Unveiling Causal Links, Temporal Patterns, and System-Level Dynamics of Freshwater Salinization Using Transit Time Distribution Theory

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.