When pollutants are introduced into the marine environment, they are subject to a number of physical processes which result in their dilution in the receiving water. Dilution is one of the main processes for reducing the concentration of substances away from the discharge point. Dilution is more important for reducing the concentration of conservative substances (those that do not undergo rapid degradation, e.g. metals) than for non-conservative substances (those that do undergo rapid degradation, e.g. some organic substances).

Dilution capacity of the receiving water can be defined as the effective volume of receiving water available for the dilution of the effluent. The effective volume can vary according to tidal cycles and transient physical phenomena such as stratification. In estuaries, in particular, the effective volume is much greater at high spring tides than at low neap tides. It is important to consider concentrations of substances in worst case scenarios (usually low neap tides except, for example, when pollutants might be carried further into a sensitive location by spring tides) when calculating appropriate discharge consent conditions. Stratification can reduce the effective volume of the receiving water by reducing vertical mixing and constraining the effluent to either the upper or lower layers of the water column.

The process of dilution can be separated into initial dilution and secondary mixing.

For many discharges from pipes, the effluent is principally freshwater, containing a mixture of pollutants and the discharge point is generally located below Mean Low Water Springs (MLWS) such that the effluent is released under seawater. Initial dilution occurs as the buoyant discharge rises to the surface because of the density differential between the saline receiving water and the freshwater effluent. Under certain circumstances of stratification or where the effluent comprises seawater (in a cooling water discharge for instance), the effluent may not rise to the surface but may be trapped in the lower layers of the water column. The design of sewage outfalls including the use of diffusers, can maximise the initial dilution by entraining as much receiving water into the effluent as possible. Guidelines for the amount of initial dilution expected for the design of discharges are set by the competent authorities.

For the many buoyant discharges, the effluent rises to the surface where it can form a 'boil'. The plume then forms and spreads and secondary mixing takes place. Eventually, the plume disperses both vertically and horizontally in the water column as the density differential becomes inconsequential and the concentration of pollutants in the water column approaches uniformity. Further dilution occurs as a result of the action of tide, wind and wave driven currents.

The concept of the mixing zone was developed to allow a sound basis for the derivation of discharge consents which can be readily related to enforceable end of pipe effluent concentrations and outfall design criteria. A mixing zone is an area of sea surface surrounding a surface boil. It comprises an early part of the secondary mixing process and is prescribed to ensure that no significant environmental damage occurs outside its boundaries. An individual mixing zone is only defined with respect to an established EQS for a particular polluting substance. The mixing zone is the area of sea surface within which the EQS will be exceeded (SEPA 1998). The relation of the mixing zone to the location of European Marine site features will be a key consideration for determining the acceptability of dilution criteria.

Dilution within the mixing zone consists of initial dilution (the dilution received as a plume rises from the discharge point to the water surface) and secondary dilution (a slower rate of dilution, occurring between the surface 'boil' and the edge of the mixing zone). To ensure that the integrity of a European marine site is not affected, the minimum size of a discharge to be consented (in terms of flow or load) should be assessed on a site-specific basis. This will depend on the substances and/or physico-chemical parameters associated with the discharge, together with the positioning of the discharge in relation to the biotopes(s) or species for which it was designated. The initial dilution of discharges also needs to be considered. For example, an initial dilution of 50 times may be considered appropriate for secondary treated sewage effluent and low toxicity industrial effluents, but highly toxic industrial effluents may require a minimum initial dilution of 100 times (95 %ile).

SEPA (1998) define the maximum size of a mixing zone as 100 m around the centre boil in any direction that the plume may travel, but for European marine sites, more stringent criteria may be applied. Clearly, the flow and concentration of pollutants within the discharge are critical to defining the size of the mixing zone.

Further information on initial dilution and mixing zones and SEPA's policy is provided in SEPA (1998). A further useful introduction to the subject is presented in the Urban Pollution Management Manual (FWR 1998).

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