Robert L. Anderson, C.E.G., Glenn K. Young, P.E., Timothy W. Crist, P.E.
Current California regulations regarding subsurface landfill gas monitoring allow some latitude in the design and operation of subsurface landfill gas monitoring systems at municipal solid waste landfills in California. Landfill gas monitoring systems are typically divided into three categories: monitoring of operating and inactive landfills, monitoring of closed landfills, and monitoring of landfills with structures either on or adjacent to the waste management units. Depending on the topography, geology, and presence of structures, the amount of subsurface landfill gas monitoring points may vary. The frequency and types of gases monitored are dependent on the results of confirmed landfill gas survey results, and the presence of receptors and structures. This paper will include a discussion on the application of various strategies for design and operation of subsurface landfill gas systems.
Keywords: Gas migration, landfill gas, methane, mitigation, subsurface landfill gas monitoring
With the development of sanitary landfilling in California in the 1950s, landfill gas problems began to manifest themselves in the form of fires and explosions. In 1986, the California Air Resources Board initiated a program to assess landfill gas from municipal solid waste landfills (Air Resources Board, July 1996). The program lasted for several years and resulted in the collection and assessment of landfill gas data for many landfills. The results of the Air Resources Board program and case histories of fires and explosions caused by methane gas generated at landfills and migrating into structures near landfills prompted the California Integrated Waste Management Board (CIWMB) to establish and implement subsurface landfill gas monitoring regulations.
CalRecycle determined that in order to assess the potential for landfill gas hazards at or adjacent to a landfill an integrated approach to monitoring must be considered. This monitoring effort was divided into three principal elements:
- Monitoring of the subsurface.
- Monitoring of surface emissions.
- Monitoring of structures.
Landfill gas monitoring in structures or as surface emissions consists of a significantly different approach to detection and evaluation of gas levels than does subsurface landfill gas monitoring. This paper concentrates on subsurface landfill gas monitoring strategies used at municipal solid waste landfills in California.
The overall goal of monitoring landfill gas migration is to assess the need for control of the movement of the gas and to provide input for the design, and successful operation of a landfill gas control system should one be needed.
Selection of the location and elevation of landfill gas monitoring probes is highly dependent on the structural and hydrogeologic conditions at the area to be studied. Landfill gas migration is affected by the permeability and transmissivity of the soil or rock that the gas travels through. Soil and rock units that are permeable to water are also permeable to gas. Landfill gas will not necessarily saturate an entire permeable soil or rock unit that is tilted since the gas is generally lighter than air and will travel up structure and accumulate against areas where there is a barrier to transmission (a cap rock or structural trap). If there is not a cap rock or a structural trap, the gas will work its way to the surface of the ground and escape into the air. If there is a pressure differential between a subsurface soil or rock unit, landfill gas may flow from an area of high pressure to an area of low pressure.
Permeability of soil or rock units may be increased by weathering, fracturing, or faulting. Placing structures either on or adjacent to a unlined landfill that is located in highly permeable gravel deposits have led to structures being monitored for gas both in the subsurface and in the structure. The partial removal of refuse from a landfill, followed by the construction of a residential development over the footprint of the landfill, has lead to differential settlement problems in several yards, fear of landfill gas seeping into houses, and exposed waste in gardens.
Influence of Bedding Attitude and Primary Permeability
As discussed above, the location of landfill gas monitoring probes depends on the attitude and permeability of geologic units. This observation is illustrated by the location of elevated landfill gas concentrations at the City of Ukiah Landfill in Mendocino County. The site is a 42-acre, 3.7 million-cubic-yard, unlined, side hill canyon fill. Local geology is made up of alluvium and continental basin deposits unconformably overlying sandstone and shale beds of the Franciscan Formation. The alluvium is made up of unconsolidated sandy to clayey loam with localized lenses of gravel. The alluvium is up to 15 feet thick. The basin deposits are made up of poorly sorted, partially indurated conglomerate with silty clay, clayey sands, and gravel. As a unit, the basin deposits dip approximately 20 degrees in a northwesterly direction (EBA Wastechnologies, April 1993). The Franciscan Formation in this area is made up of melange, a mixture of discontinuous sandstone and shale that contains blocks of greenstone, chert, and metavolcanic rocks. It is thought that the upper 120 feet of alluvium and continental basin deposits are in contact with waste on the southwestern side of the landfill.
The highest concentration of landfill gas has been detected up dip from the waste in a gravelly zone. Landfill gas has also been observed in clay zones in the same unit within the continental basin deposits up dip of the landfill, but at lower concentrations than in the gravelly zones.
Landfill gas detection monitoring at the site consists of measuring barometric pressure, methane content and oxygen content from the landfill gas monitoring system. The subsurface landfill gas monitoring well field contains 10 wells. Each well contains several probes. Landfill gas readings taken from five probes along the southwestern portion (up dip from the landfill) of the site indicated that methane concentrations vary from 4.2 percent to 30.4 percent by volume (Lawrence and Associates, August 27, 1997). Methane concentrations along the northeastern perimeter (down dip from the landfill) of the site vary from nondetect to 2.65 percent by volume Lawrence and Associates, October 3, 1996). One explanation for lower landfill gas values northeast of the landfill is that there is less waste in contact with alluvium and the continental basin deposits; and the creek that runs along the north side of the landfill may act as a partial barrier to landfill gas migration since bedding in the continental basin deposits and the Franciscan Formation tends to dip to the north.
