Grote Laak

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 join the Grote Nete. This confluence area coincides with the nature reserves Zammelsbroek and Trichtelbroek, part of the valley of the Grote Nete. Parts of this valley belong to the Natura 2000 Habitats Directive area, ‘Upper reaches of the Grote Nete with Zammels Broek, Langdonken and Goor’, in which conservation and restoration of the valley are the principal objectives. The Flemish flood risk strategy is also applicable here, to increase the natural dynamics of the Grote Nete. Although there are still many meanders that play an important role as environmental elements, a large number of them have been straightened out in the past, relics of which are still visible in the landscape. 

The undulating terrain alternates agricultural hay meadow with areas delimited by hedges, tree lines or canals, forest and heather, and mere and marsh in the depressions. The brook valleys have a high ecological value and include 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 almost 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 protected.

© 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 maintenance works is the main cause of contamination of adjacent soils. In addition, the Grote Laak is very sensitive to precipitation. With 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 and flood plains. 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 tributaries, leading to local pollution there. 

LIFE NARMENA focuses on the pollutants cadmium, arsenic and radium, in the confluence area of the Grote Laak. 

As in the case of the Winterbeek, it will be useful to compare innovative, nature-based remediation with conventional techniques. The focus area includes the zone where the Grote Laak flows into the Grote Nete and where two wetlands will be created. Proximity to nature reserves plays a crucial role in the choice of this type of remediation. 

Remediation works for the entire Laak are planned for the period 2020 - 2026. The descriptive soil survey is complete and the remediation approach, together with the subdivision of remediation works along the watercourse, has recently been proposed.   

A special technique known as dose rate measurement is used for the delimination of pollution along riverbanks and in flood areas. This innovative method could be applied because in the past, discharged water mainly contained cadmium and arsenic, and to a lesser extent radium. The presence of radium slightly increases radiation levels, compared to the background radiation. 

Also reflecting the Winterbeek approach, two free surface water flow constructed wetland are planned in the confluence area of the Grote Laak. The valley area and the biotopes that can be found here are comparable with the Winterbeek area, as are the objectives, which enables close alignment of the nature conservation measures pursued in these areas. 

Measures will be taken to retain the water within the flooding area, depending on the current groundwater level. 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 immobilisation of the contamination and reduced uptake by organisms. 

Planning

We subjected both floodplains to groundwater and surface water modelling. Through an iterative process, we selected various measures to keep the two areas all year round inundated, without creating flooding in adjacent areas. Dry and wet years were also taken into account. In this way, we create a wetland that remains functional in the context of climate change. The measures were included in the soil remediation project, for which a permit has been delivered in June 2023.

We attempted to start the works in the summer of 2024, but the wet weather conditions and high water levels of this particularly wet year did not allow this. We plan to restart the works after the breeding season of 2025. The works in the floodplains, together with the remediation works at the level of the watercourse itself, will take one year. After that, three more monitoring campaigns will take place to evaluate the effects of the wetlands on the contamination. You can find more information about the remediation of the Grote Laak at www.grotelaak.be