Weekly Monitoring Reveals the Dynamics, Sources, and Loads of Pesticides, Pharmaceuticals, and Tire-Derived Chemicals in the Mississippi River

Understanding the sources and transport of contaminants in large rivers is often constrained by low-frequency monitoring that fails to resolve short-duration hydrologic effects. Weekly water samples were collected for one year from the Upper Mississippi River to quantify the occurrence, temporal dynamics, sources, and mass loadings of pesticides, a pharmaceutical, a corrosion inhibitor, and tire-derived chemicals. Distinct temporal patterns were observed. Tire-derived chemicals exhibited pronounced concentration peaks during spring snowmelt. Pesticide concentrations were highest in early summer following agricultural application. Pesticides showed strong positive correlations with river discharge (e.g., for metolachlor, ρ = 0.78 and p < 0.001), indicating inputs from increased runoff, while the wastewater indicator metformin had a negative correlation, consistent with dilution of point-source inputs. Cross-compound correlations indicated dominant contributions from urban runoff (tire-derived chemicals), agricultural runoff (pesticides), wastewater effluent (metformin), and mixed sources (benzotriazole). Load estimates showed tire-derived chemicals had mass loadings comparable to those of neonicotinoid insecticides. These results indicate that weekly sampling leads to improved characterization of contaminant sources and load contributions in large river systems.