The ability to assess hydroxyl radical (*OH) scavenging rates of natural organic matter (NOM) (k_{NOM,*OH) is necessary for the optimization of advanced oxidation processes (AOPs) for water treatment, and to understand the role of *OH in natural systems. We have developed a rapid method to measure *OH scavenging rates in less than one hour. Using the method, we measured kNOM,*OH for NOM isolates, and performed the first known scavenging analyses of surface waters and treated drinking waters over time. The results are compared with values quantified using more time consuming methods. Our results reveal that kNOM,*OH in water samples can fluctuate with time, which has implications for AOP operation. The methods and benefits of the new measurement technique and implications of observed variability of kNOM,*OH values on AOP operations are presented. Additionally, observations regarding current NOM scavenging measurement and analysis methods are discussed.}