10 February 2011, by Tom Marshall
Organic refuse from households and industry can help turn polluted soils back into rich and varied habitats. But a shift in public and government attitudes is needed before this can happen, scientists say.
A recent paper published in Elements describes how researchers managed to restore heavily contaminated sites by applying specially designed mixes of organic waste, which would otherwise often end up in landfill sites.
'Remediating land with waste has gained more interest recently, but the science is still in its early stages,' says Professor Davey Jones of Bangor University, who led the research. 'What's unusual here is that we managed to end up with high-biodiversity grassland, whereas in the past people have often been content to remove the most harmful pollution and end up with a site covered in something like football turf.'
Jones and his colleagues worked on two sites in northern Wales. The first was a hillside near Ffestiniog where the waste rock left over from slate mining had been dumped for generations, leaving a permanently barren eyesore.
The team added compost made from a mix of solid waste from sewage works, green garden waste and sludge from a paper recycling mill, adding sulphur to create the low nutrient, acidic conditions needed for a diverse grassland habitat. Eighteen months later, a varied mix of grasses and other plants had colonised a site where nothing had grown for 60 years.
The second was an old industrial site at Shotton, contaminated with a variety of heavy metals and organic chemicals. Here, the team devised a different blend of organic materials and mixed it into the soil, creating raised piles of compost to encourage microbial activity. The site is now covered in rich meadowland.
Treating a polluted legacy
The UK's industrial history has left an unwanted heritage of contaminated soils. Often the most harmful material has been dealt with, but what remains is still too badly polluted for real ecosystems to develop. Organic matter can help clean up these soils, or at least limit the damage the pollution causes, in a variety of ways.
Some pollutants are organic, such as hydrocarbons - a chemical group including oil and petrol. These are broken down into relatively harmless compounds by specialised soil bacteria and fungi, and adding organic waste speeds this up.
In other cases the contaminants are toxic metals, and organic matter can act like a sponge, attracting particles of metal and locking them away in a form that's no longer accessible to living things.
The waste also aerates the soil and boosts its ability to hold water, accelerating the natural breakdown of pollutants, as well as immobilising pollution in an insoluble form to stop it contaminating groundwater.
Each site needs a particular blend of waste materials, with different plants seeded on top. Jones says that earlier attempts often used organic material that was too rich in nutrients. This can lead to an uninteresting mix of species - many biodiverse ecosystems depend on poor soil to stop any one species taking over.
The technique has great potential, but the major hurdle is acceptance, both from the general public and from regulators and policy-makers. Many people dislike the idea of spreading sewage and other waste on the land, even if it's not dangerous.
The public sector isn't always much more enthusiastic. 'Some regulators seem to think this is just landfill on the cheap,' says Jones. In many areas using organic waste isn't legal, partly due to the lack of long-term studies of its effects. 'It's a shame, because if you removed the regulatory barriers, the slate quarries around north Wales would mop up all the organic waste we make for a very long time.'
He says the area needs more research, to answer questions ranging from how it works over the long term and which kinds of waste work best in each situation to how far it's worth transporting different types of organic material before the costs - both in cash and in greenhouse gas emissions and other unwanted side-effects - start to outweigh the benefit of cleaning up pollution.
The study was primarily funded by the European Union, with additional support from the Natural Environment Research Council.