PNNL

Abstract

Riverbeds can have an important impact on large-scale fluxes of biogeochemically active solutes in river corridor systems. The hyporheic zone is an important area of both hydrology and biogeochemistry, particularly in a hydropeaked river system where rapid variations in river stage height, hydraulic head gradients, and residence times occur. We measured several biogeochemical and hydraulic parameters at three different subsurface depths in the riverbed of the Columbia River in Washington State, a complex hydropeaked river system with significant subsurface heterogeneity. During the study, episodes of significant dissolved oxygen (DO) change were observed. The DO signal changes were the most apparent, compared to more modest changes in other biogeochemical markers. While DO variations are often associated with changing rates of biological activity, we ultimately found that the notable DO excursions were associated with hydraulic head gradients. We observe two distinct DO responses to hydraulic head gradient perturbations, one linear, and one nonlinear and hysteretic. The system tips abruptly from one response to the other, exhibiting threshold behavior that is uncaptured in current estimations of cumulative influences of subsurface processes from reach to earth system scales. The existence of tipping point behavior in this system suggests that it may be necessary to carry out a short high-frequency monitoring campaign to asses any system for such complex responses prior to embarking on a long term, low-frequency campaign.