Based upon the level of gas found in the southwestern probes, a partial perimeter gas collection and control system made up of nine deep and six shallow extraction wells, a lateral gas collection line, and a blower/vent assembly was installed during the summer of 1997.
Influence of Ground Water Elevation
Depth of landfill gas monitoring wells is mainly dependent on two factors, the elevation of ground water and the invert elevation of the waste mass. In some instances high ground water conditions has led to landfill gas monitoring well installation as shallow as 5 feet below ground surface. An example of a landfill gas monitoring system installation at a site with high groundwater elevations and a shallow invert elevation for waste is the Caspar Solid Waste Disposal Site in Mendocino County. The Caspar Solid Waste Disposal Site is a 16-acre, 832,000-cubic-yard, unlined trench/area fill located adjacent to Russian Gulch State Park in Mendocino County. Local geology is made up of sand and clayey sand from partially consolidated marine terrace deposits and buried drainage channels unconformably overlying the Franciscan Formation. The seasonal high ground water table varies from 0 to 5 feet below ground surface. The site was built in the 1960s by excavating a series of trenches into the water table and placing garbage within the trenches. When possible, the trenches were set on fire to reduce the volume of waste and to control vectors.
Three perimeter gas probes were installed at the site in January 1988. In addition, one landfill gas probe was driven into the waste prism. No significant landfill gas was detected. However, ground water monitoring results indicated that volatile organic compounds that had been observed in other landfills with similar waste streams were showing up in ground water. The North Coast Regional Water Quality Control Board had attributed the volatile organic compounds to leachate. Five additional shallow landfill gas probes were installed along the perimeter of the site. All five probes were advanced to a depth of five feet below grade.
Landfill gas results did not indicate that landfill gas was migrating in ground water from the landfill. Since volatile organic compounds have been found in small amounts cross gradient, up gradient and down gradient near the landfill, other sources than the landfill were investigated. CIWMB staff noted that at one time drilling exploratory offshore oil wells was contemplated not far from the site. It has been suggested but not confirmed that some of the volatile organic compounds may be from source rocks near or at the site.
Influence of Secondary Permeability (Fractures)
Landfill gas monitoring in fractured rock with a thin soil cover is primarily controlled by the location and attitude of continuous fracture zones. An example of a landfill gas monitoring system installed at a site underlain by fractured rock is the McCourtney Road Landfill in Nevada County. The McCourtney Road Landfill is a 38-acre, cut-and-cover canyon fill in the Sierra Nevada foothills. Colluvial soil forms a thin (2 to 5 feet thick) veneer over weathered bedrock. Bedrock is principally made of steeply dipping, folded and faulted metamorphic rock intruded by dikes of diabase. Amphibolite, gabbro, and serpentinite has been locally fractured, faulted, and sheared. All rock types at the site have been moderately to highly weathered near the surface. Zones of kaolinized rock follow the general orientation of the two principal faults that cross the site. Landfill gas permeability of the bedrock is governed by the degree of weathering, continuity of fractures, and infilling of fractures.
Twelve landfill gas monitoring wells were installed around the site in 1995. Due to the low conductivity of intact unweathered bedrock at the site, the landfill gas monitoring wells were located in fracture and shear zones. All of the dedicated landfill gas monitoring wells showed only minor amounts or no landfill gas. None of the dedicated landfill gas monitoring wells are known to have been completed below the local high ground water elevation. The site is known to produce landfill gas since a piezometer, located about six feet south of a lined refuse cell had landfill gas levels in excess of the lower explosive levels for methane. The piezometer was determined to be too close to a waste management cell to be considered an appropriate point of compliance for monitoring landfill gas. The site was undergoing closure during the summer of 1997. CIWMB staff will be looking for changes in landfill gas readings in the monitoring wells once the landfill cap is in place. Due to the lack of significant levels of landfill gas detected in the gas monitoring system, monitoring is conducted on a quarterly basis.
Influence of Structures
The location of structures either on or adjacent to a landfill can dictate the placement and number of landfill gas monitoring probes. An example of placement selection due to structure location is the 14th Avenue Landfill in Sacramento County. The site is made up of two pit-type, unlined landfills. Both sites are reclaimed gravel pits. The site received municipal and commercial waste from the early 1960s until the mid-1970s. The eastern pit fill covers 10 acres and is from 20 to 60 feet deep. The western pit fill covers 7 acres and varies from 10 to 30 feet deep. Site geology is made up of silty sands overlying gravel deposits. Total refuse in place has been estimated to be 500,000 tons (1,000,000 cubic yards). Mass grading for the development of commercial pads at the site was done prior to the development of current landfill capping regulations. Although the soil cover over waste is thought to be in excess of 5 feet thick, no barrier layer to prevent surface infiltration of water or gas exfiltration exists at the site except where paved parking or storage areas, or building footprints exist. Seventeen buildings have been built either on or adjacent to the landfills (see figure 1).
