Future-Proofing the Mixing Zone Concept

Report no.3/26: This report describes an assessment of the evolving role of mixing zones in European water protection management, including the potential impacts on regulated industry such as fuel manufacturing operations. Mixing zones are designated areas within receiving surface water bodies where wastewater effluents may cause localised exceedances of regulatory criteria, provided that water quality standards are met at the boundary of the mixing zone. They allow in-waterbody mixing and dilution to ensure protection of the quality of the wider aquatic environment. However, the concept faces increasing scrutiny due to regulatory focus, public environmental concerns, and climate change related impacts on water systems. The work employed a comprehensive literature review, a survey of 22 refineries across 14 countries, and advanced modelling using the CORMIX tool. Findings revealed significant regulatory variation across Europe, with some countries providing detailed guidelines while others lack clear frameworks, resulting in operational uncertainty, and potentially hindering efforts to achieve the Water Framework Directive’s water quality goals. Survey data showed limited current use of mixing zones but anticipated growing reliance due to evolving standards. Climate change induced variability in water systems and the detection of emerging contaminants highlight the need for adaptive management and advanced treatment technologies. This study concludes that harmonised European-wide regulations or guidance would be beneficial to futureproof the mixing zone concept.