A case study on the correlation of micro-contaminants and potassium in the Leine River (Germany)

River monitoring with focus on the correlation of 41 micro-contaminants with K+ and its temporal and spatial variation was carried out in the Leine River (Germany). A positive correlation of concentrations of wastewater-related micro-contaminants and K+ is to be expected in receiving waters of wastewater treatment plant (WWTP) effluent since urine is a significant source of K+ in WWTP effluent. This correlation was found for compounds, which meet the following criteria: (1) WWTP effluent is the dominating source of the compound, (2) variability of its mass flux in the WWTP is negligible, and (3) the compound is persistent in WWTPs and in the environment. The excellent positive correlation of carbamazepine with K+ resulted in the fitting of a universal linear equation to the summer and winter data. For almost all other correlating compounds (1H-benzotriazole, citalopram, diclofenac, metoprolol, sotalol, sulfamethoxazole and tolyltriazole), slopes of the line fittings were steeper in winter (x-axis: K+, y-axis: respective micro-contaminant). This has been attributed to a presumed lower degree of degradation and attenuation within WWTPs and in the environment due to low temperatures as well as an increase in consumption of these compounds in the winter months. As part of this research, a sampling event along the entire flow path of the Leine River (∼280 km) was conducted to identify compounds demonstrating stable ratios of various respective micro-contaminants with K+. Among other compounds, carbamazepine, sulfamethoxazole, and tolyltriazole demonstrated the best correlations with R2 > 0.89. Potassium-equivalents of the individual micro-contaminants depended on land use and population structure of the investigated river section.