Inland freshwater salinity is rising worldwide and threatens the quality of our water resources, a phenomenon called the freshwater salinization syndrome (FSS). Simultaneously, the practice of indirect potable reuse (IPR) that augments critical water supplies with treated wastewater to enhance water security presents complexities in water quality management. This dissertation explores the complex interplay between FSS and IPR in the Occoquan Reservoir, an important drinking-water source in the Mid-Atlantic United States, within its diverse environmental, social and political contexts. Using extensive data collected over 25 years, this research quantifies contributions of multiple salinity sources to the rising concentration of sodium (a major ion associated with the FSS) in the reservoir and the finished drinking water. These sources encompass two rapidly urbanizing watersheds, a sophisticated water reclamation facility and the drinking water treatment utility. The novel application of unsteady transit time theory reveals that stream salinization can be linked to watershed salt sources using stream water age as a master variable and provides a real-time prediction model for sodium concentration in the reservoir. These results identify substantial opportunities to mitigate sodium pollution and help set the stage for stakeholder-driven bottom-up management by improving the predictability of system dynamics, enhancing knowledge of this social-ecological system and supporting the development of collective action rules. / Doctor of Philosophy / The global rise in freshwater salinity, termed as the freshwater salinization syndrome (FSS), poses a significant threat to water quality in our freshwater resources. The practice of indirect potable reuse (IPR), which involves reusing treated wastewater to supplement and secure our water supplies presents significant challenges in managing water quality. This dissertation delves into the intricate relationship between FSS and IPR, focusing on the Occoquan Reservoir-a vital drinking water source in the Mid-Atlantic United States-within its multifaceted environmental, social, and political contexts. This study uncovers the contributions of various sources of salinity to rising sodium ion concentrations (a key FSS-associated ion) in the reservoir and in finished drinking water. Sodium ions are contributed by road salts, chemicals used in water and wastewater treatment, commercial and industrial discharges, household products (e.g., laundry detergents) and human excretion. An innovative approach of examining the age of water in the stream and in the reservoir outflow enables us to trace origins of salinity within the watershed and predict the concentration of sodium ions in the reservoir, respectively. These findings reveal promising avenues for effectively addressing sodium pollution at this site. Furthermore, this research underscores the significance of convergence research, bringing diverse stakeholders together to develop collaborative strategies to manage freshwater salinization using a bottom-up approach.