Tales of the tides: Pattern-based non-target analysis of the Elbe River

The goal of this study was to identify previously undetected organic micropollutants (OMPs) in a tidal river system using pattern-based non-target analysis (NTA). Tidal rivers are complex hydrological systems with constantly changing water levels, flow rates, and flow directions, posing significant challenges for monitoring contaminant behavior and fate. In a previous study, we demonstrated that these conditions create distinctive spatiotemporal pollutant patterns for target analytes, differing fundamentally from unidirectional river systems, ranging from tidally oscillating pesticide plumes to stable pharmaceutical concentrations from continuous wastewater inputs. Here, we employed high-frequency sampling with NTA and statistical methods to unravel additional contaminants with tidal-related patterns in the Elbe River, Germany. Two sampling campaigns (August 2022, February 2023) were conducted at upstream and downstream locations spanning Hamburg's metropolitan area. We analyzed 404 hourly composite samples using high-performance liquid chromatography coupled to high-resolution mass spectrometry (HPLC-HRMS). Statistical methods (differential analysis, correlation analysis, variance analysis, hierarchical cluster analysis) prioritized non-target features based on similarity to target micropollutant patterns exhibiting tidal-related dynamics. Seven micropollutants were identified and confirmed: pharmaceutical metabolites (phenylethylmalonamide, N-desmethyltramadol, O-desmethyltramadol), industrial additives (triisopropanolamine, 1,3-diphenylguanidine), and niche chemicals (tributylphosphine oxide, ethyl centralite), several have rarely been reported in surface waters. This study provides novel insights into micropollutant dynamics in tidal rivers, highlighting the importance of high-frequency sampling for capturing contamination in tidally influenced waters. Importantly, our workflow was implemented by a single analyst using standard regulatory equipment, demonstrating that comprehensive chemical characterization of complex tidal systems is achievable for routine monitoring laboratories.