A strategy to tackle soil contamination

Soil pollution is a worldwide environmental problem. Traditionally, polluted soil is mechanically excavated and transported from the site. However, a great deal of information, like the type of contamination, its severity, location, size and depth, needs to be gathered before these heavy interventions can begin. Plus, offsite soil transport and soil cleaning processes are expensive operations.

Hence, a new practice has emerged: instead of excavating the ground, a new clean layer of soil is added on top of the contaminated area. The site, with one metre of extra soil, is then ready for construction. It will soon become a green paradise, which children and the elderly can safely use, a trendy urban core where active people can create new things and make our world better. Or will they? Is it possible to have a good and healthy life if the ground we are living on is not healthy?

A Healthy Place for Healthy People 
Let’s consider Nature as a human system. In the 5th century BC, the Greek physician Hippocrates, Father of Western Medicine, explained that every living system, evolving in its adequate environment, returns to a healthy state. What is alive has the faculty to repair itself if its natural living conditions are not disturbed. What is ‘soil’? What are the appropriate conditions for a healthy soil?

We still know very little about soil. Natural soil is a complex community of life where air, water, organic materials (humus) and mineral materials (rock particles) form a thick layer on top of earth. It is the place where numerous chemical reactions happen and materials are transformed thanks to plants and microorganisms.

Soil biodiversity is an important but poorly understood component of terrestrial ecosystems and can be easily disturbed by human activities.

There are other options to solve soil pollution besides mechanical interventions. In natural processes, microorganisms and plants play an initial role. As suggested by Hippocrates, a polluted soil, if it contains enough bioactivity, can remove pollutants itself if humans allow the natural processes to start. This is called ‘Natural Remediation’.

There are two types of pollutants: organic and heavy metals.
Biodegradation of organic compounds in polluted soil is a process involving interactions among soil particles, pollutants and microorganisms. Microorganisms include all creatures that cannot be seen with the naked eye (<0.1 mm in diameter) as well as algae, yeast, protozoa and many nematodes. They transform organic matter into nutrients for plants. The two main groups of microorganisms are bacteria and fungi. Both occur mainly in the upper layer of the ground and most bacterial species and all fungi need oxygen. Excavating the ground disturbs the biological activity while adding extra soil or any other material (asphalt, concrete) inhibits the oxygen permeability of soil and slows down the natural remediation processes.

Metals are non-degradable. This type of contamination can be remediated by other processes like introducing green plants. Some plants can bio-accumulate (phyto-extraction) as well as immobilise (phyto-immobilisation) metals in the immediate surroundings of their roots, the so-called rhizosphere.

Grass presents several advantages. It grows fast, has a very well-developed root system, which is dense and can develop up to one metre deep. Twice a year the grass is mown, to stimulate new growth: young plants absorb more pollutants.
Grass Types: Epilobium angustifolium (fireweed), Typha latifolia (bulrush), Digitalis purpurea (foxglove), Achillea millefolium (yarrow). Lolium perenne (ryegrass), Capillaris Agrostis (bentgrass), Festuca arundinacea (tall fescue).

They collect metals in their leaves or branches. These pioneer plants, grass types and trees, are ones that can grow without any problem on contaminated or nutrient-poor soil. Plants additionally stimulate the bio-activity of the microorganisms in the soil (and thus the decomposition of organic substances). 

In this sense, plants are the best means for remediation of organic and non-organic pollutants.

Trees have a root system that is both horizontal and vertical: deep contamination can be reached by the tree roots. Because trees absorb more fluid than other plants, soil contamination spreads less easily.
Trees: Salix nigra (black willow), Populus deltoids (American black poplar), Robinia (acacia) 

Except for extreme cases, both types of contamination, organic and non-organic, are not dangerous for humans and animals. Foundation piles must not touch the contamination to prevent the contaminated soil layers from spreading to the environment. In case of heavy metal contamination, produce from plants growing on the site is not fit for consumption. Also, cut grass may not be fed to animals.

Recent studies in Montpellier, France, have shown that heavy metals collected in the plants can be re-used. Claude Grison, a biochemist, has developed a system which recovers metals from the leaves of small plants. These metal particles are then re-used in the laboratory as catalysts.

They speed up chemical reactions in the making of cosmetics and medicines. One of Grison’s most recent projects is to apply phytoremediation on an old coal extraction site in Gabon, collect the heavy metals from the plants and produce drugs on site. This process of metal re-collection is further being developed for market applications.

In Montreal, Canada, scientists are working on how to re-collect metals accumulated in the branches of trees. The wood can serve as firewood, without presenting any health risks. The flying ash, which contains metal particles, is then filtered and collected for new use.

Natural remediation of soil contamination does not pretend to make the ground completely clean, but it does make it cleaner. Depending on soil hydro-geochemical properties, metal form and land use, the immobilisation efficacy induced by natural remediation may be enduring. Noticeable results are seen within a few years.

In extreme cases, natural attenuation processes alone may not be sufficient. However, accelerating these processes with human interference (assisted natural remediation) might be a viable option.

Application to soils of certain amendments that enhance key biogeochemical processes in soils have already been demonstrated in Europe and North America. Case studies have demonstrated enhanced natural remediation resulting in substantially improved vegetation growth, invigorated microbial population and diversity and reduced offsite metal transport. And, contaminated plants may soon have an economic value. Enough reasons to go green.

Hélène Leriche is a French architect and urban designer based in the Netherlands. She has been working for several years on major international urban design projects including those in India. Her interest lies in the integration of (landscape) architecture and urbanism with respect to the natural environment.

Hans van Veen is Senor Scientist in the field of microbial ecology of terrestrial ecosystems at NIOO-KNAW, the Dutch Institute of Ecology in Wageningen and former professor of soil microbial ecology at Leiden University. He has received several awards, including the the UNESCO Carlos J Finlay Award, for his contribution in microbiology.