Results of the research conducted in the Olsztyn Lake District in the years 1994–2001 showed that the outflow of nitrogen from rural areas amounts on average to 11 kg ha–1 yr–1, whereas from drained areas to 28 kg. The load constitutes a threat to surface water and it should be limited by the construction of biogeochemical barriers. The function may be fulfiled by small water reservoirs understood as a composition of water, bottom sediments and littoral plants. It was stated that such a system may store from 1.4 to 344 kg of N per 1 ha of the catchment area. Vegetation and bottom sediments contained 99% of the total stored N. Accumulated N may become released. The magnitude of accumulation depends on the elements (water, bottom sediments and vagetation) within the system, which may be a limiting factor in long-term outflows of N from arable lands.