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 fall into 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 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 remediation 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. The Grote Laak is, in effect, 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, radium, arsenic and mercury, in the area of the river mouth of the Grote Laak. 

As in the case of the Winterbeek, it will be useful to compare new, nature-based remediation with conventional techniques. It is of importance that the upstream areas are first remediated, to prevent more contaminated surface water or sediment flows into the downstream subarea. This area includes the zone where the Grote Laak flows into the Grote Nete and where a wetland will be created. Proximity to nature reserves plays a crucial role in the choice of this type of remediation. 

Remediation works for all subareas are planned for the period 2020 - 2025. 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 demarcation 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. 

Hydrodynamic modelling is currently ongoing. Different wetting scenarios are being modelled in a groundwater model that interacts with a surface water model. Modelling will take into account the effects of a number of both dry and wet years, as well as climate change scenarios. 

Also reflecting the Winterbeek approach, a free surface water flow constructed wetland is planned for the Grote Laak and Grote Nete confluence area. 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. 

The highest concentrations of metals are almost exclusively situated in the riverbanks on both sides of the Grote Laak. The deposition of cleared sludge on the banks in the past is seen as the main cause. This should have been restricted to a 5-metre-wide strip of the bank, but due to land cultivation the contents of the strip can be spread over a larger area. It is important to select a pilot flood area in which contamination is sufficiently present, in order to demonstrate the wetland remediation theory as an effective method in practice. 

Measures will be taken to retain the water within the flood 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

Different measures to higher the groundwaterlevel have been modelled in a ground and surface water models. The measures are included in the remediation concept and the soil remediation plan is being drafted. Implementation works are foreseen for summer 2023.