For any active demolition site or sanitary landfill, it is crucial to perform ground monitoring for gas migration using gas monitoring wells. This document offers essential guidance on the appropriate design and construction of gas monitoring wells to ensure compliance with quarterly monitoring regulations outlined in the Code of State Regulations 10 CSR 80-3.010(14) and 10 CSR 80-4.010(14). Proper landfill gas monitoring is not just a regulatory requirement; it’s a vital step in environmental safety and responsible waste management.
Key Elements of Effective Well Design
The design and construction of gas monitoring wells are paramount to obtaining accurate soil gas concentrations. To ensure the collection of reliable soil gas samples, every well should be engineered to minimize atmospheric air intrusion into the system. It’s important to note that in many regions, including Missouri, any monitoring wells exceeding a depth of 10 feet fall under the regulation of the Missouri Geological Survey. Installation of these wells must be carried out by a certified well driller. For further details or clarification, you can contact them directly at 573-368-2165.
The Waste Management Program highly recommends the following well designs to meet different site conditions and monitoring objectives:
- Permanent Monitoring Well: For detailed specifications, please consult the Missouri Well Construction Rules 10 CSR 23-4. Figure 1 provides a visual guide to the main components of this type of well, which is designed for long-term monitoring and robust data collection.
- Push Probe/Direct Push Well: While this design may not fully align with current codes, it’s possible to obtain a variance through MGS for its installation. Figure 2 illustrates the key components of a Push Probe/Direct Push Well, often chosen for its less invasive installation.
- Spike Probe: Technically, a spike probe isn’t classified as a monitoring well due to its limited depth of use, typically less than 10 feet below the ground surface. Consequently, it doesn’t require a variance from MGS. Figure 3 shows the essential components of a Spike Probe, suitable for very shallow monitoring needs.
Alt text: Diagram illustrating the components of a permanent gas monitoring well, including well casing, screen, filter pack, bentonite seal, and surface completion, designed for long-term landfill gas monitoring.
Spike probes can be particularly useful in situations where shallow groundwater, around 10 feet or less beneath the surface, hinders the construction of a traditional drilled well. However, this scenario should be infrequent. It’s important to understand that the department does not consider bar punch testing as an effective long-term methane monitoring method for shallow soil gas migration, except for immediate assessments of lateral gas movement.
Monitoring wells should be strategically designed to monitor all unsaturated soil and rock layers down to the level of the landfill base. Designs can range from a single riser well, screened from just below the well seal to the landfill bottom, to well clusters. Well clusters incorporate multiple risers screened across different geological units. The key is to ensure monitoring coverage for every geological unit between the surface and the waste base elevation within the landfill. Well clusters are especially beneficial for isolating distinct permeable zones that are likely pathways for gas transmission, such as sand seams, fracture zones, karst features, or mine shafts.
Crucially, gas monitoring wells must be designed to consistently prevent atmospheric air from entering the wells. To achieve this, the well cap should feature a valved or quick-connect sampling port. This allows for the direct attachment of gas sampling instruments, facilitating sample extraction directly from the well without risking air contamination. This is essential for obtaining representative soil gas samples and accurate readings.
Alt text: Illustration depicting a push probe or direct push gas monitoring well, highlighting its simpler construction suitable for quicker installation and shallow to moderate depth landfill gas investigations.
Strategic Well Locations for Landfill Monitoring
The placement of gas monitoring wells requires approval from the department’s Waste Management Program. Location decisions should be grounded in a thorough understanding of the geological and hydrological conditions at the landfill site, as well as the nature of adjacent land use. Careful monitoring well location is critical for effective and representative data collection.
Subsurface methane monitoring must be conducted around the perimeter of the waste disposal area. Regulations stipulate that the compliance point for methane migration regulatory limits is at the landfill property boundary. However, in cases where the fill area is significantly distant from the property boundary, landfill operators may opt to position monitoring wells closer to the waste mass. This proactive approach enables earlier detection of gas migration, allowing for timely intervention to prevent gas escape beyond the landfill property. In such instances, these closer wells are considered the points of compliance. Should methane levels exceed regulatory limits in these initial monitoring wells, landfill operators are obligated to implement actions as specified by the department’s Methane Gas Policy. Relocation of monitoring wells to the property boundary may become necessary as part of a gas migration investigation, but this requires prior approval from the Waste Management Program. If the newly positioned wells demonstrate compliance, formal corrective actions may not be required. Landfill operators should consider this potential relocation when initially selecting gas monitoring well locations.
Monitoring wells should be situated in areas with a higher likelihood of gas migration or where gas migration poses a potential threat to public safety or the environment. Critical locations include areas between the landfill and nearby buildings, as well as utility lines buried in sand or gravel beds, which can act as preferential pathways for gas.
Spacing between monitoring locations should not exceed 500 feet. Closer spacing may be necessary depending on ground permeability; higher permeability necessitates closer spacing. The number of potentially affected nearby features also influences spacing requirements. It’s important to avoid placing monitoring wells too close to gas extraction wells within the fill area. Monitoring wells situated within the influence zone of an extraction well might yield falsely low readings.
In some areas, monitoring may be deemed unnecessary if the potential for gas migration is minimal. For instance, a stream or valley might serve as a natural barrier, hydrologically or topographically, effectively preventing gas flow through the ground. However, in such cases, the landfill operator must provide a formal written demonstration proving the existence of a natural cutoff. This demonstration is subject to approval by the department’s Waste Management Program.
Alt text: Diagram showing a spike probe gas monitoring well, designed for very shallow subsurface gas sampling, typically used for initial site assessments or in areas with shallow groundwater.
Final Thoughts on Gas Monitoring Well Implementation
To summarize, the primary goal in gas monitoring well design is to minimize air intrusion. Air contamination dilutes samples, rendering them unrepresentative and unreliable. The selection of well designs and their locations should be driven by the specific zones requiring monitoring. To formally document new monitoring well construction, a monitoring well certification record must be completed and submitted. These forms, along with additional resources, are accessible on the department’s Well and Pump Installation Contractor Permitting webpage. Implementing a robust landfill gas monitoring system through carefully designed and placed wells is essential for environmental protection and regulatory compliance.
References
- Farquhar, Grahame, Monitoring and Controlling Methane Gas Migration, course notes presented at April 1993 Sanitary Landfill Design and Management training, offered by the University of Wisconsin, Madison, College of Engineering.
- Missouri Department of Natural Resources, Flood Grant Team, An Analysis of Landfill Gas Monitoring Well Design and Construction.
