Ultraviolet advanced reduction processes (UV-ARP) have garnered significant attention recently for the degradation of several hard to treat contaminants, including recalcitrant per- and polyfluoroalkyl substances (PFAS). The rate of contaminant degradation in UV-ARP is directly related to the available hydrated electron concentration ([eaq –]). However, reports of [eaq –] and other parameters typically used to characterize photochemical systems are not widely reported in the UV-ARP literature. Deploying monochloroacetate as a probe compound, we developed a method (R e–,UV) to quantify the time-based hydrated electron concentration ([eaq] t ) available for contaminant degradation relative to inputted UV fluence. Measured [eaq] t was then used to understand the impact of eaq – rate of formation and scavenging capacity on the degradation of two contaminantsnitrate and perfluorooctane sulfonate (PFOS)in four source waters with varying background water quality. The results show that the long-term treatability of PFOS by UV-ARP is not significantly impacted by the initial eaq – scavenging conditions but rather is influenced by the presence of eaq – scavengers like dissolved organic carbon and bicarbonate. Lastly, using [eaq] t , degradation of nitrate and PFOS was modeled in the source waters. We demonstrate that the R e–,UV method provides an effective tool to assess UV-ARP treatment performance in a variety of source waters.