Better protecting groundwater from toxic waste

A research team has developed a better way to protect groundwater from acids, heavy metals and toxic chemicals - such as those found in mining, industrial and domestic waste - which would otherwise contaminate the water for decades, rendering it unusable and undrinkable.

Research leader Professor Derek Eamus, of the National Centre for Groundwater Research and Training (NCGRT) and University of Technology Sydney (UTS), noted that “mining produces millions of tonnes of waste known as tailings that are often stored above ground”, while “industrial and domestic waste are buried as landfill”.

When rainwater travels through waste, it leaches toxic chemicals from discarded electronic equipment, batteries, detergents, solvents and pesticides. The contaminated water then drains into the aquifer below, which may be used for drinking or watering crops. Once polluted, groundwater is expensive and difficult to clean up - which is especially problematic since it makes up 97% of the world’s fresh water supply, said Professor Eamus.

Professor Eamus said one way to minimise the contamination is to cover the waste with a layer of soil, trees and plants. Known as store-release covers, the soil soaks up rainwater, allowing the vegetation to use it and release it back into the atmosphere. This siphons off enough water to prevent it from reaching the waste.

However, said Professor Eamus, “To build a cover, we have to know what type of soil and plants to use, and how thick the soil layer should be.

“Also, every site has a different climate, vegetation and soil, so a lot of it is guesswork, followed by hundreds of experiments. It can take years and years to optimise the design of a store-release cover.”

So the researchers ran a soil-plant-atmosphere model with different climate scenarios to test its effectiveness in designing store-release covers. To find out which covers work best, they looked at four factors: the depth of the soil layer, how much water it can hold, how much water a plant will use and the local rainfall. They then applied the model to three different Australian climates: cool, wet winters with hot, dry summers in Perth; the monsoonal climate in Darwin; and evenly distributed rainfall across the year in Sydney.

“We found that an effective store-release cover has to have enough capacity to store any additional rain that falls in wetter years,” said Professor Eamus. “The trees have to grow leaves that cover the entire ground and their roots have to reach the bottom of the soil cover.

“We don’t want the lower half of the store-release cover to have no roots, because water will gather there and seep through the waste. Also, having more leaves that cover the ground means more water will be used and transpired by the plant.”

By reducing the guesswork involved in building store-release covers, the process will become cheaper and more efficient. Professor Eamus added that by combining this information with the rainfall average and extremes for any location, the design can be optimised for anywhere in the world.

The study has been published in the journal Hydrological Processes.