‘Purifying green in the city’: using phytoremediation to improve groundwater quality

As part of the "Purifying green in the city" project, a green infrastructure has been realized at the Polymer Science Park on Ceintuurbaan in Zwolle that contributes to continuous groundwater quality improvement.

The project is a collaboration between the municipality of Zwolle, Bioclear earth, Deltares, and Janssen Vastgoed and aims to develop knowledge and experience in applying phytoremediation systems in an urban environment. Two different systems have been installed on the site, consisting of a helophyte filter and purifying trees. These phytoremediation systems are part of a larger pilot at three locations in the province of Overijssel under the TKI Water Technology program.

A very special helophyte filter

The helophyte filter is specially designed to ensure that specific degradation conditions are present for the degradation of chlorinated hydrocarbons (VOCl and vinyl chloride). For this, we have chosen to let the targeted groundwater flow from bottom to top through the filter bed which makes the design of this filter one of a kind. In addition, organic rich layers have been incorporated into the filter to provide the specialized bacteria with sufficient nutrients for pollutant breakdown.

The helophyte filter is fed by two abstraction filters located in the immediate vicinity. The treated groundwater is eventually infiltrated back into the soil, directly contributing to the improvement of the quality of the immediate underground environment.

Additional treatment power thanks to purifying tree pits

Two purifying tree pits in which Black alders have been planted have also been installed at the site. These tree pits consist of concrete tubes inserted up to 3 metres deep. At the bottom, the tree pits are open and are in contact with the groundwater contamination. As a result, the tree will only be able to get its water from this depth and the groundwater contamination is extracted and subsequently degraded in the root zone of the tree.

As part of the pilot, both phytoremediation systems will be monitored for functionality and degradation of chlorinated hydrocarbons over the next few years. Monitoring wells have been installed to make this possible. In addition to degradation in the subsoil, attention is also given to potential volatilization via the leaves of the planted trees and plants.