## 2 Materials and Methods
### Sampling  
Surface water samples were collected every month from March to November 2018 from five main streams of the Han River (H1-H5) and 17 sites in its tributaries (T1-T17) (Figure 1). The Han River is the largest river in South Korea. It flows through Seoul and has three large tributaries, with various small streams; the average flow is about 874 m\({}^{3}\)/s. Surface water samples were collected using pre-washed glass bottles (2 L volume) that were stored in a cool dark location before analysis.  
Samples were also collected quarterly at 12 locations (5 influents and 7 effluents) in four STPs (STP1-STP4) located in Seoul. The locations of the collection points are shown in Figure 1. The total area served by these STPs is approximately 605 km\({}^{2}\). The total treatment capacity of the four STPs is 4.98 million m\({}^{3}\)/day; the largest plant (S4) has a capacity of 1,630,000 m\({}^{3}\)/day. The total input flow of the four STPs is 4,207,000 m\({}^{3}\)/day. These STPs mainly treat domestic wastewater (99.4%) with small amounts (<0.6%) of industrial wastewater [32]. As shown in Table S1 in the Supplementary Materials, after the primary sewage treatment, including the precipitation stage, each STP applies an advanced biological treatment process (a modified Ludzack-Ettinger (MLE) or anaerobic-anoxic-aerobic (A2O) process) to remove organics, nitrogen, and phosphorus. Following the advanced biological treatment processes, a chemical coagulant is applied to reduce the total phosphorus (T-P) (<0.5 mg/L). The treated sewage water is then discharged to the Han River or its tributaries.  
To measure SMF levels in household products, various PCPs were purchased from a supermarket in Seoul and the concentrations of nine SMFs in each product were measured. A total of 36 PCPs, including 10 hair care products (e.g., shampoo), 9 dish detergents, 2 body soaps, 3 hand creams, and 12 perfumes, were collected. The perfumes were liquids, while the rest of the samples were creams and gels.  
Figure 1: Location of sewage treatment plants and sampling points in the Han River and its tributaries (STP: sewage treatment plant).  
\begin{table}
\begin{tabular}{l l c c c c c c} \hline \hline
**Synthetic Mask** & **Analytes** & **IUPAC Name** & **Log K\({}_{\text{sw}}\)** & **Solubility** & **Henry’s Law Constant** & **Vapor Pressure** \\
**Fragance (SMF)** & **(Harde Name)** & & & **(mg/L)** & **(Pz-m’/mol)** & **(Pa)** \\ \hline  & & DPMI & & & & & \\  & (Cashmeran) & 1,1,2,3,3-pentamethyl-2,5,6,7-tetrahydrodonn+4-one & 4.9 & 0.17 & 9.9 & 5.2 \\  & ADRI & 1-(6-tert-butyl-1,1-dimethyl)-2,3-dimethyl-0-4+yl)ethanoneone & 6.6 & 0.015 & 1801 & 0.020 \\  & (Celestiolide) & AHMI & 1-(1,2,3,3-6-hexamethyl-2H+inden-5-yl)ethanoneone & 6.7 & 0.027 & 646 & 0.024 \\
**Polycyclic** & (Phantholide) & 4,6,6,7,8,8-hexamethyl-1,3,4,7-tetrahydrodocytopenta(glischromene & 5.9 & 1.75 & 11.3 & 0.073 \\  & (Galischeide) & & & & & \\  & (Traselide) & 1-(1,1,2,6-tetramethyl-3-propan-2-yl)-2,3-dihydroden-5-yl)ethanone & 8.1 & 0.085 & 85.1 & 1.2 \\  & (Grandide) & 1-(3,5,5,6,8-8-hexamethyl-6,7-dihydroxy)ethanolen-2-yl)ethanone & 5.7 & 1.25 & 12.5 & 0.068 \\ \hline  & MA & & & & & \\  & (Musik amberte) & 1-tert-butyl-2-methoxy+methyl-3,5-dimethylobenzene & 4.0 & 0.79 & 0.071 & 0.00173 \\
**Nitro** & MX & & & & & \\
**musk** & (Musk xylene) & 1-tert-butyl-3,5-dimethyl-2,4,6-trimitrobenzene & 4.9 & 0.49 & 0.018 & 0.00003 \\  & MX & & & & & \\  & (Musk ketone) & 1-(4-tert-butyl-2,6-dimethyl)-3,5-dinitrophenyl)ethanone & 4.3 & 1.9 & 0.0051 & 0.00004 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physicochemical properties of synthetic mask fragances (SMFs).