Agricultural fields within the protection perimeter of the Férin (Nord département) water abstraction area, which draws water from La Craie aquifer (2023)
© BRGM
The need
Since 2017, the Conurbation of the Douaisis municipal area (Agglomération du Douaisis) has been exploiting an abstraction area with five operational boreholes located in La Craie aquifer near La Sensée Canal, in the municipality of Férin (Nord département). However, the quality of the groundwater used to produce drinking water has been compromised by the presence of certain pesticides. In particular, bentazone concentrations can vary greatly over time and from one location to another within the abstraction area.
The results
The study carried out by BRGM aimed to determine the routes and dynamics of pesticide transfer to the abstraction area, and notably bentazone. The abstraction supply zone is unusual in that a large part of its surface area is covered by tertiary formations that confine the underlying La Craie aquifer. Upstream of the abstraction supply area, in this confined zone, the groundwater is virtually free of nitrates and pesticides, despite significant agricultural activity. When the groundwater becomes unconfined, La Craie aquifer is vulnerable and its quality deteriorates. Downstream, in the abstraction area, the presence of quaternary alluvium contributes to the removal of nitrates from the groundwater.
Despite this purification process, the logs of nitrate concentrations over time show that these can increase under certain operating conditions. In particular, more intense pumping since 2011 has modified the hydrodynamics of the aquifer by mobilising water from different sectors or of different origins, at flow rates that are potentially too high to allow nitrates to be removed when the water enters the abstraction area. Conversely, local anaerobic conditions can slow down the degradation of certain contaminants, particularly bentazone.
As a result, the nitrate and pesticide concentrations – particularly bentazone – vary significantly but distinctly in each borehole. The study demonstrated the co-existence of transfer routes with different flow rates: the rapid transfer routes explain the contamination observed in the form of peaks, essentially resulting from recent sources or inputs, while the slower transfer routes explain the more persistent presence of certain compounds. Consequently, measures can be taken to limit the contamination of the water that reaches the boreholes. However, the geological and pedological context, which facilitates the transfer of contaminants close to the abstraction area, makes selecting and implementing the appropriate measures more complex.
La Sensée Canal (Férin, 2023)
© BRGM
Using the results
The operational results of this study made it possible to propose a number of ways in which the Conurbation of the Douaisis municipal area (Agglomération du Douaisis) could improve its groundwater quality and optimise its monitoring process:
- Installation of a network of dedicated piezometers;
- Improving knowledge of pesticide use throughout the abstraction supply area, particularly in the alluvial zone, and raising awareness among stakeholders in this sector;
- Continued (and even reinforced) self-monitoring on a monthly basis, incorporating all the physico-chemical parameters and key elements for all the abstraction points and certain piezometers, in order to improve understanding of the hydrodynamic system;
- Improving knowledge of interactions between surface water and groundwater in the vicinity of the abstraction area, particularly with regard to interactions with the alluvial aquifer zone, by monitoring surface water quality.
The partners
- Agglomération du Douaisis
- Agence de l’eau Artois-Picardie
