Guidance development: Transitional areas of exceedance

1. Guidance development: Transitional areas of exceedance WG-E(2) meeting, Brussels, 17/10/2007

2. Guidance development: Transitional areas of exceedance Mixing zones WG-E(2) meeting, Brussels, 17/10/2007

3. Complete mixing: Cambient= Ceffluent/Dilution Ceffluent> Cmixing> Cambient Discharge characteristics: Ceffluent(mass/volume) discharge rate(volume/time) River flow (volume/time) Dilution = (discharge rate + river flow)/ discharge rate Dilution and EQS compliance • What if EQS < Ceffluent ?  Compliance • What if EQS > Cambient ?  Non compliance • What if Cefluent>EQS>Cambient ?  Assess the mixing zone

4. B (width) h (depth) V (ambient velocity) Outfall Vertical mixing u* (friction velocity) Longitudinal mixing Complete mixing Hydrological mixing processes Mixing zone models - complete mixing (=dilution) models - mixing zone models - catchment-scale models These models can either be: - steady state models (single input values) - stochastic (probability distributions as input values) - dynamic (time series as input values) • Flux characteristics: • discharge flow rate? • discharge velocity? • discharge buoyancy? • Outfall design: • outlet diameter? • configuration of the diffuser(s)? • position of the port? • orientation of the port? • Ambient characteristics: • ambient velocity? • depth & river width? • roughness of the river bottom? • river morphology? • stratification/uniform ambient water? + seasonal variations

5. Examples of bad mixing Sharp increase of the concentration at the border of the mixing zone High river velocity, low turbulence Low velocity, low turbulence High buoyancy Laminar effluent flow Laminar effluent flow requires a very long mixing area Bad dilution: i.e. sharp increase of the concentration at the border of the mixing zone It is not possible to act on water body characteristics and hardly on effluent buoyancy  Modify outfall design or reduce pollutant loads

6. Administrative mixing zone  hydrological mixing zone

7. Size and location definition of the administrative mixing zonethe administrative mixing zone should protect the aquatic life and human health The administrative mixing zone should allow the free passage for swimming and drifting organisms The administrative mixing zone should protect non-motile organisms and should not overlap sensitive area (e.g. spawning grounds, nurseries, water supply intakes, shellfisheries, shore line, bathing areas, etc.  if it is not possible, the discharge should be reduced or should not be allowed

8. Allocated Impact Zone (AIZ) Acute effects should be avoided inside the AIZ, but exceedance of chronic EQS is allowed Zone of Initial Dilution (ZID) Acute and chronic effects are allowed inside the ZID Outside the AIZ, chronic EQS must be met The definition of the adminitrative mixing zoneshould remain simple E.g Chronic and acute mixing zones (US-EPA)

9. Application of mixing zones for risk assessment or discharge permits - TGD - IPPC BREF - US-EPA - SEPA - UK - AT - NL - DK (and LT) - DE • Not a comprehensive survey If interested, MS are encouraged to share their experience and to help in the development of a dedicated guidance