Measurement of the piezometric level before sampling (January 2023)
© BRGM
The need
AMPA is often found in the raw water in the Varras water catchment point (Mauny, Seine-Maritime), which draws water from a karst chalk aquifer. There are at least two possible reasons for this contaminant being found in the water, since it is both a degradation product of glyphosate (a herbicide used in agriculture and by certain local authorities) and a breakdown-product of certain detergents.
Due to these two possible origins and the hydrogeological environment, there are different ways that this compound could be transferred to the groundwater at the drinking water abstraction point: diffuse infiltration linked to land use and/or more localised infiltration of discharges from waste-water treatment plants downstream. Consequently, determining the relative significance of each of the potential transfer routes was essential to enable the Water Authority to implement appropriate management of this contamination.
The results
To achieve this goal, BRGM proposed an original multi-tracer approach using chemical and isotopic tools, some of which are specific to waste-water, in addition to conducting surveys to improve knowledge of how the hydrogeological system functions in the catchment basin.
Several sources of AMPA in the environment were identified: agricultural drainage, the rainwater basin of a commercial area and discharges from a waste-water treatment plant, the latter appearing to be a continuous source. Colour tracing, carried out during the first autumn period, highlighted the hydraulic relationship between the receiving environment (ditch) and the catchment point. The combined use of boron isotopes and organic co-tracers made it possible to better analyse and pinpoint the hydraulic functions of this ditch throughout the year, including after re-profiling work, and to assess the impact of the discharges over an entire hydrological cycle.
The results obtained enabled us to understand how the hydrogeological system functions and then reinterpret the historical data. This analysis highlighted the fact that weather conditions (rainfall and intensity), periods of aquifer recharge or discharge and more general hydrodynamic conditions (piezometric level) have an impact on the quality of the water measured at the catchment point in terms of AMPA and glyphosate.
We learned a great deal from this partnership. As well as providing us with the knowledge we need to tackle the specific issue of protecting water resources, BRGM's expertise enabled us to highlight certain issues we need to look at in greater detail, such as developing our piezometric network. The results also enabled us to communicate information to the public and raise awareness about the origin of pollutants that impact water quality.
Gauging campaign to estimate flows (January 2023)
© BRGM
Using the results
The SERPN will continue to implement the management initiatives and communication campaigns it has undertaken over the last few years, notably focusing its efforts on the farming community and local authorities. The results of the study also provide an opportunity to engage dialogue between the various stakeholders in the sector, as certain actions fall under the competency of other players in the area. This will enable the various actions to be prioritised with all the stakeholders involved.
The partners
- SERPN (Syndicat d'Eau du Roumois et du Plateau du Neubourg
- Agence de l’Eau Seine-Normandie
- Eure département
- Metropole Rouen Normandie
