Deciphering Controls on Nutrient and Contaminant Migration Within Floodplains: The Critical Role of Redox Environments on Metal-Organic Complexes

Authors

Christian Dewey^1,2, Rene Boiteau², Marco Keiluweit³, Cam Anderson³, Hannah Naughton¹, Scott Fendorf¹* (Fendorf@stanford.edu)

Institutions

¹Stanford University, Stanford, CA; ²Oregon State University, Corvallis, OR; ³University of Massachusetts, Amherst, MA

Abstract

Whether of natural or anthropogenic origin, the fate and transport of metal nutrients and contaminants in soils and sediments is controlled by a complex network of biogeochemical reactions coupled with hydrologic processes. Dissolved organic matter (DOM) exerts a major control on metal mobility in surface and subsurface systems, albeit one that is poorly understood. Divergent OM transformation pathways drive variation in the chemical composition of DOM across watersheds. Yet, how this variation influences the functional composition and metal binding properties of DOM, along with the fate of metals and carbon, remains largely unexplored.

The overarching goal of the project is to determine the effect of redox conditions resulting from differing hydrologic regimes on formation and transport of metals and carbon. To meet the research goal, researchers used a combination of field measurements and laboratory experiments to examine the relationships between redox conditions, functionality of dissolved organic matter, and metal speciation. Continuous monitoring of floodplain biogeochemical conditions through climatic extremes at East River, along with a newly developed analytical method, provides a unique look at metal-OM complexes. To fractionate and quantify unknown metal-organic complexes, the team developed a novel LC-ICP-MS approach, which enabled quantitation of multiple metals bound to chemically distinct fractions of DOM. Researchers also paired field experiments with multiomics (metabolomics and metatranscriptomics) techniques to examine the fate of nutrients and contaminants and compare seasonal flooding impacts in extreme low and high river discharge years, foreshadowing climate change projections. During flooded periods, microbial reduction of iron mobilized previously mineral-bound organic carbon, enhancing export of dissolved organic carbon (DOC). At the same time, flooding decreased carbon dioxide (CO₂) production and selectively preserved chemically reduced organic solutes due to metabolic constraints on microbial respiration. The onset of low-water conditions leads to re-oxygenation of floodplain soils, resulting in entrapment of DOC by newly precipitated iron minerals. Hydrologic extremes were dominated, however, by the beaver activity and the formation of a beaver dam during low water conditions, resulting in divergent water flow that yielded enhanced nitrogen removal and carbon preservation.

This work is advancing a process-based understanding of nutrient and contaminant fate and transport within watersheds, focusing principally on the dynamic hydrologic states of riparian zones. Ultimately, this work is helping to advancing a robust predictive understanding of how hydrologic changes in watersheds affect water quality and element cycling, including carbon and metals.