DESIGN, CONSTRUCTION, AND LOCATION OF LANDFILL GAS MONITORING WELLS

Solid Waste Management Program fact sheet
09/2014
Division of Environmental Quality Director: Leanne Tippett Mosby
PUB2054

Overview
This document was prepared by the Missouri Department of Natural Resources’ Solid Waste Management Program (SWMP) to provide guidance for the proper design and construction of gas monitoring wells to comply with the quarterly monitoring required by 10 CSR 80-3.010(14) and 10 CSR 80-4.010(14).

For any active demolition or sanitary landfill, ground monitoring for gas migration must be performed using gas monitoring wells.

Well Design
Proper design and construction of gas monitoring wells is critical in obtaining true soil gas concentrations. All wells should be designed to minimize air intrusion into the system so accurate soil gas samples can be collected. All monitoring wells that are deeper than 10 feet are regulated by the department’s Missouri Geological Survey (MGS)  formerly known as the Division of Geology and Land Survey (DGLS) and must be installed by a certified well driller. For further information on this subject, call 573-368-2165.

SWMP recommends the following well designs:

Spike probes may be used where shallow groundwater, approximately 10 feet or less below the surface, prevents construction of a drilled well. However, this circumstance should be rare. The SWMP does not consider bar punch testing for shallow soil migration to be an effective monitoring method for other than instantaneous monitoring to evaluate the extent of shallow lateral gas migration.

Monitoring wells should be designed to monitor all unsaturated soil and rock down to an elevation equal to the bottom elevation of the landfill. Wells can be designed with a single riser screened from just below the well seal to the bottom of the landfill. They can also consist of a well cluster with multiple risers screened across different geologic units, as long as monitoring is provided for every geologic unit between the ground surface and an elevation equal to the bottom of waste in the landfill. Well clusters are valuable for isolating separate distinct permeable zones that would be likely to transmit gas (sand seams, fracture zones, karst features, mine shafts, etc.) interval.

Gas monitoring wells must also be designed to prevent intrusion of atmospheric air into the wells at all times; the cap should have a valved or quick-connect sampling port for the direct attachment of the gas sampling instrument, so that samples may be drawn directly from the well.

Well Locations
The location of gas monitoring wells must be approved by the SWMP, and should be based on a characterization of geologic and hydrologic conditions at the landfill site and on the adjacent land uses.

Subsurface monitoring for methane must be conducted around the perimeter of the disposal area. The solid waste regulations state that the point of compliance for regulatory limits of methane migration is at the landfill property boundary. However, at some sites where the edge of the fill area is far from the property boundary, landfill owners/operators have chosen to locate gas monitoring wells closer to the waste mass to provide early detection so that action can be taken to prevent gas migration from the landfill property.  In these situations, the gas monitoring wells are considered to be points of compliance. Landfill owners/operators will be required to take the actions specified by the SWMP’s Methane Gas Policy dated June 2012 if methane is detected above regulatory limits in the original monitoring wells. The monitoring wells may be relocated to the property boundary as part of the gas migration investigation, with prior approval of the SWMP. If the newly installed wells are in compliance, the owner/operator will not be required to carry out formal corrective actions.  The owner/operator should keep this in mind when choosing gas monitoring well locations.

Monitoring wells should be located in areas where gas migration is most likely to occur or to become a threat to the public or the environment. The wells should be located in critical areas such as between the landfill and adjacent buildings and sand or gravel bedded utility lines.

Monitoring locations should be spaced no more than 500 feet apart, with closer spacing depending on the permeability of the ground (the more permeable, the closer the spacing) and on the number of nearby features that could be potentially affected. Gas monitoring wells closer to the waste mass should not be placed directly opposite gas extraction wells on the fill area; monitoring wells may give a falsely low reading if they are in the zone of influence of the extraction well.

Monitoring may not be necessary in areas where the potential for gas migration is low. For example, a stream or a valley may form a natural (hydrologic or topographic) cutoff to prevent the flow of gas through the ground.  However, in these situations, the owner/operator must make a formal written demonstration that a natural cutoff exists, and that demonstration must be approved by the SWMP.

Conclusions
All wells should be designed to minimize air intrusion into the system, which can dilute the sample, making it unrepresentative. Selection of well designs and locations should be based upon what zones are to be monitored. A monitoring well certification record shall be used to report new monitoring well construction. Monitoring well certification record forms can be obtained at http://dnr.mo.gov/forms/index.html  or monitoring well construction can be reported online at https://dnr.mo.gov/mowells/.


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.
Technical Bulletin - Figure 1 and Figure 2
Technical Bulletin - Figure 3