Sources, Transport, and Sediment-Water Distributions of Contaminants of Emerging Concern in a Mixed-Use Watershed

Abstract

Understanding the sources and transport of contaminants of emerging concern (CECs) is crucial for risk assessment and mitigation. The goal of this research was to augment this knowledge by characterizing the spatiotemporal variability of a diverse set of CECs in a mixed land-use watershed. The study area (South Zumbro Watershed, Minnesota) presented a gradient of land uses that facilitated the investigation of agricultural and urban/residential sources and transport of CECs. Concentrations and loadings of CECs in water samples were analyzed in light of spatial, temporal, hydrologic, and physicochemical variables. Contaminant mass balances were analyzed to characterize the proportional sources and instream transport of CECs. Sediment-water distributions of CECs were assessed in terms of their magnitude, variability, and predictability. Three distinct CEC groups emerged with respect to sources, transport, and seasonal/temporal patterns. The first group (i.e., atrazine, acetochlor, metolachlor, and daidzein) was characterized by agricultural/upstream-dominated sources and transport. These CECs were input primarily via upstream routes with loadings and concentrations that were greatest during high flows. For the second group (i.e., acetaminophen, trimethoprim, DEET, caffeine, cotinine, and mecoprop), a mix of wastewater and runoff transport was demonstrated by frequent detections in effluent and upstream samples, with peak loadings/concentrations associated with high flows and areas of greater population density/development. For the third group (i.e., sulfamethoxazole, carbamazepine, erythromycin, tylosin, carbaryl, and 4-nonylphenol), the detections, concentrations, and loadings were explained by effluent-dominated sources and transport. These CECs showed expected trends of stable loading across events, with the greatest concentrations and detection frequencies associated with low flows and the wastewater treatment plant. Average measured sediment-water distributions exceeded equilibrium hydrophobic-based predictions for five of seven detected CECs by at least an order of magnitude. The consistency and predictability of the measured distributions improved with increasing CEC hydrophobicity and persistence. Thus, spatiotemporal analysis can be used to characterize and track CEC sources and transport, even for ubiquitous CECs. These results augment existing knowledge of CEC sources, fate, and transport by describing dominant sources, transport, and temporal patterns for different types of CECs. This will enhance monitoring, exposure/risk assessments, and management of CECs in surface water ecosystems.