Katherine H. Markovich has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education plans at the University of Arizona. The key goal of the research is to investigate the relationship of groundwater age and sustainability. The two have often been conflated, but there has yet to be a mechanistic study linking the distributions of groundwater age encountered in a pumping well with the sustainability of extracting water from that well. This work will bring together stakeholders in Tucson such as water managers, native communities, urban residents, and industry representatives to establish mutually agreed upon parameters for groundwater sustainability, such as acceptable spatial and temporal scale of overdraft. The results of this stakeholder engagement will be explored using a combination of age tracers for residence time estimation and integrated hydrologic modeling. The study is important in linking groundwater age to sustainability using advanced modeling and age-dating techniques, as well as to advance national health and welfare, since access to clean water is paramount for the prosperity of communities in semi-arid and arid regions of the United States. <br/><br/>Multiple age tracers will be linked with a sophisticated integrated hydrologic model to explore the full spatial and temporal scales of flow path and storage response to groundwater extraction, using the Tucson regional aquifer system as a case study. Extraction wells will be sampled for common age-dating tracers (Tritium-Helium and Carbon-13) to estimate the fraction of young and old water encountered in Tucson extraction wells, respectively, using a novel tracer (Argon-39) to estimate the intermediate age fraction. The integration of these three tracers will reveal a more complete distribution of flow paths contributing to a well, and thus provide insight as to the recharge and hydrogeologic characteristics of the regional aquifer system. These results will be used to validate an integrated hydrologic model (ParFlow-CLM) of the Tucson area via a particle tracking code (EcoSLIM). This step will result in a model that captures the dominant flow systems contributing to Tucson extraction wells. The posterior model will then be used to explore the relationship between groundwater age and sustainability, and project this relationship forward under climate change and population growth scenarios. This result will fill in the gaps in our understanding of flow path distributions to wells using non-overlapping age tracers and link those results to a sophisticated regional aquifer model that includes the upland recharge and contributing areas.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.