MITIGATING FLOW REDUCTION WITH WASTEWATER REUSE: PROMISE OR PERIL FOR STREAM HEALTH?

In many parts of the world, climate change reduces summer baseflow and promotes flow-intermittency. The resulting reduction in dilution capacity of point sources is often considered a threat to aquatic ecosystems. Therefore, water managers often store treated wastewater in summer and release it in winter when flow is high. Today, this strategy is being questioned, as the stored wastewater could be used to sustain flow during summer. Few studies compare the hydrological, chemical and ecological effects of different wastewater discharge strategies, especially on temperate headwater streams. We chose 3 effluent-fed headwater streams in Brittany (France) and monitored discharge hourly, chemistry bi-weekly, biofilm communities monthly and aquatic invertebrate communities annually. In 2025, we implemented reduced wastewater discharges while increased or pulsed discharges are planned for 2026. In 2025, treated wastewater strongly contributed to the stream discharge (maximum between 39% and 87% from June to September). Effluents significantly altered the water quality, with an increase in mean temperature and nutrient concentrations (+2.2 °C, +21.5 mg/L for nitrate, +0.9 mg/L for phosphate). Diatoms dominated the communities and we observed a considerable increase of 517% in mean concentration from downstream to upstream. Aquatic invertebrate samples are still under analysis. We conclude that, even reduced, treated wastewater already substantially contributes to the natural discharge and highly influences the stream’s chemical and ecological conditions.

Presentation Preference: Poster

Primary Presenter: Tanguy Nardon, INRAE (tanguy.nardon@inrae.fr)

Authors:

Tanguy Nardon, INRAE  (tanguy.nardon@inrae.fr)

Rémi Dupas, INRAE  (remi.dupas@inrae.fr)

Ronan Nédelec, Rennes Métropole  (r.nedelec@rennesmetropole.fr)

Alexandrine Pannard, University of Rennes  (alexandrine.pannard@univ-rennes.fr)

Luc Aquilina, University of Rennes  (luc.aquilina@univ-rennes.fr)