Discharge test for fresh and tidal waters

Despite the continued reduction of emissions from point and diffuse sources the desired water quality cannot (yet) been met at various locations.In order to realize a further improvement of the water quality a clearer alignment between the water quality objectives and the necessary emission reduction method is needed: the so-called emission-immission approach.
The tool includes the assessment of the admissibility of the discharge remaining after application of the source approach (best available techniques) from a specific pointsource, for the receiving surface water. According to Art. 4 (4) of the Directive 2008/105/EC (directive on priority substances) the mixing zone approach is required when evaluating discharges of priority substances (2011).
The discharge test determines the concentration in the near vicinity of the point of discharge and assesses whether or not the concentration (CL) at the border of the mixing zone, a limited area in the near vicinity of the point of discharge, meets the EQS standard and the increase in concentration (ΔCL) does not lead to a significant deterioration in water quality.


Since 2011 the discharge test can also be applied for the assessment of discharges on tidal waters.

For the evaluation of discharges at tidal waters complex processes have to be taken into account. The necessary calculations to assess discharges are more complex than in the situation of fresh waters. Tidal movements, fresh-salt transitions, stratification can play an important role. In the new instrument, a distinction is made between:

  • Freshwater (without tide);
  • Canal/estuary/tidal river with net discharge (riverflow);
  • Dead end canals and harbours (without net flow);
  • Coastal discharge to wide estuaries with clear tidal gullies;
  • coastal discharge (to open sea);
  • Open sea discharge.

For the assessment of a discharge to tidal waters, using the discharge test, usually many water system data are needed. For the main RWS (tidal) waters and (tidal) ports these data are stored in a database that is linked to the discharge test.

For the various conditions are different calculation methods necessary. Hence for the salt key is chosen to use a decision tree. This decision tree gives in a transparent manner to how a discharge in a given situation must be assessed and what calculation methods in chosen conditions are applicable.