Abstract
The thin layer between rivers and their underlying sediments is one of the most reactive regions in freshwater systems. Predicting the fate of important waterborne materials, such as nutrients and contaminants of emerging concern, therefore requires a mechanistic understanding of how mass moves and reacts within this thin layer. I will highlight research that addresses three key features of rivers that make such predictions challenging: high heterogeneity, scaling, and hydrologic variability.
I will first present new theory that reveals how distinct features of the hyporheic zone control exposure to contaminants from the local to the whole-river scale. Results highlight two dominant controls of exposure times to contaminants. First, while the shallow benthic biolayer in the subsurface is very effective at degrading stream-borne contaminants, compounds that propagate below this layer are sequestered in deeper locations that are less reactive and slower mixing. Back-diffusion from deeper locations causes exposure times to persist for orders of magnitude longer than would be predicted by classical reactive transport models. Second, predictions must account for the persistence of daughter products that can be as toxic as their parent compounds, and which can extend exposure times by over 100% in porewaters.
I will conclude by presenting a recently developed lumped model that predicts time-varying discharge in rivers, which is an important step for relaxing the uniform flow assumption plaguing most analytical reaction models. The new model reveals that a tug-of-war between two fundamental processes—water uptake by vegetation, and water storage within hillslopes—determines how temporal patterns of precipitation are translated to temporal patterns of river flow. Counterintuitively, many of the 671 streams studied show greater variability of river flow than variability of precipitation. We find that this amplifying effect of catchments is likely to strengthen in response to climate change.
Bio
Dr. Kevin Roche joined Boise State University in Fall 2020. While his research has spanned disciplines ranging from fluid mechanics to microeconomics, it is unified by a need for improved predictive models that respect the natural variability of hydrologic processes. His work involves a combination of (1) novel observations at scales ranging from the laboratory (mm – m) to the field (m – km); and (2) developing mechanistic models that establish a parsimonious link between these scales. He uses this combined experimental and modeling approach to improve the physical basis of stream- and watershed-scale models of contaminant and nutrient fate.
Prior to Boise State, Dr. Roche was a Fulbright Junior Scholar at the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) in Barcelona, Spain, where he developed theory of reactive transport in rivers. He also worked as a postdoctoral scholar at the University of Notre Dame, where he worked on projects ranging from field experiments in streams to theoretical modeling of climate change and conflict.
Dr. Roche received his B.S. in chemical engineering from Purdue University. After undergraduate studies, Dr. Roche worked as a process control engineer at Eli Lilly and Company (Indianapolis, IN), where he improved automation to increase water efficiency in pilot-scale manufacturing facilities. He then served for three years in the US Peace Corps (Guinea, Costa Rica) before joining Northwestern University for his graduate studies.