Xuehang Song, Xingyuan Chen, John M. Zachara, Jesus D. Gomez-Velez, Pin Shuai, Huiying Ren, Glenn Hammond

Residence Time Distributions (RTDs) exerts an important control on biogeochemical translations in watershed systems. RTDs tend to follow time-invariant exponential, lognormal, or heavy-tailed RTDs that have power-law behaviors for long tails in headwater or low-order streams. However, there is increasing recognition that RTDs can be more complicated and time-variable in response to dynamic hydrological forcing. In this study, we use particle tracking to estimate RTDs along the Hanford Reach of the Columbia River and to quantify the influences of river stage fluctuations on RTDs and biogeochemical reaction potentials. Particle tracking is conducted using the velocity fields from high-resolution three-dimensional groundwater flow simulations. The effects of dynamic hydrological forcing on the RTDs were evaluated by applying time-varying river flow boundary conditions and continuously releasing particles in different time windows. Our results revealed that dynamic stage fluctuations created rapidly changing losing-gaining conditions in the river, which led to highly transient RTDs and resulted in multiple modes of RTDs. Dam-induced high-frequency (sub-daily) flow variations increased the fraction of short (sub-daily) residence times of the RTDs. Deviation of the reactant consumption under the single-mode assumption compared to the multimodal RTDs was relatively small (~5%) and the maximum deviation appeared when the Damköhler number was close to one. Dam-induced high-frequency stage variations potentially increase the biogeochemical reactions by 27%. These findings suggest that current large-scale hydrobiogeochemical models (reach to basin scales) could be improved by accounting for dynamic hydrologic exchange flows and associated transient RTDs influenced by both short- and long- term river stage fluctuations.