When a municipal solid waste (MSW) landfill is declared “out of commission,” it means the site has reached its final permitted capacity and stopped accepting new waste. This marks a formal transition from the operational phase to a highly regulated, multi-phase closure process. Closure is not abandonment but the beginning of a decades-long commitment to environmental protection and site management designed to isolate the waste from the surrounding environment.
Preparing for Closure: Regulatory Requirements
The transition to closure is governed by environmental laws, primarily the minimum standards set by the Resource Conservation and Recovery Act (RCRA) Subtitle D. Owners must notify regulatory agencies and the public of the intent to close the facility well in advance of the final waste delivery date. This initiates the formal process and triggers regulatory oversight.
A crucial requirement is financial assurance, mandating that the owner set aside funds to cover all closure and long-term care costs. Mechanisms like trust funds or surety bonds ensure money is available for decades of post-closure maintenance, even if the operating company faces financial distress. This demonstration is based on a detailed closure plan outlining necessary construction, monitoring, and maintenance.
Sealing the Site: The Final Cap System
The most significant physical component of closure is installing the final cover system, or cap. The cap is a multi-layered barrier engineered to minimize water infiltration and control the escape of landfill gas.
Cap Preparation
Before placement, the waste mass must be graded to create a stable foundation. Slopes are designed to promote surface water runoff and minimize erosion. A smooth surface prevents water from pooling, which would otherwise increase the generation of contaminated liquid inside the landfill.
Cap Layers
The barrier layer is typically a low-permeability material, such as two feet of compacted clay or a synthetic geomembrane (HDPE). This layer prevents rainwater from percolating through the waste, which would create leachate. Above this, a drainage layer (geocomposite or gravel) collects and channels any penetrating water, preventing hydraulic pressure buildup. The uppermost component is a protective soil layer, usually at least two feet thick, which supports a vegetative cover like grass to minimize erosion.
Long-Term Management of Environmental Outputs
Once the cap is complete, two major environmental outputs must be managed for decades: leachate and landfill gas (LFG).
Leachate Management
Leachate, the liquid formed when water passes through the waste and picks up contaminants, is collected by a system of pipes and sumps at the base of the landfill. This liquid is continuously pumped out and transported to a specialized wastewater treatment facility, preventing groundwater contamination.
Landfill Gas (LFG) Control
The decomposition of organic matter continually generates LFG, a mixture mainly of methane and carbon dioxide. Methane is a potent greenhouse gas and a potential explosion hazard if it migrates into surrounding soil or structures. An active LFG collection system, involving vertical wells and horizontal pipes, captures the gas before it escapes. The collected gas is either safely flared (burned off) or processed and converted into a renewable energy source like electricity or natural gas.
Post-Closure Monitoring and Land Conversion
Post-Closure Care Period
Following physical closure, the site enters the post-closure care period, mandated by federal regulations to last a minimum of 30 years. This involves continuous monitoring of environmental control systems and the cap’s integrity. Technicians inspect the cap for erosion, subsidence, or cracks, performing necessary repairs to maintain its barrier function.
Groundwater is regularly sampled from monitoring wells to check for contamination. The gas and leachate management systems are also maintained and monitored to ensure effective operation and safe gas pressures. Regulators can extend the 30-year period until the waste mass has chemically stabilized and outputs have significantly diminished.
Land Use Restrictions
Because of the unstable nature of the buried waste and ongoing presence of LFG, the future use of a closed landfill is subject to strict land-use restrictions. Deep excavation or the construction of heavy foundations is prohibited to avoid disturbing the cap and creating pathways for gas migration. Suitable reuses require minimal structural development, such as public parks, golf courses, or solar energy farms.