SGER: Y-90/Sr-90 disequilibria: A new method for determining water age in a drinking water distribution system

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Waples 0630847 This project will evaluate a new and innovative technique to measure water age in drinking-water distribution systems based on 90Sr/90Y disequilibrium. 90Sr is a ubiquitous contaminant from nuclear weapons testing, and 90Y is the daughter product of its decay. Because of the relative rates of decay of these radioisotopes, secular equilibrium is developed such that following initial (disequilibrium) conditions where only 90Sr is present, the two isotopes eventually reach equal activities. The technique measures the time since disequilibrium was established by determining the ratio of activities of the two isotopes. In water-treatment plants that use flocculation and filtration to remove suspended matter, 90Sr/90Y disequilibrium is achieved because 90Y is particle-reactive and is effectively removed from water, whereas 90Sr is unreactive and passes through the treatment system. Preliminary data from the water distribution system for the City of Milwaukee suggests that the technique will work. The PI will test the technique using smaller distribution systems that will provide results easier to analyze (in terms of water residence times) than the complicated Milwaukee system. Laboratory studies will be done to address an uncertainty concerning the reliability of the proposed procedure, i.e., the influence of isotope sorption onto organic and inorganic films on pipe walls in distribution systems. Knowledge of water residence times in drinking water distribution systems is important in understanding opportunities for bacterial growth and decay and changes in the chemical quality of drinking water after it leaves a treatment plant and before it is consumed by the public.