Pilot study on controlling contaminated groundwater with “Purifying trees” in Almelo

Three special trees have recently been planted in a green strip on the industrial site on the Turfkade in Almelo. The green strip was a waste dump in the 1970s. Unfortunately, a contaminated area of groundwater has developed underneath this site.

The plume is contaminated with volatile chlorinated hydrocarbons that have arisen not only from the landfilled waste, but also from a business park further upstream.

This location has been selected for a special pilot in order to study the effectiveness of the purifying trees and, in particular, their contributions to groundwater plume control and remediation. In this case, it was decided to focus the remediation on dichloroethenes and vinyl chloride contamination at a depth of 4.5-5.5 m below ground level.

How does the purifying tree technology work?

With The purifying tree technology, we can reach contaminated groundwater levels that are normally inaccessible to the root zone. For this purpose, the tree is placed in a special container with a kind of straw inserted in the soil. The groundwater is drawn up into the root zone by the vegetation, creating a hydraulic connection between the contaminated groundwater and the root zone. The container is filled with a special soil medium that creates good growing conditions for the roots and for the growth of micro-organisms capable of breaking down the contamination. When the tree starts to grow, the roots release nutrients that also boost the micro-organisms in the root zone (the rhizosphere). The soil and rhizosphere act as a kind of bioreactor in which microbiological degradation can take place.

An advantage of the chosen pilot location is that bacteria (Dehalococcoides spp.) capable of breaking down VOCl are already present. However, due to a shortage of nutrients (carbon source) for this group of micro-organisms, there has been little decomposition so far. The soil in which the trees are planted contains a high level of carbon source in the form of organic matter. The nutrients in the groundwater, combined with the organic matter in the root zone of the purifying trees, are sufficient to start the breakdown of VOCl. The design takes into account the possibility of adding extra nutrients and/or carbon source at a later stage, should this be necessary.

A green solution for groundwater contamination?

The aim of this project is to investigate whether this technique is suitable for managing and breaking down groundwater pollution in a sustainable and cost-effective way.

In this pilot project, the tree species European White Elm, Common Oak and Poplar were chosen to investigate whether other species than the commonly used Poplar are just as efficient. Furthermore, this pilot project will demonstrate whether the residence time of the contamination is sufficient for the complete breakdown of VOCl in the breakdown zone around the purifying trees.

If the pilot is successful, this type of sustainable phytoremediation solutions will be used in other locations in Almelo. In any case, the pilot trees will remain in place, even when they have completed their cleaning activities!