Current and future chloride concentrations in a large river - will a disaster happen again?

Chlorides are considered as an important factor promoting the development of the golden algae ( Prymnesium parvum) and their presence was crucial during the Oder River ecological disaster in 2022. Since industrial waters from mining activities in the Upper Silesia region are discharged not only into the Oder River, but also into the other large European River (Vistula), there is growing concern about a possible re-occurrence of such an event. Combining catchment modeling and in-situ monitoring, the impact of mining discharges from 17 active and inactive hard coal mines on the calculation profile of the Vistula River, located in a potential risk zone of golden algal blooms, has been investigated. Moreover, future chloride concentrations have been predicted through model simulations for two time horizons: near- (2024–2050) and far-future (2074–2099), and for two Representative Concentration Pathways: RCP 4.5 and RCP 8.5. The results showed significant variability in chloride concentrations with monitoring data, indicating concentrations already exceeding those observed during severe algal bloom events. Furthermore, despite the selected scenario and time horizons, the simulated chloride concentrations may still promote golden algae development under both dry- and wet-climate scenarios, with extreme values potentially surpassing 3000 mg/L. Our results emphasize the urgent need for proactive water management strategies to reduce chloride contamination, and to protect aquatic ecosystems in this river. Failure to act in this area may lead to a repetition of catastrophic ecological events. • Chlorides fuel Prymnesium parvum blooms as observed in the 2022 Oder River disaster. • Mining wastewater threatens the Oder and Vistula Rivers with chloride loads. • Chloride levels in river water already exceed thresholds linked to severe golden algae blooms. • Future models predict chloride concentrations >3000 mg/L under multiple climate scenarios. • Urgent water management is needed to prevent future ecological disasters.