The NSW Environment Protection Authority (EPA) Environmental Guidelines: Solid Waste Landfills state that “the site capping (of a landfill) should ensure that the final surface provides a barrier to the migration of water into the waste, controls emissions to water and atmosphere, promotes sound land management and conservation, and prevents hazards and protects amenity”. The Environmental Guideline identifies benchmark techniques to achieve these goals. The EPA recommends that the final capping should have five parts including a seal bearing layer, a gas drainage layer, a sealing layer, an infiltration drainage layer and a revegetation layer. This traditional approach to capping of the landfill aims to seal the surface so that rainfall cannot infiltrate the waste and landfill gas cannot escape to the atmosphere. This is a highly engineered and costly means of achieving the desired environmental goals. Experience in the Hunter Region is that final covers created with a compacted clay sealing layer as recommended by the NSW EPA do not always achieve these required environmental goals. Problems relating to conventional covers include the availability of suitable materials, particularly for the low permeability sealing layer, the potential for cracking of the sealing layer particularly during extended dry conditions, the response to differential settlement within the waste and problems with revegetation. Thus opportunities exist to explore alternative options for the final cover of landfills.
One type of alternative cover that is of increasing popularity in the United States is the evapotranspiration (ET) cover. In the United States, ET covers are in place or on trial at a variety of different types of landfill including those used for hazardous and municipal waste. Evidence suggests that these can perform well in a variety of climatic conditions.
The principle behind ET covers is the effective management of water balance in the cover medium. Precipitation is balanced by evapotranspiration, water storage in the cover medium and influx to the underlying waste. In areas where the potential for evapotranspiration approaches or exceeds rainfall, the influx of water through the ET cover can be very low or neutral and can effectively control migration of water into the waste.
This paper considers the potential for the use of ET covers in the Hunter Region.
There are two key considerations:
- Climatic factors – is evaporation greater than rainfall? Are there any critical climatic factors such as prolonged periods of high rainfall and low evaporation? Sample water balances are presented to demonstrate the applicability of ET covers in the Hunter Region.
- Cover media factors – the water holding capacity of the cover medium, availability of materials and ability of the media to support vegetation. A variety of materials available in the Hunter, including naturally occurring soils and waste materials from power generation, coal mining and the waste management industries are assessed for their potential for use in ET covers.