Description du sujet de thèse

Climate change and intensification of climate extreme events along with increasing population and urbanisation, amplify pressure on the fresh water resources In that context it will become difficult to reconcile water availability and water demand for Human as well as for ecosystems both in terms of quantity and quality. The 2022 intense drought has led the French government to promote the use of non-conventional water resources as urban stormwater or treated wasterwater.
The infiltration of treated wastewater is a nature based solution which allows to recharge the aquifer (Managed Aquifer Recharge: MAR) and limits the saline intrusion in coastal areas, while offering an additional natural epuration through the soil and aquifer : this is the SAT: Soil Aquifer Treatment. Treated wastewater contains chemical and microbial contaminants, which are then introduced into the soil and the aquifer. Once in the aquifer, the fate of these contaminants is controlled by physico-chemical (dilution, filtration, sorption/desorption processes) and biological (loss of viability, stress, viral lysis, adhesion to biofilms, competition with indigenous microbial communities) processes, yet little is known about these processes and the key factors influencing them.
The thesis is based on an in situ approach of the SAT system of Agon-Coutainville. Treated wastewater is infiltrated into reed beds before reaching the sand aquifer and finally reaching the sea (at a distance of around 800 m). Piezometers located along a transect : in the reed beds, both upstream and downstream, provide access to the water table and sediments (aquifer sands). The SAT of Agon-Coutainville has been studied since 2016 by METIS and BRGM, in particular by Guillemoto (2022) in his PhD, which allowed a first characterisation of the reactive transport processes for organic trace compounds (TrOCs) (Guillemoto et al., 2022) and an initial hydrogeological model at the SAT scale (Guillemoto et al., 2023).
About microbiology, the health risk has not yet been explored, even though the discharges into the sea are close to bathing and fishing areas; similarly, the role of the autochtonous microbial community in the degradation of chemical contaminants remains to be studied.
The aim of the PhD project is to characterise and model the reactive transport processes of chemical and microbiological contaminants in the aquifer with infiltration of treated wastewater, based on in situ observations of the SAT system at Agon-Coutainville, for different hydroclimatic conditions.
The thesis is based on an interdisciplinary approach combining hydrogeology, contaminant chemistry, microbiology and microbial ecology.
The objectives are
i) characterisation of the exposure levels of the receiving environment to chemical and microbiological contaminants along the flow of the SAT system for contrasting hydro- climatic conditions
ii) assessment of the SAT's chemical and microbiological purification efficiency (characterisation of biotic and abiotic processes)
iii) assessment of the impact of infiltration of treated wastewater on the functions and diversity of autochtonous microbial communities
iv) modelling of reactive transfers of chemical and microbiological contaminants and development of contamination scenarios
v) proposal or validation of relevant chemical and microbiological indicators of the SAT's purification efficiency.

Contexte de travail

Main Lab : UMR 7619 METIS, Sorbonne Université 4 place Jussieu, 75005 Paris (https://www.metis.upmc.fr/)
+Field trips at Agon-Coutainville
+ Analyses at UMR 6143 M2C -Rouen
Supervisors : Danièle Valdés (UMR 7619 METIS, Sorbonne Université)
Thierry Berthe (UMR 6143 M2C, Université de Normandie, Rouen)
Julie Leloup (UMR 7618 iEES, Sorbonne Université)
+ collaboration avec Laurent Moulin (Eau de Paris, Recherche et Développement)
Ecole Doctorale : Sorbonne Université - ED 398 – GRNE (Géosciences, Ressources Naturelles et Environnement)