What is leachate?

Leachate is the liquid that drains or 'leaches' from a landfill. Leachate can be generated from the precipitation of water into landfill sites via rainfall or other surface water sources and from underground aquifer activity. The Darebin Parklands landfill has both sources of leachate generation with the majority of water coming into the landfill site via underground aquifer activity and the associated Alphington water table.

The best analogy to describe the leachate situation at Darebin Parklands is to imagine the old quarry hole at the parklands is a bathtub. Instead of putting your rubbish in bins, throw it all into the bathtub until ¾ full. Don't hold back, everything: chemicals, paints, organics, inorganics, you name it.

Now turn on a tap to a trickle. The water mixes with the rubbish to make a cocktail of polluted water (leachate), which over time will fill up the bath and eventually spill out onto the floor.

Except the floor in our case is the Darebin Creek.

So what does DCMC do about it?

First and foremost we contain it and prevent any seepage getting into the Darebin Creek.

Secondly we treat it via aeration and bio-filtration using reed beds and bacteria.

Then we move it through a series of wetlands where it is polished with macrophytes (aquatic plants) through luxury uptake (nutrient removal).

So what happens from there?

Evaporation from wetland areas and sub surface irrigation outside of the landfill site is the main method of disposal.

How much leachate is there?

That depends on water table activity and rainfall, which of course are interlinked, but a rough average is around 350,000 litres a week.

What's the leachate composed of?

The Darebin Parklands leachate is maturing and the main constituent is Total Dissolved Solids (TDS). The most common chemical constituents of TDS are phosphates, nitrates, soduim, calcium, chloride and potassium.

We treat TDS as salts meaning the leachate is brackish in character.

There are other contaminants within our leachate such an heavy metals and tanins and dyes but mostly at lower levels.

How long will it take for the leachate to stop generating?

That is the interesting question. We have had expert consultants trying to answer this point. It has been generating for over 30 years so far and it is predicted that it will mature and become more brackish over time, but in short there is no real definitive time scale as it depends on the breakdown of polluntants in the landfill site.

To put a figure on it though we have been told it will continue for at least the next 50 years.

Is there anyting that DCMC can do to stop the leachate generating?

In short no.

If we intercepted and diverted the water table and recapped the old tip site we could reduce the leachate generation but this is a hugely cost prohibitive exercise and impractical as it would mean the purchase removal of most homes along the western boundary of the tip site.

What are the main issues with the Darebin Parklands leachate?

During the warmer months algal blooms and the threat of botulism in the treatment wetlands are the main issues. Cyan bacteria (blue green algae) and botulism are the major threats to waterfowl. Many of the worst looking algal blooms in the wetlands are in fact Spirulina that can be found on sale in health food shops.

All algal blooms represent an excess of nutrients within the leachate and can prove difficult to manage.

Botulism on the other hand occurs when oxygen is depleted within a wetland and can cause major public health concerns.

The DCMC is currently installing aerators into the wetlands that have had botulism outbreaks in the past in an effort to remove this threat from Darebin Parklands.

What has DCMC got planned for the future regarding leachate management?

DCMC is always looking to improve the water quality of the leachate and is always investigating treatment solutions to problems that occur. Being proactive to the overall treatment quality and improvement is the main focus.

But in saying that, reactive management sometimes is required due to the dynamic nature of the leachate itself.

Currently we are investigating improved bio-filtration methods and aeration techniques.

We are proud of our record of protecting the Darebin Creek from the leachate and that the system is aesthetically integrated into the natural environment.

The ongoing management of the leachate at Darebin Parklands is a main priority of DCMC and will continue to do so in the future.

© Darebin Creek Management Committee 2008
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