A three-year river monitoring was carried out in the Bystrzyca River catchment (Poland) dominated by arable areas and wastewater treatment plants. The concentrations of potassium (K) – “a neglected nutrient” – in fresh waters were determined. Analysis of variance (ANOVA) and principal component analysis (PCA) showed spatial and seasonal variability in potassium concentrations. The highest potassium levels were observed in sites located directly below the outlet of the sewage treatment plant of Lublin. The impact of smaller sewage treatment plants on the water quality of the Bystrzyca River was low. Contents of K+ in waters situated in areas experiencing crop and animal production were significantly lower in comparison to ‘sewage’ points. There was a differentiation of correlations between concentrations of K and NH4-N, NO3-N and PO4-P depending on the source of pollution. The potassium levels in river waters usually decreased in early spring.