Measures need to be investigated regarding what is required to achieve good water status in lakes and streams. Treatment requirements must be made when planning areas for new building and rebuilding. The requirements differ depending on in which recipient catchment area the study area is located. To know if pollutant treatment is necessary in existing and exploited areas, calculated acceptable load (kg/year) per recipient and substance should be used. This acceptable load has been calculated since many years in StormTac Web. However, in this new development of the model the acceptable load (kg/year) is distributed over the total area (ha) in the watershed area to the recipient, to be used as guideline for treatment of an existing or planned area. This results in specific calculated acceptable load (kg/year) for the area of exploitation which is a much more relevant and effective criteria to work against when designing treatment facilities for the area, compared to working against outlet concentration criteria (µg/l).

If the calculated load from the area is greater than its calculated acceptable load, a treatment action is required to decrease the load from the area. An action type can be any type of stormwater treatment facility, supplemented with a flow detention function if necessary. To get adequate treatment, different types and series-coupled facilities of the same or different types can be tested in different combinations. Another type of action to cope with required treatment may be to reduce the rate of exploitation.

This new feature of StormTac Web can be used as an effective tool for municipalities working with their action plans to design treatment facilities in their planned areas of exploitation to achieve good water status in the receiving waters.