A landfill is considered closed when it has reached its permitted capacity or final elevation grade. This triggers a formal, highly engineered, and strictly regulated process known as closure and post-closure care. The procedure is mandated by environmental laws, such as the Resource Conservation and Recovery Act (RCRA) in the United States, to protect public health and the environment from the hazardous byproducts of decomposing waste. This transition involves sealing the waste mass, managing continuous emissions, and committing to decades of environmental monitoring.
Engineering the Final Cover System
The closure process begins with the construction of a final cover system, often called a cap, which acts as a protective shield over the waste mass. This engineered barrier drastically reduces the amount of precipitation that can infiltrate the buried waste. Reducing water entry minimizes the formation of contaminated liquid known as leachate.
The cap is a multi-layered structure designed to achieve low permeability and erosion control. A typical design includes a barrier layer, often a thick sheet of high-density polyethylene (HDPE) geomembrane or compacted clay. This low-permeability layer is the primary defense against water infiltration, ensuring the waste remains as dry as possible.
Above the barrier layer, a drainage layer (usually sand or a geosynthetic net) is installed to quickly channel any water that penetrates the upper layers off the surface. The outermost layer is a vegetative layer, consisting of topsoil capable of supporting shallow-rooted grasses. This soil layer prevents erosion of the underlying components and uses evapotranspiration to remove moisture before it reaches the barrier layer.
The entire surface is carefully graded, often with a minimum slope of three to five percent. This grading promotes rapid surface water runoff and prevents ponding that could compromise the cap’s integrity.
Managing Leachate and Landfill Gas Emissions
Even after closure, the decomposing waste continues to generate two primary byproducts: leachate and landfill gas (LFG). Leachate is a highly contaminated liquid formed as water percolates through the waste, picking up dissolved and suspended materials. This liquid is collected by a network of perforated pipes and sumps installed at the base of the landfill.
The collection system uses gravity to channel the leachate to a central point where submersible pumps remove it. This ensures the liquid level resting on the liner system remains below a specified limit, typically one foot. The collected leachate is then transported for treatment, often sent to a public wastewater treatment plant. Alternatively, it is treated on-site using advanced systems like Membrane Bioreactors (MBR) or Reverse Osmosis (RO) to remove complex contaminants such as ammonia, heavy metals, and persistent chemicals.
Landfill gas is produced by anaerobic decomposition and consists of approximately 50 percent methane and 50 percent carbon dioxide, with trace amounts of other volatile organic compounds. Methane is a potent greenhouse gas and an explosion hazard, necessitating its constant collection. An active gas collection system uses a network of vertical wells and horizontal trenches drilled into the waste mass, connected by a header piping system.
Blowers create a vacuum to draw the gas out of the landfill. The collected gas is either destroyed or put to beneficial use. Flaring is the most common method, safely combusting the methane and converting it to less potent carbon dioxide and water. Alternatively, the gas can be cleaned and processed to generate electricity, used for thermal energy, or upgraded to pipeline-quality Renewable Natural Gas.
Regulatory Requirements for Post-Closure Care
Once the final cover is in place and the collection systems are operational, the landfill enters a lengthy, mandated phase of post-closure care. Federal regulations require a minimum post-closure care period of 30 years, though state environmental agencies have the authority to extend or shorten this duration based on the site’s performance data. This period is dedicated to maintaining the integrity of all containment and monitoring systems.
Extensive environmental monitoring, particularly of the groundwater, is a main activity. A network of strategically placed monitoring wells compares the water quality downgradient of the landfill against unaffected background water. Samples are collected following strict protocols, including purging the wells to remove stagnant water, and are analyzed for indicator parameters to detect any potential release from the waste mass.
Operators must also continuously monitor the stability of the final cover system, performing periodic inspections and repairs to correct issues like settling, erosion, or animal intrusion. Furthermore, the effectiveness of the leachate and gas management systems must be tracked, with regular adjustments made to gas extraction well settings and ongoing treatment of collected liquids.
To ensure these costly, long-term obligations are met, the facility must demonstrate financial assurance, even if the operating company becomes insolvent. This involves setting aside funds (e.g., trust funds, corporate guarantees, or surety bonds) sufficient to cover the estimated costs of closure and 30 years of post-closure care. Upon completion of the closure construction, a final certification must be submitted. This certification is typically signed and sealed by an independent, registered professional engineer, confirming that the work was completed according to the approved plan.
Reclaiming the Land for New Uses
The land on a closed landfill site has severely limited development potential due to the presence of buried waste and the necessary protection of the final cover system. The continuous settling of the waste mass and the need for gas venting infrastructure prevent the construction of heavy buildings. Any new use must not compromise the integrity of the engineered cap or interfere with the monitoring equipment.
Acceptable, compatible uses often transform the former liability into a community asset or an energy generation site. Common examples include public parks, golf courses, and wildlife habitats, which require minimal ground disturbance. Increasingly, closed landfills are being converted into “brightfields,” which are large-scale solar farms. The open, unshaded land is perfectly suited for photovoltaic panel arrays.