For facilities that pre-date Subtitle D, landfill gas (LFG) control requirements can present several menacing challenges. These older sites, which often have only in-situ or constructed liners rather than a relatively impermeable Subtitle D liner, can be prone to gas migration. To make matters worse, many older facilities also lack the luxury of large buffer zones between them and adjoining residential areas.
Similar concerns beleaguered officials in the City of Mesquite, Texas, which owns and operates a municipal solid waste (MSW) landfill approximately 10 miles southeast of downtown Dallas. Originally a sand and gravel quarry, the 40-acre tract began accepting refuse in 1969.
Today, the permitted refuse footprint occupies approximately 136 acres, 60 of which will be lined meeting Subtitle D requirements. The remaining 76-acre portion will likely reach final elevations by late 1995.
On-Site Monitoring Monitoring along the perimeter's gas probes revealed that explosive gases were present above compliance limits at the facility's property boundary. With homes as close as 150 feet from the permit limits, officials realized the potential risk to property and the urgent need for remediation.
In response, city officials used the facility's approved Landfill Gas Management Plan (LGMP) to implement a remediation program, which included on- and off-site monitoring, an active gas extraction system and below-grade slurry walls.
In December 1993, Team Consultants Inc. installed nine permanent gas monitoring probes near the site's perimeter. Probes were spaced 300 feet apart along a boundary shared by residential homes and 700 feet apart in remote areas. All permanent probes consist of full depth, slotted PVC casings, with a filter of fine gravel extending from the seasonal low groundwater level to approximately five feet below the surface.
In January 1994, Team completed the first quarterly sampling. Only one permanent probe, remote from residential structures, indicated methane gas concentrations above the allowable level. In addition, methane migration around that probe was confined to city-owned property.
Team's second quarterly monitoring, completed in April 1994, revealed methane levels above the lower explosive limit (LEL) in all perimeter probes. Landfill officials immediately notified state, city and county health agencies, fire and emergency response agencies and all adjoining homeowners.
Meanwhile, every home within 200 feet of the landfill boundary was surveyed for methane. In the lawns of the nearest homes, officials conducted additional sampling using bar hole probing techniques.
Despite negative preliminary results, these off-site points were added to the regular monitoring program. In addition, monitoring frequency at on- and off-site locations was increased from monthly to daily.
Active Gas Remediation The city's next step was to contract with Ecogas Inc. to begin Phase I, installing an active remedial gas extraction system to protect the homes adjacent to the landfill. In July 1994, 12 gas extraction wells were installed on maximum 100-foot centers approximately 100 feet inside the refuse footprint. The 12 wells were connected via well heads to an above-ground header pipe terminating at a portable skid-mounted candlestick flare station.
Although considered conservative, this design approach aimed to ensure the rapid control of migrating landfill gas. High density polyethylene (HDPE) pipe was used for the well casing, well screen and above-ground header; the well heads were constructed of schedule 80 PVC pipe and fittings. After weekly system monitoring, three additional extraction wells were added.
At the request of state regulators, Team also installed 11 soil-gas vapor probes along the residential street west of the site entrance. These probes consisted of 3/4-inch, porous, stainless steel tips driven into the ground to a depth of 15.5 feet.
Prior to installation, a small-diameter Teflon sampling tube was connected to each tip. Each probe's annulus space was backfilled with sand to approximately three feet below the surface and a bentonite grout seal was placed above the sand to the ground's surface. Finally, meter boxes flush with grade completed the probes' installation.
Sampling of the off-site probes indicated methane present above the LEL in only one location, while 10 locations were not affected or had minimal impacts. Subsequent daily monitoring found methane in the off-site probes to be erratic, sometimes at levels of significant concern.
These methane levels remained inconsistent until the off-site soil-gas vapor probes were replaced with full-depth permanent probes two months later. Following this modification, two of the 11 locations consistently showed elevated methane levels.
In hopes of identifying the methane source, officials sampled and tested these two locations extensively. Eventually, one location's detectable methane was depleted; the other has remained persistent.
Qualitative analyses of samples from the two off-site probes indicated different methane sources; neither of them resembled samples of LFG taken upstream of the temporary flare station. As a result, officials concluded that methane in the final off-site probe was unrelated to the landfill. Potential off-site sources included a commercial gas line and a sewer main that run adjacent to the off-site probes.
Slurry Wall Construction Although Phase I of the gas extraction system aimed to protect adjacent homes, the landfill's remaining perimeter still did not comply with Subtitle D requirements. Meanwhile, the city was faced with state Regulation V's requirements to control landfill surface emissions.
To solve this problem the city hired HDR Engineering Inc., Omaha, Neb., to design the remaining phases of the total gas extraction system. Phase IIA was designed as a looped, temporary system targeting the perimeter of the landfill's 76-acre older section. Included in this phase were 35 additional extraction wells.
Due to the urgency of possible off-site migration, the city requested that Phase IIA be installed prior to the final cover for the 76 acres. In response, the engineers designed a temporary above-ground header and condensate management system.
The final system also will include underground road crossings, with manual sumps located at low elevations in the header to collect and store condensate. The city selected J.W. Operating Co. to install this phase of the system.
Prior to placing the final cover, all well heads and sumps will be raised and the temporary header removed. The final system will consist of above-ground bell and spigot PVC pipe and an automated condensate management system.
Because this site's characteristics include deep alluvial deposits with cohesionless soils, Team recommended that a soil-bentonite slurry trench wall be used as a barrier to methane migration (which generally occurs through porous formations). The slurry wall was constructed along the perimeter adjacent to residential areas.
This portion of the slurry wall, which Geo Con Inc. installed, began approximately 100 feet west of the landfill's northeast corner and continued along the northern boundary toward the northwest corner. Next, the wall turned along the western site boundary and continued southward to a point approximately 400 feet south of the southern limit near the adjoining residential area.
By installing both the slurry wall and the landfill gas extraction system, Mesquite Landfill officials believe they've given concerned homeowners some peace of mind.