In 1986, the Sacramento County Health Department required parcel owners to install landfill gas monitoring wells. Monitoring of the wells was conducted to determine if landfill gas was seeping into the structures built on or adjacent to the site. Landfill gas monitoring probes have shown that methane ranged from 15 percent to 49 percent by volume. In addition, temporary landfill gas probes, less than two feet deep, have been used by CIWMB staff to look for near surface hot spots. One building built over waste has suffered up to 5 feet of settlement. The building owner was required to install landfill gas monitoring and an alarm system. County and CIWMB staffs have detected up to 25 percent methane by volume in cracks in the floor. Subsequent to the gas detection, 28 passive vents were installed in the ceiling of the building. Since the installation of the vents, gas levels in the building have dropped.
Influence of the Location of Waste
The location of in place refuse should be considered when installing a landfill gas monitoring system. An example of a landfill gas monitoring system where the location of landfill gas monitoring wells equipped with multiple probes was selected based on the location of refuse left in place is the Braito Landfill in Solano County. The site located in two canyons northwest of the Carquinez Strait. Several draws off of the main canyon were used for disposal of solid waste. Site geology is represented by colluvium overlying Panoche Group sandstone, claystone, and shale. Locally the sandstone, claystone, and shale are fractured. The site was operated as a cut-and-cover landfill from 1955 until 1978 (Levine and Fricke, September 1996). The site was closed in place sometime between 1978 and the beginning of grading operations for residential development in the early 1980s. During grading operations, several pockets of residential, industrial (including tannery waste), and inert waste were partially removed.
The northern and eastern sides of Blake Court, a small cul-de-sac off of the northeastern side of the main canyon are thought to overlie a deposit of refuse. Depth of groundwater in the Blake Court area is approximately 20 feet below ground surface. No homes were built on Blake Court. Four houses along Rose Drive, adjacent to the entrance to Blake Court, were vacated due to concerns about differential settlement and exposed waste in the bottom of the settlement feature in the back yard of at least one of the houses. The houses have been monitored for three years on a monthly basis for methane with an organic vapor analyzer. No methane level above 500 ppm by volume has been detected in the homes.
After reviewing available information on the location of remaining pockets of waste, landfill gas monitoring wells were concentrated around the Blake Court area. As part of the CIWMB’s effort to monitor landfill gas in the area of Blake Court, 66 landfill gas monitoring probes in 27 landfill gas monitoring wells have been installed. From June 1991 through August 1997, over 400 gas samples were taken. Over 30 organic compounds have been detected in soil gas at levels low enough to be quantifiable. All landfill gas monitoring wells and utility corridors in the Blake Court area are monitored for methane and volatile and semivolatile organic compounds quarterly. There have not been indications of gas migration away from localized pockets of waste. This may be due to the generally fine-grained nature of the fill used in building the residential development. A soil vapor extraction program has been proposed to reduce gas levels should waste at Blake Court be excavated. The soil vapor extraction program is currently under consideration for implementation.
Current subsurface landfill gas monitoring regulations and practices are heavily influenced by topographic and hydrogeologic features and the location of nearby structures. Geologic units that provide a pathway for gas migration due to the permeability, attitude, and continuity of the bed should be targeted for monitoring. Shallow groundwater will result in shallow landfill gas monitoring wells. Any permeable soil or rock unit in contact with the landfill that is not saturated with water is a possible conduit for gas migration and is a preferred target for landfill gas monitoring well selection consideration. Landfill gas monitoring well locations and depths are dependant on the location and depth of fractures, faults, or other soil or rock defects that are continuous and permeable to gas. The location of structures both on and adjacent to landfills should be taken into account when placing gas monitoring wells and probes. The well and probe locations should be the areas most likely to intercept landfill gas in the subsurface, in structure foundation elements, and utility corridors. Landfill gas monitoring frequencies are dependent upon the level of gas detected, the presence of receptors, and the need for monitoring the performance of a site’s landfill gas collection and control system.
The authors wish to thank Ms. Charlene Herbst and Ms. Brenda Niemeyer for reviewing the paper, providing suggestions, and preparing the manuscript.
Air Resources Board Compliance Division, July 1996, Landfill Gas Control Facilities
EBA Wastechnologies, April 1993, Proposed Article 5 Detection Monitoring Program and Financial Assurance Provisions, Page A-3.
Lawrence and Associates, August 27, 1996, Amended Gas-Mitigation Corrective Action Plan for Perimeter Gas Migration at the City of Ukiah Sanitary Landfill, Mendocino County, California 25 pages with attachments.
Lawrence and Associates, October 3, 1996, Construction/Testing of Five Gas Monitoring Wells, Ukiah Solid Waste Landfill, 2 pages with attachments.
Levine-Fricke, June 7, 1996 (revised September 11, 1996), Prior Investigations Report and Remedial Investigation Work Plan, North Canyon OU, Former Solano County Sanitary Landfill, Benicia California, page 6.
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