Diurnal changes of nutrients and carbon forms in different land use catchments located in Carpathian Foothills (Southern Poland)

Hydrological and chemical studies show that in addition to natural factors (geological structure, land cover), the chemical composition of water is strongly influenced by human activity. Typically, changes in physical and chemical characteristics during floods have been studied. It is worth noting that during non-flood days, when there are no floods, there are also changes in the chemical composition of the water. The aim of this study was to find out the changes in the concentrations of nutrients and forms of carbon in water in variously land used catchments during non-flood days.

The research was carried out from 4 to 8 July 2024 in small flysch catchment (22 km²) of Stara Rzeka located in the Carpathian Foothills in southern Poland. The study points were located in selected catchments with different land uses and anthropopressures: a forested catchment, an agricultural catchment and in front of and behind the Stara Rzeka stream wastewater treatment plant. Every 2 hours, water was sampled using an ISCO autosampler (n=100). In the laboratory, the concentrations of 14 ions (Ca, Mg, Na, K, NH₄, Li, HCO₃, SO₄, Cl, NO₃, NO₂, PO₄, F, Br) were determined by ion chromatography. In addition, results were also obtained for total carbon (TC), total inorganic carbon (TIC) and total organic carbon (TOC) (Vario TOC Cube).

The results show that in the forest catchment average concentrations of NO₃ were lower (1.82 mg·dm^-3) than in the agricultural catchment (36.32 mg·dm^-3). On the contrary, average TOC concentrations were higher in the forest catchment (4.56 mg·dm^-3) than in the agricultural catchment (1.42 mg·dm^-3). It is worth noting that the average TC concentrations were higher in the agricultural catchment (55.17 mg·dm^-3) than in the forest catchment (41.27 mg·dm^-3). Analysis of diurnal concentrations indicates that in the agricultural catchment, as water flow decreases, NO₃ concentrations increase, whereas as flow increases, NO₃ concentrations decrease. In the Stara Rzeka catchment, discharges of treated wastewater change the diurnal rhythm of nutrient compounds including: NO₃, PO₄, which is expressed in a greater amplitude of concentrations. In the longitudinal hydrochemical profile of the Stara Rzeka downstream of the sewage treatment plant, a rapid increase in concentrations of NO₃ and PO₄ was observed. Reference these diurnal concentrations of ions to the Polish ministerial regulation on water quality (Dz.U. 2021 poz. 1475) indicates that water taken at different times of the day (day/night) downstream of the wastewater treatment plant were of different quality (very good, good and below good status).