Reference, completed project

Behaviour of groundwater in chalk and transfer of n-nitrosomorpholine (NMOR) and morpholine (MOR) contaminants in Seine-Maritime

In 2012, a substance of the nitrosamine family (n-nitrosomorpholine, i.e. NMOR) was detected in the groundwater that supplies drinking water to the towns of Bolbec and Gruchet-le-Valasse, in Seine-Maritime. A monitoring plan, set up at the instigation of the Regional Safety Agency (ARS), quickly revealed a greater than expected extent of the pollution (found in abstraction structures located at two extremities of the department).

26 September 2020

Piezometric map of the Commerce catchment area completed in March 2019.

The need

Following the cessation of pollutant discharges into the natural environment by the industrial company causing the pollution, and given the persistence of pollution in this environment and in the drinking water abstraction structures (AEP), the technical group for monitoring pollution asked BRGM to carry out a study to better understand the behaviour of the chalk hydrosystem (groundwater and rivers) in the Bolbec area and downstream. The objective is to gain a better understanding of the possible pathways that morpholine (MOR) and NMOR can take in their migration towards the drinking water supplies, in order to set up effective monitoring of pollution changes and take protective measures to resume drinking water production. This study between 2016 and 2020 was carried out in conjunction with Caux-Seine Agglo, Le Havre Seine Métropole and the Seine-Normandie Water Agency.

The objectives of the study were as follows:

to establish as best as possible underground and surface flow characteristics and changes over time,
to describe the interactions between groundwater and surface water and their temporal variations
to establish the path of the MOR/NMOR and provide the means to help search for possible areas that may contain one or more residual pollution source(s),
to evaluate as best as possible the quality changes in the aquifer in terms of MOR and NMOR
to propose measures to be put in place in order to protect the drinking water supplies and optimise pollution monitoring.

The results

The geophysical (gravity map, electrical and seismic profiles) and geological (boreholes) investigations of the first phase enabled a better understanding of the structure and nature of the subsurface within which the contaminated water circulates, and then highlighted the various potential transfer pathways from the pollution sources to the abstraction points. Hitherto unknown faults and structures were identified and a new structural sketch map, more complex than initially determined in the area, was established.

These structural elements helped to improve the interpretation of the results of the study's second phase (hydrological and hydrogeological investigations) and to better understand the aquifer–river relationships and the multiple pathways taken by morpholine and nitrosomorpholine in their migration to drinking water abstraction structures.

These results will help the authorities draw up a map of the complex pollutant pathways and create an integrated resource protection/rehabilitation system.

Using the results

This work provides communities with reliable information to help them adjust the parameters of the monitoring and protection system to be set up. This system will re-establish the sector's drinking water supply under good conditions and help measure the gradual rehabilitation of the water body's chemical state.