Australian scientists are suggesting erection of underground barriers to prevent contamination of aquifers by poisonous fluids leaching out of rubbish dumps.
Researchers in the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE), based in South Australia, are developing a solution to one of the most urgent problems faced by modern society worldwide.
Leaky tips are a worldwide problem as human populations swell, especially in newly industrialising countries where there are few controls over tip design or contents. For the sake of society's health, practical low cost solutions are urgently needed.
"The contamination depends on what was in the garbage placed in the tip. It usually contains old industrial and household chemicals, solvents and oils, antibiotics and endocrine disruptors, medical drugs, personal care products, biological pollutants and heavy metals," explains CRC CARE project leader Professor 'Vigi' Vigneswaran of the University of Technology Sydney.
The solution to this little-noticed assault on society's good health is the 'permeable reactive barrier,' an underground wall that filters and cleanses the toxic flow before it can enter the aquifers which are used by communities for drinking, domestic, stock or industrial water.
While modern tips are designed to prevent the flow of toxic leachate from reaching groundwater, older tips were not, and many are still quietly and insidiously feeding the pollution from decades ago back into the community of today.
According to Professor Ravi Naidu, Managing Director of CRC CARE, Vigi's barrier can be buried twenty metres or more into the ground and be up to two metres thick.
'It's is a sophisticated sandwich of materials which adsorb, break down or otherwise render harmless the contaminants in the leaching tip water,' Professor Naidu explains
Traditionally permeable reactive barriers have consisted of iron particles or slag which helped remediate the contaminated leachate. Prof. Vigi's design tailors the barrier precisely to the particular pollutants in the leachate and subjects them to a series of processes which ensure they are destroyed or rendered harmless.
"For example first we use biosorption to remove and neutralise biodegradable hydrocarbon compounds. This uses a material on which biofilm forms - and these do the job of breaking down the pollutants," Prof. Vigi explains.
The second layer deals with much tougher pollutants - organic compounds that either do not degrade or only do so very slowly. These are known as POPs (persistent organic pollutants). The second layer of the barrier subjects these to fierce oxygen free radicals which break down the poisons into harmless by-products such as water, CO2 and nitrogen.
The third layer is designed to deal with toxic heavy metals such as arsenic or the lead and mercury left by the millions of discarded batteries and other poisonous metals from old consumer electronics. These metals are scavenged out of the leachate by an adsorption layer consisting of activated carbon, wood waste or a special clay which either locks them up or oxidises them into less harmful compounds which can be removed by biosorption.
Depending on the type of contaminants in the leachate, these layers may be arranged in different sequences to obtain the best performance.
"Garbage tips can leak for decades, even centuries and contaminate water supplies kilometres downstream. As our cities expand, tips which once lay beyond the edge of a city are swallowed up by it, and their contaminants begin to travel under the soil where people live and children play," Prof. Vigi explains. "This is a particular issue in communities which use groundwater for drinking water or growing vegetables."
He says the laboratory work carried out into ways of dealing with the problem shows strong promise that these relatively low-cost barriers can be designed to keep toxic leachate out of groundwater in the longer term.