Landfills are typically closed by sealing them with impermeable layers of clay or plastic geomembranes, a process designed to contain waste and prevent the escape of gases and leachate. However, this traditional ‘cap’ remains barren for decades and requires ongoing maintenance.
The new research suggests that an alternative approach — applying a thick layer of soil and planting native vegetation — can achieve comparable containment performance. The study compares gas flux, water infiltration and structural stability of green caps against conventional caps over a multi-year monitoring period.
“We found that a well-designed vegetated cap can control methane emissions and rainwater infiltration just as effectively as a traditional engineered cap,” Sorbello stated. “In addition, the site becomes a green space that can support local wildlife, absorb carbon dioxide, and be used for recreation or community gardens.”
The economic case is also promising. Green caps can be cheaper to install and maintain, especially where local soil and native plants are used. They avoid the high material and transportation costs of synthetic liners, and they reduce long-term liability by turning a liability into an asset.
However, the approach is not without its challenges. Green caps require careful design for site-specific conditions — particularly in arid climates where water availability is limited, or where the waste is highly reactive. Methane oxidation in the soil layer must be actively managed, and deep-rooted plants must be selected to avoid compromising the cap integrity.
Environmental groups have welcomed the research. “Too many closed landfills remain ugly, fenced-off scars on the landscape,” said a spokesperson for the Green Infrastructure Alliance. “This study provides the evidence we need to start turning brownfields into community assets.”
Landfill operators are more cautious. “Green caps are promising, but we need to see long-term data across different waste types and climates before we abandon proven methods,” said a representative of the Solid Waste Association.
The research team plans to expand trials to landfills in different geographic regions, and to develop design guidelines for engineers and local councils.