Town arrests gas migration

Years after Mansfield, Mass., closed and capped its East Street landfill, the town discovered that it had a problem: Gas was migrating from the 15-acre site, and it exceeded allowable regulatory levels. The problem was corrected last winter, when Mansfield constructed a perimeter barrier and new venting system. By handling the work in-house, the town not only solved its environmental problems, but did so for nearly half the cost of using a contractor.

Landfill gas — mostly methane and carbon dioxide — is odorous and potentially explosive. As landfill material decomposes, the gas that is generated tends to move along the path of least resistance, through the waste and into the air or surrounding soils.

When the East Street landfill was capped in 1987, a passive gas venting system was installed. Nevertheless, some gas migrated beneath the site’s recycling area and paved parking lot, searching for an exit.

The Massachusetts Department of Environmental Protection directed Mansfield to reduce the landfill’s threat to nearby areas. The town assessed the problem, and, with the help of Cambridge, Mass.-based Camp Dresser & McKee, determined that only the site’s north side was affected. It would be able to contain the gas with a perimeter barrier and release it with a new venting system.

Although planning the solution and receiving state approval for it took almost a year, construction of the project took one week. It involved excavating a 15- to 20-foot-deep, 500-foot-long trench, and installing a 40-mil HDPE barrier along the outer trench wall. Crushed stone was placed in the trench to enhance movement of the gas, and PVC pipes were installed for venting. The barrier ensures that the migrating gas is contained within the property line, and the vented trench allows the gas to escape into the atmosphere.

The town estimated that the cost of hiring a third party to design and construct the project would reach $150,000. Therefore, it served as its own contractor, using subcontractors and public resources, to reduce costs and speed installation. “On projects of this nature, we have found that the town has the construction and management expertise to cost-effectively perform the task at hand,” says Lee Azinheira, Mansfield’s director of public works. “We can save between 30 and 50 percent of the contractor’s bid price.” The East Street landfill project cost $79,000 to complete.