Future-proofing the mixing zone concept in water resources management: an industrial perspective.

Mixing zones allow limited exceedances of Environmental Quality Standards within defined areas of receiving waters, yet their implementation across Europe remains uneven and increasingly scrutinised. This study evaluates how environmental change, regulatory evolution, and industrial water-use practices may influence the future viability of mixing zones. A cross-European assessment was conducted using (i) regulatory and technical guidance, (ii) a survey of 22 fuel manufacturing facilities in 14 countries, and (iii) CORMIX near-field modelling to test sensitivity to effluent and ambient parameters. Results revealed substantial variability in permitting conditions, discharge limits, and review frequencies. Only four facilities used mixing zones, though several anticipated future reliance as concentration-based criteria tighten. Modelling demonstrated that effluent density, ambient density, flow regimes, and temperature strongly affected mixing zone dimensions and dilution, with climate change and water reuse practices requiring careful considerations. These findings highlight the need for harmonised guidance, incorporation of mass-loading considerations, and climate-resilient assessment methods to ensure the long-term applicability of mixing zones in (industrial) wastewater management.