Grote Calie

The Winkelsbroek reserve encompasses approximately 24 hectares of important wetland complexes. It is part of the Natura 2000 protected Habitats Directive area, ‘Forest and heathland east of Antwerp’, which mainly consists of a beautiful mix of forests, heather landscapes and stream valleys. Many ponds were historically used for fishing or peat extraction. After years of nature management in Winkelsbroek, the area offers many opportunities for flora, fauna and recreation. 

The Grote Calie is historically contaminated by a former tannery located in Oud-Turnhout. Principally chromium (Cr) has been found in high concentrations in the soil and sediments of the stream. This metal exists as trivalent and hexavalent chromium. Hexavalent chromium is more soluble and therefore more easily taken up by plants and organisms than trivalent chromium. The dominant oxidation state of chromium depends on various environmental factors, such as the pH of the soil. 

Chromium contamination is primarily present in the sediments of the Calie, but it can also be expected in solid soil in the streambed, riparian soils and flooding areas. 

Several exploratory studies and sampling campaigns have already been taken place, not only to estimate chromium concentrations, but also to assess their bioavailability and potential toxicity. 

We also monitor nutrient concentrations, like phosphorus. The concentrations currently measured can cause eutrophication and disturb certain types of soil and water flora and fauna. This is especially significant for the target habitats and species in Winkelsbroek. The origin of the nutrients still has to be investigated. Nutrients can enter the area via leaching of upstream agricultural lands or via the groundwater through seepage from nearby agricultural activity.

At the Grote Calie site we apply phytostabilisation. This is a specific type of phytoremediation. Plants and their microorganisms stabilise and/or fix certain pollutants (metals) in the soil, rhizosphere or roots, instead of absorbing them. In the case of metal pollution, this will reduce the mobility of metals, reducing their bioavailability and consequently their effects on fauna and flora. If necessary, plant-associated bacteria are added, equipped with appropriate mechanisms to help the plants survive in an environment of high levels of toxic substances such as metals. The plants are selected to be non-invasive in the environment and do not have a negative impact on the native flora and fauna. 

BAP will be applied on a former agricultural land and in the nature reserve. Each case will have specific requirements. 

The constructed wetland is an artificial swamp or flood zone, in which the water is purified by means of natural geochemical and biological processes. 

The constructed wetland consists of a sediment trap in the watercourse and a flood zone on the left bank of the Grote Calie. The flow slows down as soon as it enters the sediment trap. At this point the sludge has time to settle. The contamination is mainly bound to sludge particles and will therefore remain in the sediment trap. Especially at high flow rates, for example in winter or after heavy rain, the flood zone will be addressed. The vegetation that grows in the wetland will help to retain the contaminated sludge in its root zone. 

The constructed wetland occupies one hectare and is located just upstream of Winkelsbroek, in order to capture as much as metal pollution and nutrient as possible before the stream enters the nature reserve. 

Planning

The baseline monitoring of sediment, surface and groundwater and the soils of riverbanks and flooding area has been completed. A feasibility study on phytoremediation has also been carried out by the University of Hasselt. This includes a feasibility screening of phytoremediation, selection of the most appropriate phytoremediation mechanism (with a focus on phytostabilisation of chromium), identification of the most appropriate plants and bacteria and a thorough feasibility evaluation with greenhouse pot experiments. 

Both pilot projects are included in the first phased soil remediation project of the Grote Calie. Planting of the BAP fields has been completed in two fases (2021 and 2023) and the inoculation of the root zone has been carried out in spring 2024.

We also installed the constructed wetland in the period 2022-2025. First, we carried out the excavation works to shape the flood zone and sediment trap. The wetland also has a elevated section, for maintenance works. The sediment trap has been reinforced to prevent leaching. Finally, we planted marsh vegetation. In the coming years, we will monitor the functioning of the wetland and the pollution in surface water.

The Alder Carr restoration is planned for the summer of 2025. In summer 2026 Natuurpunt will carry out some hydrological measures in certain ponds in Winkelsbroek. The peat pit remediation will be carried out in the After-LIFE, after the effectiveness of the wetland has been proven.