter pack thickness
- Seal placement
Errors in measurement can compromise data interpretation.
Clean Installation Techniques
To avoid introducing contaminants:
- Equipment must be decontaminated between boreholes
- Clean materials must be used
- Handling of well components must be controlled
Appropriate Screen Selection
Screen slot size and length must be selected based on site-specific geology. Incorrect selection can result in:
- Sediment infiltration
- Poor sample recovery
- Biased analytical results
Proper Development
Inadequate well development can lead to:
- Turbid samples
- Inaccurate laboratory results
- Reduced well performance
Development should continue until water clarity and field parameters stabilize.
Documentation and Reporting
Accurate and detailed documentation is a regulatory requirement and a cornerstone of a defensible Phase II ESA.
The qualified professional overseeing the work must record:
- Total borehole and well depth
- Screen interval and slot size
- Filter pack and seal depths
- Installation methods and materials
- Development procedures and observations
In Ontario, guidance from the Ministry of the Environment, Conservation and Parks must be followed to ensure compliance with provincial standards.
Monitoring well locations must also be surveyed and included on a site plan, typically with reference to a coordinate system and elevation datum.
All information is compiled into the Phase II ESA report, forming part of the permanent environmental record for the site.
Regulatory Considerations in Ontario
Monitoring well installation in Ontario is subject to stringent regulatory oversight. Ontario Regulation 153/04 establishes requirements for environmental site assessments, including:
- Use of qualified persons (QPs)
- Adherence to prescribed sampling and installation methods
- Documentation standards for Records of Site Condition
Additional technical guidance provided by the Ministry of the Environment, Conservation and Parks outlines best practices for well construction, sealing, and abandonment.
Failure to comply with these requirements can result in:
- Rejection of the ESA report
- Regulatory penalties
- Liability risks for property transactions
Common Challenges and Solutions
Borehole Collapse
In unstable soils, boreholes may collapse before installation is complete. This can be mitigated by:
- Using drilling methods that support the borehole (e.g., hollow stem augers)
- Installing wells immediately after drilling
Smearing and Cross-Contamination
Drilling can smear contaminants along borehole walls. Proper sealing and development help minimize this effect.
Improper Seal Installation
If the bentonite seal is not properly installed or hydrated, vertical migration pathways may form. This risk is reduced by:
- Verifying seal placement depth
- Allowing adequate hydration time
Poor Well Yield
Low-permeability soils can result in slow groundwater recovery. Solutions include:
- Using shorter screens
- Installing wells in more permeable zones
- Allowing additional recovery time before sampling
Conclusion
Monitoring wells are an essential component of any Phase II Environmental Site Assessment involving groundwater investigation. Their proper installation ensures that groundwater samples are representative, hydraulic data is accurate, and long-term monitoring can be conducted effectively.
From material selection and design to installation, development, and documentation, each step must be executed with precision and in accordance with regulatory standards such as Ontario Regulation 153/04.
By following best practices and adhering to guidance from the Ministry of the Environment, Conservation and Parks, environmental professionals can ensure that monitoring wells provide reliable, defensible data that supports informed decision-making and regulatory compliance.
Ultimately, well-installed monitoring wells form the backbone of groundwater investigations, enabling a clear understanding of subsurface conditions and supporting the successful management of contaminated sites.