LIFE NARMENA - Grote Laak
Background and description of the area
The Grote Laak is a non-navigable waterway which rises in Leopoldsburg. It has a total trajectory of 29.7 km, flowing through the municipalities of Ham, Tessenderlo and Laakdal to mouth into the Grote Nete. This confluence area coincides with the nature reserves Zammelsbroek and Trichtelbroek, part of the valley of the Grote Nete. This valley includes areas within the Natura2000 Habitats Directive "Bovenloop van de Grote Nete met Zammels Broek, Langdonken en Goor", in which conservation and restoration of the valley is the principal objective. The Flemish flood risk strategy is also applicable here to increase the natural dynamics in the Grote Nete. Although there are still many meanders that play an important role as environmental elements, a large part of them have been straightened out in the past, of which the relics are still visible in the landscape.
The undulating terrain consists of an alternation of agricultural hay meadow, area delimited with hedges, tree-lines or canals, forest and heather with in the depressions mere and marsh. The brook valleys have a high ecological value, which includes moist Franguletea on nutrient-rich soil, nitrophilic alluvial alder forest, reed vegetation and marsh marigold fields. This area is also very attractive to a wide range of bird species due to the diversity of vegetation types.
The high water levels and regularly recurring floods of the Laak make flood areas inaccessible. Due to the diversity of flora and fauna, this area is of great ecological importance and the quality of the surface water running through it must be guaranteed.
© Vic Van Dyck, Natuurpunt
The poor surface water quality is reflected in sediment contamination. This is mainly the result of discharges. Until the 1990s, the Grote Laak was heavily charged with industrial discharges of heavy metals and chlorides. Since then, both the discharge flow rate and the load from heavy metals and chlorides have been greatly reduced. Industrial point discharges are the major cause of the pollution, although diffuse pollution, mainly from domestic wastewater, also plays a role.
The deposition of contaminated sludge on the riverbanks during remediation works is the main cause of contamination of adjacent soils. In addition, the Grote Laak is very sensitive to precipitation. In case of excessive precipitation, the stream overruns its banks and flows into the floodplains adjacent to the watercourse. In this scenario, contaminated sediment can also end up on nearby soils. In principle, mainly the Grote Laak, is polluted from the point of discharge to the river mouth. At a high flow rate and mainly in the downstream part of the river, contaminated water could potentially be pushed into its side-rivers, leading to local pollution there.
LIFE NARMENA focuses on the pollutants cadmium, radium, arsenic and mercury, in the area of the river mouth of the Grote Laak.
Remediation and planning
Similar to the Winterbeek, it will be useful to compare the new nature-based remediation with the conventional techniques. It is of importance that the upstream areas are first remediated to prevent that more contaminated surface water or sediment flows into the last sub-area. This area includes the zone where the Grote Laak flows into the Grote Nete and is subjected to the creation of a wetland. The proximity to nature reserves plays a crucial role in the choice of this type of remediation.
The remediation works for all sub-areas are planned for 2020-2025. The descriptive soil survey (BBO) has been finalized and the remediation concept, together with the subdivision of the remediation works along the watercourse, has recently been elaborated.
For the demarcation of the pollution on the riverbanks and flood areas, a special technique is used: dose rate measurements. This innovative method could be applied because the discharge water in the past mainly contained cadmium and arsenic, but also to a lesser extent radium. The presence of radium slightly increases the radiation compared to the background radiation.
The hydrodynamic modelling is currently ongoing. Different scenarios are being modelled in a groundwater model in interaction with a surface water model. Several dry and wet years, and even climate change scenarios will be taken into account.
Free surface water flow constructed wetland
Just like in the Winterbeek, we plan a free surface water flow wetland in this confluence area. The valley area and the biotopes that can be found here are then comparable and this is therefore closely in line with the nature conservation objectives pursued in this area.
The highest concentrations of metals are almost exclusively situated in the riverbanks on both sides of the Grote Laak. The deposition of clearance sludge on the banks in the past is seen as the main cause. In principle, this was done on a bank strip of 5m, but due to land cultivation, this strip can be spread over a larger width. It is important to select an pilot area where the contamination is far enough present in the flood area, in order to demonstrate the theory behind the wetland as a remediation method.
Depending on the current groundwater level, measures will be taken to retain the water into the flood area. This will change the redox state of the subsoil. Reduction processes will produce sulphides that bind to the metals. This greatly reduces mobility and bioavailability, leading to immobilization of the contamination and reduced uptake by organisms.
- 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 cadmium and arsenic in soil and sediment.
- A decrease of 50 to 93% in cadmium concentration in surface water.
- Reduction in CO2 emissions.
- Increased protection and improvement of biodiversity.