Lake Onega is the second largest lake in Europe after Lake Ladoga. This paper is a part of the project concerning a general plan of water protection, as expressed in the Water Framework Directive. The aim of this paper was to present an investigation of the status of Lake Onega and to present steady state and dynamic modelling approach in order to assess the impacts of different loading scenarios of water quality of Lake Onega. In the project more catchment and water quality models were used but in this paper these models were chosen. The presented steady state model was the mass balance model of Vollenweider and the dynamic model is the box-type model AQUATOX. While Lake Onega preserves a good status of water as a whole, the problems with pollution and eutrophication exist in Petrozavodsk and Kondopoga Bays where anthropogenic loading is more pronounced.

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The Baltic Sea is one of the most eutrophied European seas. High nutrient input mainly via river discharges and leaching from agriculture along with the reduced water exchange with the open ocean have, during past decades, led to enhanced growth and biomass of primary producers. Characterized by a large catchment area, pollution that originates from land finally ends up in the Baltic Sea. The nutrient loading from the Kokemäenjoki river basin (27000 km 2 ) was simulated by applying different mathematical models. Phosphorus and nitrogen loading from the Kokemäenjoki river basin to the estuary was estimated by calculating nutrient loading from point and diffuse sources using the CATCHLOAD model and by calculating retention in rivers and lakes using the CHAIN-LAKE model. Simulated nutrient fluxes are finally the input data for an estuary model. Two nutrient reduction scenarios, based on requirements of the current environmental policy have been carried out: 1) waste water scenario which is based on the implementation of the EU directive for N removal from inland wastewater treatment plants, and 2) diffuse pollution scenario from agricultural areas which is based on good management practices of the Finnish Agri-Environmental Programme. In the waste water (WW) scenario, N removal efficiency on the major waste water treatment plants of Tampere would rise from 40% to 70%. In the agricultural scenario (AGRI), total P load would decrease by 20% and total N by 37% from fields to the river. The scenarios affected the annual nutrient output from the Kokemäenjoki river by reducing total N on an average by 4% (WW), and total N by 7% and total P by 6% (AGRI). If a total nitrogen reduction of 70% were required the loading of nitrogen to the Baltic Sea would not decrease much. From the point of view of Baltic Sea protection, special attention should be paid to coastal river basins, where water protection measures could be concentrated on selected land use forms with high nutrient loading.

In the study, submodels for describing degradation of organic chlorine (measured as AOX) coming from a mill producing bleached sulphate pulp were developed. The submodels coupled to a hydrodynamic model can then be used in assessing the behaviour of AOX in receiving waters. The data used in developing the models was based on mesocosm-scale experiments carried out in two lakes. Waste waters of two mills were used at two dilutions (2% and 10%). Part of the mesocosms were darkened in order to study the effect of light on degradation. In the first phase of the modelling first-order kinetics with a constant coefficient were used. The next steps were to include the effect of temperature and light in the model. Including temperature and light did not much contribute to the modelling results. It was found that more than half of the degradation was caused by the light-dependent component of the degradation reaction. The reaction obeys first order kinetics satisfactorily. However, the reaction coefficient is greater in the beginning of the experiment than later. A correction function describing the decrease of the reaction coefficient was included in the model. The correction function is based on the idea that the most readily decaying components of AOX disappear from the solution in the beginning and therefore the apparent reaction coefficient decreases. Only data on AOX was available in this study but the approach can be applied for different chloro- organic compounds and also for other organic substances.