This study investigated theflux stability of select chemical and biological sewage markers, including caffeine, total
nitrogen (TN), total suspended solids (TSS), E. coli,and enterococci, and their suitability in assessing the severity of rainfall-derived infiltration and inflow (RDII) in a residential sewershed. To quantify and compare marker flux stability, concentrations of the candidate markers in two dry-weather periods were determined and the one-day lag autocorrelation coefficients (r) of their massfluxes were calculated. TN (r =0.82
0.88) exhibited higher and more consistentflux stability than TSS (r= 0.49
0.82), caffeine (r= 0.56
0.58),E. coli(r=0.36
0.87), and enterococci (by culture;r= 0.40
0.52), all of which except enterococci by qPCR (r=
0.10
0.21) showed significant autocorrelation. Sewageflows and marker concentrations were also monitored in two wet-weather periods, and the severity of RDII (RRDII) were calculated using eitherflow measurements or marker concentrations independently. Corresponding
to its outstandingflux stability,RRDIIvalues estimated by TN predicted all severe RDII instances and gave the highest and most consistent correlation (r= 0.74
0.78) among the different sewage markers. Overall, the study illustrated the feasibility of using the flux stability of sewage markers in assessing the severity of RDII and thereby deterioration levels in sewer systems.