The aim of this study was to pilot the determination of the state of contamination with residues of plant protection products of freshwater ecosystems in areas of intensively used agriculture in the Podlasie voivodship. The analytical studies sought residues 431 active ingredients (a.i.) of pesticides. The qualitative and quantitative determination was performed using liquid chromatography technique coupled with tandem mass spectrometry. 85% of the samples tested showed the presence of residues. From among 431 substances tested, only 23 active substances were detected, of which the most numerous group were herbicides (11 (a.i.). The results show that there are significant concentrations of pesticides in the water lance (Lemna L.), which confirms that this plant is an excellent bioindicator of freshwater ecosystems contamination.