Contaminant covariation as a potential indicator of hydrological connectivity in an urban canal-river system,...

Urban canals frequently convey untreated wastewater to receiving rivers, but in the absence of flow data, tracing this contamination remains a major obstacle for regulatory agencies. To test whether contaminant-mixture structure can serve as a hydrological tracer, we conducted twelve monthly synoptic campaigns across nine stations in the Mae Kha Canal-Ping River system (northern Thailand). Canal stations exhibited a robust, covarying wastewater fingerprint-dominated by ace-sulfame, caffeine, and gemfibrozil-that was entirely absent from the upstream river. This canal signal decayed predictably downstream, yet several wastewater-exclusive compounds remained detectable in the river, confirming persistent urban influence despite substantial in-channel attenuation. Notably, atrazine followed a contrasting river-dominated pattern, demonstrating that external agricultural inputs modify-but do not erase-the canal-derived signature. Our findings introduce a practical, flow-independent framework: by analyzing the covariance of persistent and labile tracers, mixture composition can distinguish urban hydraulic connectivity from simple dilution, even in ungauged catchments. This approach offers regulatory bodies a low-cost diagnostic tool to prioritize canal interceptors and validate river-quality models where conventional monitoring infrastructure is sparse.