Landfill Capping Solutions: Conventional or Evapotranspiration?

Landfill cells which have been filled to capacity with material such as rubbish from kerbside collections or contaminated soil require capping to contain the contents, which are potentially hazardous.

While capping neither eliminates nor destroys pollutants, it can prevent their spread by holding them in place, shielding people, wildlife and the environment from harmful substances.

Fyfe provides a range of environmental services including contamination assessment, remediation and management across Australia. With regard to landfill, we provide various services including development of closure plans and the monitoring and management of leachate, groundwater conditions and fugitive gas emissions.  The capping/encapsulation of contamination onsite can also be applied as a remediation and management measure on development sites for example.

In addition, we provide advice on landfill instrumentation and cap integrity, on landfill characterisation and on the capping of ‘legacy’ landfills. In these cases, there are several possible caps we can use, ranging from conventional options such as compacted clay or composite clay and geosynthetic caps to evapotranspiration (ET) caps.

Conventional caps

These effectively contain waste but may not effectively stop water from infiltrating to the landfill contents, which leads to the formation of large volumes of leachate. Treating this leachate, which is often highly contaminated and a risk to local groundwater resources, is expensive.

Landfills containing organic matter, such as food and other wastes found in ordinary municipal household waste, also generate considerable volumes of methane as the organic matter breaks down. While this methane is often captured and used to generate energy, one of the problems with conventional landfill caps is that they contain methane rather poorly, which is why conventional landfills are a significant source of fugitive methane emissions. There is also a range of challenges involved in constructing and maintaining conventional landfill caps. This is one reason why greener, alternative options are increasingly explored.

ET caps

Evapotranspiration takes a different approach. Instead of aiming for an impenetrable barrier, soil-based ET caps work by using a thick, uncompacted layer of soil to store water like a sponge until it is transpired by the carefully selected plants planted on the surface. While the soil layer is permeable to methane, the root zone of the plants also develops a microbial flora that breaks the methane down, thereby reducing or even eliminating emissions.

ET caps are less likely to be damaged by weather and seasonal changes, for instance, and once established can continue to perform over a wide range of seasonal conditions, ranging from deep frost to short-term drought. They are also cheaper to construct and, apart from extended drought, when the plants may need to be watered, need very little ongoing maintenance. However, like conventional caps, ET caps require ongoing monitoring to ensure the soil layer is working effectively and leachate levels are within acceptable standards.

The best cap depends on the site and the waste

There are pros and cons to each capping approach, and the right one for a site depends on many factors including rainfall in the area, local landfill regulations and environmental compliance, and the nature of the waste being managed.

Dr. Brent Davey, one of Fyfe’s Principal Environmental Consultants, notes that ET caps are a particularly attractive option where the visual amenity of the site is important, because they benefit from having a mixture of grasses, shrubs and trees on the surface, rather than just grass – conventional caps cannot tolerate plants with roots that penetrate the clay.

Brent’s recent webinar on the differences between ET and convention caps, and the pros and cons of each, is an excellent source of information.

Fyfe offers a full range of environmental solutions and services, including capping and ongoing management and monitoring of landfills. Our environmental experts and engineers design solutions that serve our clients’ goals while maintaining the integrity of the local environment.

For more information or to discuss your landfill or onsite contamination capping/encapsulation options, please contact us