Impact of Urban Agglomeration on the Longitudinal Hydrochemical Profile of the Mleczna River (Central Poland)

Catchments draining urban agglomerations are particularly exposed to significant anthropogenic pressure. Along watercourses, various types of wastewater are discharged, including both area pollution sources (e.g., parking lots, streets) and point sources, such as the discharge of treated wastewater from large municipal sewage treatment plants.

Field studies were conducted in Radom, along the Mleczna River, using the hydrochemical mapping method. Sixteen points and hydrological nodes were identified along the river's longitudinal profile, at locations where wastewater or other watercourses discharge. The studies were carried out twice (November 8-9, 2024), and 66 water samples were collected. During the fieldwork, selected physicochemical parameters (electrical conductivity, pH, dissolved oxygen content, and water temperature) were measured in two series, and water samples were taken for further laboratory analysis. In the laboratory, the chemical composition of 14 major ions (Ca, Mg, Na, K, NH₄, Li, HCO₃, SO₄, Cl, NO₃, NO₂, PO₄, F, Br) was analyzed using ion chromatography (DIONEX ICS 2000). The Elementar Vario TOC Cube analyzer was also used to determine the content of organic carbon (TOC), inorganic carbon (TIC), total carbon (TC), and total nitrogen (TNb).

Spatial variation analysis of physicochemical properties and ion concentrations indicates that anthropogenic influence is noticeable across all parameters along the longitudinal profile of the Mleczna River. It is worth noting that along the hydrochemical profile, there is primarily an increase in the concentrations of chloride and sodium. These ions are associated with the water and wastewater management of the city of Radom. This is especially evident at hydrological-chemical node number 9, where stormwater from the city’s drainage system is discharged. Anthropogenic pressure is reflected in a change in the hydrochemical type – particularly noticeable in the suburban area, where treated wastewater is discharged from the sewage treatment plant into the Pacynka River, a tributary of the Mleczna River (node 15). The natural hydrochemical type, related to the geological structure, changes from a typical, simple HCO₃-Ca type to a four-ion, complex HCO₃-Cl-Ca-Na type.