Monitoring Well Installation (Single and Nested)
What is Monitoring Well Installation?
Monitoring well installation is a key component of environmental and hydrogeological investigations. It involves installing wells into the subsurface to observe groundwater levels, collect water samples, and track changes in quality or contamination over time. There are two common types:
- Single Monitoring Wells
- Nested (or Multi-Level) Monitoring Wells
Both are used for groundwater monitoring, but they differ in design, complexity, and the number of zones they monitor.
Single Monitoring Well Installation
A single monitoring well is constructed to monitor one discrete depth interval (aquifer or zone of interest) in the subsurface.
Installation Process:
- Drill the borehole to the target depth using auger, mud rotary, sonic, or direct push methods.
- Install well casing and screen:
- Casing (usually PVC or stainless steel) extends from surface to just above the screen.
- Well screen (slotted or perforated) is positioned across the monitoring zone.
- Place filter pack (sand or gravel) around the screen to allow groundwater to enter.
- Seal the annulus above the screen with bentonite or grout to prevent vertical water flow.
- Surface protection: Lockable cap, monument casing, or protective well cover.
Uses:
- Monitor water table or potentiometric surface
- Sample for contaminants or nutrients
- Track seasonal water level changes
- Monitor a specific aquifer or lithological layer
Nested Monitoring Wells (Multi-Level Wells)
Nested wells involve installing multiple monitoring points within a single borehole, allowing for vertical profiling of groundwater conditions at different depths. They can be:
- Multiple casings installed in the same borehole (traditional nested)
- Multilevel systems like Westbay®, Solinst CMT, Waterloo systems (more advanced)
Installation Approaches:
1. Traditional Nested Wells
- Multiple small-diameter casings with separate screens installed at different depths in a single borehole.
- Annular seals (bentonite or cement) are placed between each screened interval to prevent cross-contamination.
- Requires careful design and precise placement of seals.
2. Multilevel Monitoring Systems
- Single outer casing with multiple isolated sampling ports or sensors along its length.
- Engineered systems with isolated flow paths to each zone.
- Easier to install and maintain data integrity across zones.
Uses:
- Monitor vertical groundwater gradients
- Detect contaminant plume migration across stratigraphy
- Evaluate hydraulic connectivity between layers
- Support complex groundwater models
Key Design Considerations
- Screen length and placement: Should match target water-bearing zone
- Annular seal integrity: Critical to avoid vertical flow or cross-contamination
- Material selection: PVC for general use; stainless steel for chemical compatibility
- Access and protection: Must be secure and accessible for sampling or datalogging
Comparison: Single vs. Nested Monitoring Wells
| Feature | Single Well | Nested Well |
| Number of Zones | One | Two or more |
| Complexity | Simple | Moderate to high |
| Cost | Lower | Higher (due to drilling and sealing requirements) |
| Risk of Cross-Contamination | Low | Higher unless properly sealed |
| Applications | General monitoring, compliance | Detailed site characterization, contamination studies |
Monitoring Well Use Cases
| Sector | Purpose |
| Mining | Monitor drawdown and tailings seepage |
| Construction | Track dewatering effects near excavations |
| Landfills | Detect leachate movement and groundwater impact |
| Environmental remediation | Track contaminants (e.g., TPH, VOCs, metals) |
| Agriculture | Monitor nutrient leaching and water quality |
Summary
| Attribute | Single Monitoring Well | Nested Monitoring Well |
| Definition | Monitors a single depth interval | Monitors multiple depths from one borehole |
| Design | Simple screen and casing | Multiple screens or engineered multi-level systems |
| Use | Water level, quality monitoring | Vertical profiling, complex site characterization |
| Risk | Low cross-contamination | Higher unless sealed carefully |
| Cost & Complexity | Lower | Higher but more data-rich |
Properly designed and installed monitoring wells are vital tools for understanding groundwater behavior and informing remediation, compliance, and resource management decisions.