Antibiotic resistance is a public health threat that is exacerbated by the dispersion of antibiotic resistance genes (ARGs) into aquatic environments. Urban stormwater runoff has been recognized as a source of and a mechanism by which intracellular ARGs (iARGs) can be transported into receiving environments. The prevalence and behavior of extracellular ARGs (eARGs) in stormwater, however, has yet to be considered. This study quantified four iARGs and eARGs under baseflow and stormflow conditions. Urban stormwater runoff was found to be a source of all the ARGs examined, with the absolute concentration of all iARGs and two eARGs (ermF and tetC) increasing significantly (p \u3c 0.05) between baseflow and stormflow. The occurrence of iARGs and eARGs in stormwater runoff was also investigated across three seasons to differentiate temporal trends. All eARGs were found to be most abundant in the fall, while the iARGs did not display a consistent seasonal trend. Following, spatial patterns of the ARGs were elucidated by targeting four sampling locations, including direct runoff from a curbside storm inlet and a stormwater outfall, and two receiving environments, the Menomonee River and Lake Michigan. Stormwater was found to have the largest impact, in terms of the percent increase in ARG concentrations between baseflow and stormflow, on the outfall location where on average the iARGs and eARGs increased 16% and 12.3%, respectively. The variability in seasonal and spatial trends between the iARGs and eARGs suggests a difference in sources and transport mechanisms of the ARGs into the environment. Moreover, the results of this study revealed that eARGs are relevant contaminants to consider when determining the threat of antibiotic resistance originating from stormwater runoff
