What is Phytocapping?

What is Phytocapping?
Phytocapping is a capping technique that utilises vegetation as a natural pumping system to reduce water being stored in the soil and then gravity fed into the covered waste. This in turn reduces the amount of water reacting with the buried waste and being released as leachate.

The Legislation
Provision of a final landfill cover is a legislative requirement under relevant State and Territory Guidelines. Phytocapping is an alternative option to traditional clay capping methods for closed landfills towards effective leachate management and containment of environmental pollution.

How does it work?
Trees extract water through the soil that they grow in for their development. The key process that uses this water is photosynthesis. Excess water that is not used in this process is released into the atmosphere through transpiration through the leaf surface. Some water received via rainfall is also stored in the canopy and directly evaporated into the atmosphere without reaching the ground. Vegetation that has a high capacity for respiration through photosynthesis is highly suitable for phytocapping.

What are the benefits of Phytocapping?
There are economic and environmental benefits to phytocapping.  Financially, establishing and managing a vegetative cap cost less overall than delivery and management of traditional clay-based cap where available of suitable clay material is sparse. Cost savings shown as we move away from a traditional cap where spend per metre is approximately halved if planned ahead.

Environmentally, increased vegetation can assist with provision of habitat for all species and can often provide key linkages within the landscape. Phytocaps are most beneficial where the potential evapotranspiration rate exceeds the average rainfall.

Social benefits are also available if recreational aspects can be built into the redevelopment of the site. This is particularly applicable in urban areas, with land previously unusable for recreational benefits becomes newly available.

How do we monitor a phytocap to ensure its efficiency
Phytocaps  have been successfully constructed/established across a number of closed landfills in Australia. Ultimately, phytocapping reduces the cost of ongoing capping for the life of the closed landfills. Type 2 landfills require lysimeter monitoring to determine the effectiveness of the cap. There are 3 elements of the phytocap that will influence the long-term effectiveness of the capping system viz.climate, soil type and vegetation that are critical for water balance and management within the site.

Design Process
Pre-qualification tests and modelling needs to be undertaken to determine the suitability of a phytocap for the site. The design can factor in post-closure recreational aspects to promote additional use of the site, particularly those closer to developed areas. A pre-qualification test will require determination of the source of local soil, its physical and hydraulic properties and prediction of the hydrological balance using mathematical models such as HELP, HYDRUS, LEACHM etc.

Key Considerations of Design
Vegetation used should be selected with preference to faster growing species that have high stomata to leaf area ratio. Local climate factors are also critical for plant selection, given colder climate species will not be adapted to higher temperatures and vice versa. Being able to manage threats to the revegetation are an important consideration. Competition from weeds, watering through drought periods and limiting impacts from browsing need to be integrated into the overall design of the area.

Monitoring is an essential activity to collate data relating to the effectiveness of a phytocap. Establishment of a lysimeter will assist in monitoring the water balance of the site through an indicative measurement of net percolation. Data on height and stem diameter of established plants will assist in measuring effectiveness of selected species over others in consideration with climatic factors. This will influence species selection for other phytocaps within the local area, if required.

Maintenance costs will be higher in the first few years of a phytocap but will decrease over time as the natural system establishes and aspects of ecosystem function recover. It is important to monitor development of the trees and comparable weed growth in the initial stages as competition between the two could be a limiting factor in growth. Regular inspections will also assist in recognising other limiting factors such as drought stress, nutrient deficiency and impacts from browsing or illegal timber harvesting.

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