The most significant factor behind the eutrophication of L. Tuusulanjärvi is the external phosphorus loading. It is on average four tonnes per year. This is too much: about two times higher than the critical load. The basis of permanent improvement of water quality of lake is thus a sufficient reduction in the external loading from diffuse sources. It consists of leaching from agricultural areas, untreated sewage of scattered settlements, stormwater from densely populated areas, in addition to the natural run-off and precipitation.
In recent years, there have been large efforts to reduce the nutrient loading from the fields of the catchment area. Most of the farmers are entitled to environmental compensatory aid of EU directives and protective zones bordering drainage channels have been established. The project has offered financial support to farmers for planning and establishing more efficient water protection measures, as protective belts wider than the protective zones awarded under agricultural environment protection aid.
One way to reduce the non-point loading is by building wetlands and settling ponds. Ten wetlands have been built at the brooks in order to withhold the stray solids and nutrients. Wetland construction has been funded by municipalities, government, EU special funding and also local private funds. The wetland of Rantamo-Seitteli (24 ha), finished in 2009, is one of the largest water protection wetlands constructed in Finland.
Decrease of direct inflow of storm waters from urban areas has been supported and municipalities of Järvenpää and Tuusula have built sedimentation pools in new residential areas. Järvenpää has directed the main inflow of stormwaters through the shoreline vegetation.
There are still several hundred households in the catchment area the lake which are not connected to the municipal sewery. This does pose a threat to the lake’s condition since the phosphorus contained in their wastewaters is more available for the algae than the one contained in agricultural run-offs. Therefore, it has relatively a larger effect on the eutrophication. In little townships household sewages could be connected to the municipal sewage disposal system by transfer sewage or to a joint local cleaning system along a joint sewage. In scarcely populated areas individual solutions have to be found for the treatment of wastewaters.