Annual changes in the chemical composition of stream water in small catchments with different land-use (Carpathian Foothills, Poland)

The aim of the study was to identify the factors that influence annual changes in the chemical composition of stream waters. The research area was located in the marginal zone of the Carpathian Foothills (Poland) in the Stara Rzeka catchment (mixed land-use) and its two sub-catchments: Leśny Potok (wooded) and Kubaleniec (farmed). Hydrochemical studies were carried out during the 1998-2004 water years and with separate recording frequencies for individual parameters. Measures used included specific conductance (SC), pH and the concentration of the main ions (Ca2+, Mg2+, Na+, K+, HCO3-, SO42-, Cl-) and nutrient compounds (NH4+ , NO2-, NO3-, PO43-). Tendencies towards changes in chemical parameters were estimated using the Seasonal Kendall Test. Elimination of flow rate impact on the chemical composition of stream waters was achieved using the residuals from the LOWESS analysis. During the analysed period, a statistically significant increase in SC and the concentration of the majority of main ions occurred. Additionally, there was an increase in the concentration of nutrient compounds in watercourses undergoing the anthropogenic impact (Kubaleniec, Stara Rzeka), as opposed to the Leśny Potok stream draining the wooded catchment. The SC changes were determined by a decrease in the annual river run-off and simultaneous ion concentration (natural factor). In the Stara Rzeka catchment, the increase in NH4+ concentration was caused by the increase in sewage discharge into the river (anthropogenic factor). Another factor that contributed to the annual changes of water chemistry was gradually deeper and deeper water-circulation (circulation factor), responsible for an increase in the concentration of Na+ and Cl- and a decrease of HCO3-3- and Ca2+.