The research methodology for determining the sources of nutrients responsible for the eutrophication of
rivers and seas, as well as the extent of their load in particular drainage basins, has for many years been at the
centre of vigorous discussion. In the Oder and Vistula river basin, apart from the calculation of monthly and annual loads of nitrogen and phosphorus, based on the discharge and chemical monitoring data of waters, the
MONERIS (Modeling Nutrient Emissions in River Systems) model has also been applied in determining nutrient
sources. This article, on the basis of a comprehensive review of the professional literature, shall cast a critical
eye over six issues that have been at the centre of past robust discussion: 1) determining the balance of N and P
in agriculture, 2) the effects of a significant improvement in sewage treatment, 3) impact of technology on agriculture, 4) determination of nutrient retention in drainage basins, 5) impact of tile drainage practices on the
leaching of nutrients, 6) as well as the accuracy of calculations made according to the MONERIS model. It
would appear that for practical purposes it is sufficient to determine given loads of N and P from the drainage
basins of particular rivers, as well as to adjust the above mentioned model, or indeed – resign from the unproven
methodology of determining nutrient sources in rivers.