Nitrate concentrations and loads, and their nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) stable isotope ratios were analysed in samples collected in SE Poland, from the River San and two of its tributaries. The river waters studied were in fact nitrate-poor (with ca. 2 mgN L^-1), with loads calculated indicating an agricultural impact (range ca. 2-40 kg N ha^-1year^-1). Nevertheless, the isotopic signature (δ¹⁵N and δ¹⁸O within 4.5-7‰ and 9.5-17‰ respectively) for nitrate implies the influence of wastewaters and manure. This is confirmed by modeling using IsoSource 1.3 software, which reveals shares of nitrate from the nitrification of wastewaters and manure in the range 48-80%. Multiple regression methodology was then used with results obtained in the present study and cited in litt., in order to estimate the impact of the basin’s agricultural area and the relationship between nitrate isotopic composition and nitrate content in surface waters. Therefore, the results suggest that stable isotope analysis can supply key information as regards the origins of nitrate, beyond that available using classical models based solely on catchment management and concentration data.