Changes in the quality of a small lowland river in Poland under the influence of pollution and a dam reservoir located in its course

Abstract The study on a small lowland river was conducted in order to determine changes in water quality under external loading (inflow of treated wastewater, runoff from agricultural areas) and the location of a dam reservoir in its course. The 21 km long River Pogona (Greater Poland, Poland) is a small tributary of the Obra Canal in Kościan. In 2004, in its valley, the Jeżewo Reservoir, with an area of 78 ha, was utilized to supply the flow in the Obra Canal during the dry season and to irrigate agricultural areas. The river is also a receiver of treated sewage from the sewage treatment plant in Karolewo. The research, carried out in 2017 at six sampling stations in its course, showed that the discharge of treated wastewater resulted in an increase in conductivity, concentration of ammonium nitrogen, nitrites and total nitrogen in the waters of the River Pogona. On the other hand, the location of the Jeżewo Reservoir in its course contributed to an increase in water temperature, pH, dissolved oxygen, chlorophyll-a and nitrite concentration in the outflowing river. The results showed that the inflow of treated wastewater from the sewage treatment plant, surface runoff from the catchment area and the Jeżewo Reservoir formed in its course, influenced seasonal and spatial changes in the water quality of the River Pogona.