Using Naturally Occurring Radionuclides To Determine Drinking Water Age in a Community Water System

Drinking water quality in a community water system is closely linked to the age of water from initial treatment to time of delivery. However, water age is difficult to measure with conventional chemical tracers; particularly in stagnant water, where the relationship between disinfectant decay, microbial growth, and water age is poorly understood. Using radionuclides that were naturally present in source water, we found that measured activity ratios of ⁹⁰Y/⁹⁰Sr and ²³⁴Th/²³⁸U in discrete drinking water samples of known age accurately estimated water age up to 9 days old (σ_est: ± 3.8 h, P < 0.0001, r² = 0.998, n = 11) and 25 days old (σ_est: ± 13.3 h, P < 0.0001, r² = 0.996, n = 12), respectively. Moreover, ⁹⁰Y-derived water ages in a community water system (6.8 × 10⁴ m³ d^–1 capacity) were generally consistent with water ages derived from an extended period simulation model. Radionuclides differ from conventional chemical tracers in that they are ubiquitous in distribution mains and connected premise plumbing. The ability to measure both water age and an analyte (e.g., chemical or microbe) in any water sample at any time allows for new insight into factors that control drinking water quality.