Using trees to restore heavy metal values to within acceptable limits

This is a cost effective soft (i.e., non-destructive) remediation technique that is appropriate to low to moderate contamination scenarios only. It is unlikely if contaminant levels are moderate to high. Metal uptake for some tree species is low when compared to the potential amounts of metals present in contaminated soils. However, not all of the pollutant has to be removed, as the ‘bioavailable’ part is the most important for minimising transfer of pollutants to crops or groundwater. This will be more effective for metals with greater lability, such as cadmium and zinc.

Removal of metals by short rotation coppice (SRC)

We have also undertaken research examining the removal of metals by short rotation coppice (SRC) on a site which had been subjected to a heavy application of sewage sludge. Results indicated that the SRC was highly effective at removing zinc but less so for copper, nickel or lead.

Using trees for organic contaminant remediation

Some tree species are effective at remediating soils contaminated with certain organic chemical wastes. Although the range of wastes that can be remediated in this way is relatively small in comparison to the plethora of organic wastes, they include some solvents, petrochemicals, wood preservatives, explosives and pesticides. Organic contaminants can be completely destroyed by mineralisation to carbon dioxide and water. This is preferable to storing or immobilising them and is the goal of tree based remediation strategies.