Landfill Leachate by Marilyn Steiner


You may have heard the term ‘leachate’ in connection with landfills. It might help to understand what we mean by this term, and why we have serious concerns about where this substance is going in relation to the Mangrove Mountain Landfill at Central Mangrove.

The nature of leachate

Landfilled solid waste contains a variety of substances which when water passes through can be dissolved into the water. The solution percolates slowly to the base of the mound where it eventually leaches out. It is first black in colour and anoxic (depleted of oxygen, top picture), then brown or orange coloured (bottom picture) as it takes in oxygen and releases iron salts. It has a highly offensive smell.



The composition of the leachate can vary depending on the material in the landfill and the age of the landfill, but is primarily composed of decomposition products, particularly organic materials. It may contain methane, carbon dioxide, aldehydes, alcohols and sugars. It may also contain inorganic components (sulphates, chlorides, iron, aluminium, zinc and ammonia), heavy metals (lead, nickel, copper and mercury) and PCBs, dioxins etc. Despite stricter controls in developed countries, illegal activity or poorly regulated intake makes it impossible to predict with certainty what is in the landfill or will be in leachate.

Measuring the toxicity of leachate

Common features of raw leachate are high concentrations of ammoniacal nitrogen (NH3-N).  NH3-N is stable under anaerobic conditions and is a major long-term pollutant and the primary cause of acute toxicity. It is highly toxic to aquatic organisms at a concentration as low as 100mg/L.

Two tests commonly conducted to measure the relative oxygen-depletion effect of a waste contaminant are the COD (chemical oxygen demand) test, which measures organic pollutants that increase chemical oxygen demand, and the BOD (biochemical oxygen demand) test, which measures the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present.

The problem of leachate

Leachate not contained and treated poses a potentially serious threat to the surrounding soil and to groundwater. Many landfills, particularly older ones, have been built without appropriate engineering design, impervious liners or a leachate interception and collection system, including those in Australia. These landfills are a ticking time bomb as they are not being adequately monitored. Post-closure monitoring is required for decades, but who finances this and carries it out?

Leaving aside the deliberate or inadvertent discharge of leachate into creeks and water bodies, leachate entering groundwater may travel quite slowly and accumulate in drinking water and aquatic environments undetected for several years before problems become evident. Effects on the aquatic environment may be acute or chronic and can severely reduce biodiversity. Chronic effects  on people may never be adequately explained, because it may require long-term health studies which no-one will fund, as evidenced recently in the case of perfluorinated chemicals leaving government army bases and negatively affecting the health and livelihood of nearby residents. There is essentially no solution once these pollutants escape out into the general environment, so prevention and the precautionary principle need to be vigorously applied, which is not what is presently happening.

Amount of leachate generated

The amount of leachate generated can vary but will be greater during periods of high rainfall. Poorly managed landfills in which waste is not adequately compacted during filling and which are not capped with a clay lining allow more percolation of water through the waste. The resultant leachate may exit through the sides as well as at the base of the waste mound.

Various methodologies are employed to estimate the amount of leachate that might be generated. This is necessary to calculate the size of the leachate holding pond. Local factors such as mean annual rainfall will have an influence. As a rough guide, one estimate states that one tonne of landfilled solid waste generates about 0.2 m3 (200L) of leachate.

Mitigating the risk

In developed countries, governments regulate the operation of landfills to a greater or lesser extent. There are many landfills which fall short of modern standards, however. While most licensed landfills in NSW are owned by local government, about 30% (58) are privately owned. The local authority requires a Development Consent and an Environmental impact Study. The EPA then licences the facility. The NSW EPA has published Environnmental Guidelines for Solid Waste Landfills. A Landfill Environmental Management Plan and Leachate Management Plan are now part of the consent conditions. Oversight is then provided by the EPA and local Council with a regular reporting system.

