Release Timing and Duration Control the Fate of Photolytic Compounds in Stream‐Hyporheic Systems

Predicting environmental fate requires an understanding of the underlying, spatiotemporally variable interaction of transport and transformation processes. Photolytic compounds, for example, interact with both time‐variable photolysis and the perennially dark hyporheic zone, generating potentially unexpected dynamics that arise from time‐variable reactivity. This interaction has been found to significantly impact environmental fate but is commonly oversimplified in predictive models. Our primary objective was to explore how time‐variable photolysis and hyporheic storage interact across a range of photolysis rates to control the fate and transport of photolytic solutes in stream‐hyporheic systems. In this study, we released a photolytic compound in a natural system at different times of day and simulated variable release timing and durations of photolytic compounds spanning half‐lives ranging from about 2 min to 900 hr. To contextualize these results, we simulated five photolytic compounds with varied half‐lives to systematically study the interaction between release timing, duration, and reactivity. We found that the environmental fate and transport of photolytic compounds are highly variable as a function of release timing, which controls when, where, and for how long solute is stored in the hyporheic zone or exposed to in‐channel photolysis. This knowledge can be used to improve predictions for photolytic compounds or assess potential impacts for an anticipated discharge or treatment. Plain Language Summary The ways compounds are naturally transported, stored, and removed from the environment are controlled by the interactions between the unique properties of the compound and the dynamic processes within the environment. Photolytic compounds (those that decay in sunlight), for example, travel through both the stream, where time‐variable solar intensity controls removal, and the shallow groundwater in direct connection with the stream, where compounds are always shielded from solar radiation. While these interactions are known to significantly contribute to the fate of photolytic compounds, they are often oversimplified in predictive models. Through a series of field experiments and numerical models, we assessed the environmental fate of photolytic compounds across various site conditions (time of day and duration of release) and compound properties (photolysis decay rate). Ultimately, when, where, and for how long mass can be shielded from solar radiatio