LIFE NARMENA - Grote Calie
Background and description of the area
The main source of the stream Grote Calie lies in Arendonk and it flows into the Aa around Tielen. The study area of LIFE NARMENA includes the Calie, its banks and floodplains between Zevendonk up to the nature reserve Winkelsbroek. Here, the Calie flows mainly through agricultural areas of ecological importance.
The reserve Winkelsbroek 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", that 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 area of the Grote Calie is also one of the selected zones of the spatial planning project of the Flemish government "Water-Land-Schap". It will address water-related climate adaptation at landscape level, in collaboration with the end users of the area, to make it more resilient for water related issues.
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 can exist as trivalent and hexavalent chromium. Hexavalent chromium is more soluble and therefore more easily taken up by plants and organisms than the trivalent one. 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 is also expected to be found in solid soil under the river, riparian soils and flooding areas.
Several exploratory studies and sampling campaigns have already taken place, not only to estimate chromium concentrations, but also to assess their bioavailability and potential toxicity.
Nutrient concentrations, mainly phosphorus, are monitored as well in the context of NARMENA. The concentrations measured so far can cause eutrophication and disturb certain soil and water flora and fauna. This is especially important 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.
Remediation and planning
In the research area of the Calie, LIFE NARMENA provides two nature-based remediation techniques: Bacteria-assisted phytoremediation (BAP) and a constructed wetland (CW). In addition to the pilot tests of both techniques, the project also includes the remediation of a natural peat pit in Winkelsbroek and the remediation and upgrading of a terrain enclosed in Winkelsbroek to extend the Alder Carr vegetation.
Bacteria-assisted phytostabilization (BAP)
In this technique, the selected plants absorb the contaminants and break them down to a harmless form and/or store it in the plant tissues. If necessary, plant-associated bacteria are added, equipped with appropriate degradation mechanisms to help the plants survive in an environment of high levels of toxic substances such as heavy metals. The plants are selected in such a way that they are not invasive in the environment where they are applied and do not have a negative impact on the native flora and fauna.
Phytostabilization is applied in LIFE NARMENA. Plants and their microorganisms stabilize 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.
A BAP will be carried out on agricultural land and in the nature reserve. Both cases have their specific requirements.
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The constructed wetland is an artificial kind of swamp through which a part of the Calie river is led. The water is purified according to the principle of "Horizontal Subsurface Flow" and by means of natural geochemical and biological processes.
The main building materials for a constructed wetland are an impermeable layer (e.g. heavy clay), a porous medium (gravel, stones) and native reeds to filter and fix the pollution. In addition, an inlet and outlet construction is required, in which there is usually a sieve and/or a sediment trap at the entrance.
The constructed wetland will occupy a small hectare and is ideally located just upstream of Winkelsbroek, in order to capture as much as metal pollution and nutrients as possible before the nature reserve.
The baseline monitoring of the sediment, soils of riverbanks and flooding area and surface- and groundwater are ongoing. A feasibility study on phytoremediation is also currently being carried out by the University of Hasselt. This includes a feasibility screening of phytoremediation in the context of the Calie, selection of the most appropriate phytoremediation mechanism (with a focus on phytostabilization of chromium), identification of the most appropriate plants and a thorough feasibility evaluation. The ideal locations for both remediation methods are also being determined. The remediation work is scheduled to start beginning of 2021. Spring as the start of planting has been strategically chosen for the growing season of the chosen plants.
- An area of 34.4 hectares with improved flood resistance (being the total of the three pilot areas).
- A volume of 26,700,000 m³ improved water quality (being the total of the three pilot areas).
- An increase of 165,000 m³ of water storage capacity (being the total of the three pilot areas).
- A decrease in bioavailability of 50 to 90% of tri and hexavalent chromium in soil and sediment.
- A decrease of 50 to 93% in chromium concentrations in surface water.
- Reduction in CO2 emissions.
- Increased protection and improvement of biodiversity.