Atmospheric Deposition Dominates Trifluoroacetic Acid Loading in a Large Urbanized River: Insights from Mass Balance and Causal Inference.

Trifluoroacetic acid (TFA), an ultra-short-chain PFAS, is an increasingly widespread contaminant formed partly through atmospheric transformation of fluorinated replacement compounds. However, the relative roles of atmospheric deposition and hydrological transport in regulating TFA in large urbanized rivers remain poorly constrained. Here, TFA was investigated across environmental and drinking-water matrices in the Nanjing Reach of the Yangtze River under contrasting hydrological conditions. TFA was detected in all compartments, reaching 524,380 ng/L in surface water and persisting from source water to finished tap water. Watershed-scale mass balance showed an annualized net downstream TFA flux increase of 1,500 Mg/yr, of which direct atmospheric deposition accounted for 73.8%, making it the largest quantified local input source, whereas upstream transport remained the dominant regional background load (approximately 91.0% of the total input budget). Interpretable machine learning identified mainstream discharge as the strongest predictor of TFA variability, and double machine learning estimated the largest positive effect for atmospheric deposition, supporting coupled atmospheric loading and hydrological regulation of TFA concentrations and downstream export.