The objective of the project is to lay a rainwater pipeline between Paris Charles de Gaulle Airport and the river Marne. The pipeline will be 9.2 km long, with a diameter of 1400 mm to 1800 mm at crossings. The project is located in the Beuvronne valley, through which a shallow alluvial aquifer flows.
Geotechnical and hydrogeological field investigations were carried out and revealed the presence of six wetlands along the route of the future pipeline. They also showed that the bottom of the trench will be below the water table. Pumping operations will hence be necessary to dry the trench.
Such operations can generate impacts on rivers and wetlands: drying up of wetlands and rivers, deterioration of river water quality, which can have consequences for fish fauna. Impacts can also occur during the operation phase due to the reworking of the land in the trench.
The approach adopted to assess the impacts consists in building a hydrogeological model covering the entire route of the pipeline. This approach is technically complex but is a means of assessing impacts on the various sensitive zones while taking variations in context (proximity of water courses, hydraulic conductivity variations, etc.) into account. With the aid of substantial data from the field investigations, it was possible to design a multi-layer model representing the different formations concerned by the project, draw a piezometric map over a wide area around the future pipeline itself, and calibrate the model in transient conditions by integrating periods representative of the various hydro-climatic conditions that can be encountered, such as high water (flood of the Marne) and low water.
The calibrated model was then used to evaluate the area of hydrogeological influence of the project, the extent of the drawdown cones and the time required for the water table to return to its initial level. It was also possible to specify depth of groundwater drawdown below ground level, an essential parameter for assessing impacts on wetlands, since the hydrological, biogeochemical and ecological functions of wetlands are directly related to shallow waterlogged soils (the first 50 cm in particular). Hydrogeological impacts were then translated into impacts on wetlands by combining the expertise of hydrogeologists and environmentalists.
The study thus made it possible to address the concerns of the various stakeholders (authorities, pipeline owner, etc.) and to recommend appropriate measures to mitigate the project impacts on wetlands, including operational measures during the construction phase (length of open section, prior pumping time, work period, materials and methods for backfilling the trench, etc.) and specific monitoring.