Former Landfill Sites

A number of issues must be addressed in the redevelopment of former landfill sites. These relate primarily to the management of leachate and landfill gas but also to surface runoff from the site. They include:

Assessment
Assessment of leachate quality and quantity requires identification of leachate seeps and subsurface leachate flow paths, followed by sampling and analysis of the leachate for a range of indicator compounds, and measurement of flows - preferably under both wet and dry weather conditions.

An assessment of risks due to the presence of landfill gas involves consideration of the age and composition of the fill, identification of potential gas migration pathways and, if deemed necessary, measurement of gas composition and pressure - also under a range of meteorological conditions.

Control Options
A number of options for leachate management and treatment are available.

Reduction of the volume of water infiltrating to a landfill, generally by placement and maintenance of a low-permeability cover, can substantially reduce the volume of leachate produced. However, maintenance of such a cover may be expensive if settlement rates are still high, as differential settlement of the fill causes cracking of the cover, and results in the formation of depressions in which water may pond and subsequently infiltrate.

Design of a leachate collection system for a completed landfill is likely to involve design of appropriate surface or, more probably, subsurface perimeter drains.

Once collected, leachate must be stored, treated if necessary, and finally disposed of - on or off site. For low strength leachate, storage in adequately lined open ponds is normally possible. Depending on leachate strength and salinity, disposal of untreated leachate by irrigation on to revegetated areas of fill may be practicable, and will assist the rates of both vegetation re-establishment and fill stabilisation. If irrigation proves impracticable, alternatives include disposal to surface water courses following on-site aerobic / anaerobic treatment, and treatment / disposal off site, subject to acceptance by Sydney Water.

Simple constructed wetland systems have been found to be cost-effective solutions to leachate treatment at a number of landfill sites and should be carefully considered.

Control of residual landfill gas may involve passive gas barriers, active venting, and modifications to the design of structures. The measures adopted also need to be site-specific.