Source apportionment of nutrients in Estonian rivers

Eutrophication caused by excess nutrient loads is the main problem for Estonian surface waters. The assessment of the type of human activity on the catchment area that may cause an impact on the status of a water body is needed for successful implementation of the Water Framework Directive. The lack of necessary information often makes it difficult to perform this task. The simple export coefficients approach has been used in this investigation for evaluation of the impact of different sources of nutrients on the water quality in Estonian rivers. Attention has mainly been concentrated on the diffuse pollution sources (natural and anthropogenic). Nonlinear regression was used for the estimation of the export coefficients and retention of nutrients. Results revealed that the export coefficients vary within a wide range of limits even in such a small country as Estonia. Soil type, topography, climatic conditions and water flow are the main natural factors influencing export coefficients. It was demonstrated that the use of export coefficients estimated for other regions may lead to wrong conclusions about the impact of different diffuse sources on the water body status.

• The scheme proposed in this paper, which was elaborated in compliance with the Guidance [3], for the evaluation of the influence of diffuse and point sources on river status may be successfully used for creating of the preliminary list of unmonitored waterbodies at risk. This list will allow us to select the top priority waterbodies for further investigations. • This scheme includes different approaches for the estimation of the impact of point and diffuse sources on the hydrochemical status of water bodies. Our results showed that simple methods (the mixing model for point sources and export coefficients for diffuse sources) could be used successfully for the implementation of the WFD. • The analysis of the 10-year data of 25 Estonian rivers showed that export coefficients vary from year to year depending on water flow. The equations for the calculation of export coefficients for Estonian rivers for different types of land-use and different hydrological conditions are proposed in the paper. • Our calculations reveal that the impact of diffuse sources on the status of rivers is much higher than the impact of point sources for nitrogen. Both the diffuse and point sources are significant in terms of phosphorus.