A leachate collection and retaining system is now considered essential for modern landfills taking solid waste. All leachate must be collected by lining the base of the landfill, before filling, with multiple layers of impermeable material which allows gravity feed of the leachate to a lined holding pond. No leachate should enter the groundwater or soil below. The system is very complex and must be designed and approved by engineers. The collected leachate cannot be released without appropriate treatment at a licensed facility to detoxify it. This can prove very expensive. Some larger landfills in other countries find it more economic to treat the leachate on-site, but this is not fool-proof. Nor is the collection and holding system, which has a finite life and may be damaged if improperly maintained. Recirculation of leachate by re-injection back into the waste mound used to be common practice, but tended to increase the concentration of contaminants, and should only be performed where there is a suitable lining at the base of the waste.

Mangrove Mountain Landfill

So where does that place the Mangrove Mountain Landfill in Central Mangrove? The landfill started life in 1998 as an idea to import inert waste to create and pay for expansion of a 9-hole golf course into an 18-hole golf course. It was supposed to take 10 years. However, because waste is a money-making venture, the landfill began to take precedence over the golf course, and the 80,000 m3 of approved fill volume ballooned into 800,000 m3 by 2014 with the golf course no further towards completion.

Inert waste does not generate leachate of concern, so initially there was no requirement for a lining. None was ever placed under the waste mound. But the material being trucked in was poorly regulated, so was no longer inert. Leachate can be seen puddled around the base of the mound in early Google earth overhead views. Two so-called leachate ponds appear on a 2013 diagram, one labelled disused and the other since buried, but both were unlined. In 2009, lining suitable for leachate collection was used in a new dumping area on the south side of the main waste pile, and a 100,000L leachate holding pond installed to retain it.


This leachate collection area serves only 15% of the total waste pile. Given the 500,000 tonnes of estimated waste present in 2014, the amount of leachate (estimated at 200L/tonne) is 100 million litres, which is going into a 100,000L holding pond. This is stretching credibility, and would require frequent emptying so as not to overflow. Persistent MDA enquiries to the EPA uncovered the fact that there were no records of leachate being collected from this pond or of any being trucked to treatment facilities, as is required under landfill guidelines. The inescapable conclusion is that this toxic liquid has, for the past several years, been either entering the groundwater, or been pumped back onto the waste pile (the unlined one), or is being pumped into various rainwater retention ponds surrounding it, which have themselves been observed being pumped out into ponds created on top of the waste pile (another no-no), and then evaporated through misters set up around the summit. There is evidence of polypipes running between these ponds in various overhead photographs.  This is against all guidelines for safe disposal of leachate.

Following heavy rain or top of mound irrigation, leachate has been observed running from the sides of the waste mound, to be directed into at least two stormwater holding ponds. The MDA believes also that water from these ponds has been leaving the site and entering two creek systems, Stringybark Creek and Hallards Creek, which supply drinking water to 330,000 Central Coast residents. An extensive band of dead vegetation was discovered downhill of the landfill in the NW corner where pumping of liquid from the landfill had created a channel which entered a tributary of Hallards Creek and created a plume of discoloured water in Ourimbah Creek in July 2015. No charges have been laid against the Operator.

The NSW EPA and Gosford City Council, both with responsibilities for legislating and overseeing management of this dump, have allowed it to become a megadump in a major water catchment. The agencies have been falling over themselves disclaiming responsibility, each blaming the other for failing in their statutory responsibilities. The EPA, whose representatives stated to the MDA recently that its Environmental Guidelines for Solid Waste Landfills were ‘only guidelines’,  still claims it can mitigate the risk in issuing a new licence (now pending), and points to water sampling which they claim shows no negative effects. The fact is that the EPA engagement has been driven only by an outraged community openly expressing its concerns, and the water sampling has been infrequent and inadequate at best and does not take into account long term chronic effects on ecosystems and human health. Results of EPA sampling of ponds within the landfill have been withheld. The highly improper leachate disposal was going on under the nose of EPA officers for years, with not one fine issued.

This is why the MDA is calling for a Commission of Enquiry into the landfill and a moratorium on further activities. Major deviations from the original development consent should require lodgement of a new DA and Environmental Impact Study, and an independent analysis of its present management. Only then will this travesty of a development be halted.