## 2 Materials and Methods
### Analytical Determination  
HPLC systems with different sensitivity and analysis modes were used. For the kinetic experiments at 10 mg L\({}^{-1}\), the HPLC-UV system consisted of a Shimadzu (Shimadzu Corporation, Milano, Italy) LC-20 AT solvent delivery module equipped with a DGU-20A3 degasser and interfaced with a SPD-20A UV detector. The wavelength selected for analysis was 238 nm for all GCs. Each sample was diluted (30% \(\
u\)/\(\
u\)) with MeOH and injected (20 \(\mu\)L) into a 250 \(\times\) 4.6 mm, 5 \(\upmu\)m GraceSmart RP18 (Sepachrom) coupled with a similar guard-column. The mobile phase was ultrapure water and ACN mixture, 70:30 for CORT, HCORT, PRED, and PREDLO; 65:35 for BETA, DEXA and TRIAM at a flow rate of 1.0 mL. The isocratic elution was maintained for 20 min, and then, after washing with 100% ACN for 5 min, the initial conditions were re-established. The instrumental quantification limits were 0.3 mg L\({}^{-1}\) for CORT, HCORT, and DEXA, 0.09 mg L\({}^{-1}\) for BETA, 0.2 mg L\({}^{-1}\) for PRED, PREDLO, and TRIAM. For the kinetic experiments at 50 \(\upmu\) L\({}^{-1}\), the HPLC-ESI-MS/MS system consisted of an Agilent (Cernusco sul Naviglio, Milano, Italy) HPLC apparatus 1260 Infinity coupled with an Agilent 6460C ESI-MS/MS spectrometer. Each sample (5 \(\upmu\)L) was injected into an Agilent 120 EC-C18 Poroshell column (50 \(\times\) 3 mm, 2.7 \(\upmu\)m) with a similar guard-column. The column temperature was maintained at 50 \(\pm\) 1 \({}^{\circ}\)C. The mobile phases were (A) ultrapure water (0.1% \(\
u\)/\(\
u\) acetic acid) and (B) MeOH (0.1% \(\
u\)/\(\
u\) acetic acid). A linear gradient from 40% to 84% B was applied in 12 min, followed by a column re-equilibration time of 8 min. The flow rate was 0.6 mL min\({}^{-1}\). GCs identification and quantification were performed in negative electrospray ionization (ESI). Source parameters were set as follows: drying gas temperature 300 \({}^{\circ}\)C (N\({}_{2}\)); drying gas flow 5 L min\({}^{-1}\) (N\({}_{2}\)); nebulizer 45 psi; sheath gas temperature 250 \({}^{\circ}\)C; sheath gas flow 11 L min\({}^{-1}\); capillary voltage at 3500 V (positive mode) and 3000 V (negative mode); nozzle voltage 500 V positive, 0 V negative; electron multiplier voltage (EMV) 0 V for both polarities. MRM conditions (precursor ion [M + AcO]\({}^{-}\) adduct) for each compound are reported in Table S1. MassHunter Software from Agilent was used for data processing. The HPLC-ESI-MS/MS analyses for photoproducts identification were performed by using a surveyor HPLC system (Thermo Finnigan, San Jose, CA, USA), equipped with a 150 \(\times\) 2.0 mm, 4 \(\upmu\)m Jupiter 4U Proteo column (Phenomenex). The mobile phase consisted in: (A) ultrapure water (0.1% \(\
u\)/\(\
u\) formic acid) and (B) ACN (0.1% \(\
u\)/\(\
u\) formic acid). The starting concentration of eluent B was 2%, increased to 100% by 40 min with a linear gradient; this concentration was maintained for 5 min to wash the column. The flow rate was 0.2 mL min\({}^{-1}\). The MS/MS system consisted of an LCQ ADV MAX ion-trap mass spectrometer, with an ESI ion source operating in ion-positive mode with the following instrument conditions: source voltage 5.0 kV; capillary voltage 46 V; capillary \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters/Ions** & & **Tap Water** & **River Water** \\ \hline pH & & 7.7 & 7.9 \\ Conductivity at 20 \({}^{\circ}\)C & \(\upmu\)S cm\({}^{-1}\) & 271 & 293 \\ TOC \({}^{\star}\) & mg L\({}^{-1}\) & \(<\)2 & 6.9 \\ Cl\({}^{-}\) & mg L\({}^{-1}\) & 5.0 & 4.0 \\ NO\({}_{3}\)\({}^{-}\) & mg L\({}^{-1}\) & 0.6 & 1.5 \\ SQ\({}_{4}\)\({}^{2-}\) & mg L\({}^{-1}\) & 5.0 & 13 \\ HCO\({}_{3}\)\({}^{-}\) & mg L\({}^{-1}\) & 182 & 195 \\ Ca\({}^{2+}\) & mg L\({}^{-1}\) & 35 & 54 \\ Mg\({}^{2+}\) & mg L\({}^{-1}\) & 10 & 7.5 \\ Na\({}^{+}\) & mg L\({}^{-1}\) & 12 & 5.5 \\ \hline \hline \end{tabular} \({}^{\star}\)TOC total organic carbon.  
\end{table} 
Table 1: Physico-chemical characterization of tap and river water samples.