Effect of Hydrological Forcing on the Biogeochemical Transformation of Carbon and Greenhouse Gas Emissions in Riparian and Streambed Sediments

Authors

Martial Taillefert1* (mtaillef@eas.gatech.edu), Anthony Boever1, Chloe Arson1, TingTing Xu1, Chengwu Jiang1, Thomas J. DiChristina1, Tianze Song1, Daniel I. Kaplan2, Kenneth M. Kemner3, Christa Pennacchio4, Stephen J. Callister5

Institutions

1Georgia Institute of Technology, Atlanta, GA; 2Savannah River National Laboratory, Aiken, SC; 3Argonne National Laboratory, Lemont, IL; 4DOE Joint Genome Institute, Berkeley, CA; 5Environmental Molecular Sciences Laboratory, Richland, WA

Abstract

Hydrological processes in riparian and hyporheic sediments create strong biogeochemical gradients and redox microniches that are metabolically influenced by temporal changes in precipitation, temperature, and stream discharge. The complex temporal and spatial variability of these processes and their effect on the transformation and exchange of carbon, nutrients, and greenhouse gases (GHGs) with surface waters are difficult to account for in reactive transport models. Reactive transport in these systems is traditionally simulated on the continuum scale using upscaled empirical parameters that are not able to reproduce the effect of biogeochemical reactions on pore scale heterogeneities and their feedback on biogeochemical rates. In this project, state-of-the-art in situ physical and geochemical measurements were combined with metaomic signals of the active microbial populations in riparian and hyporheic sediments of Steed Pond at Savannah River National Laboratory to predict the role of hydrological forcing on the spatiotemporal transformation of carbon, nutrients, redox processes, and GHG emissions along this gaining and losing wetland stream. Two in situ electrochemical systems and a suite of monitoring wells were deployed at two locations along Steed Pond to monitor the temporal variations in pore water redox biogeochemistry at four different depths and relate these changes to the local hydrology. Simultaneously, sediment samples were collected at each site under different hydrological conditions to extract DNA, RNA, and proteins for in situ multiomic analyses that will shed light on the microbial metabolic processes affected by hydrological changes. In parallel, homogenization models were developed to calculate the stiffness and diffusivity tensors of porous media with thin ellipsoidal flow paths and small spherical voids. Upscaling was extended to permeability to predict the effect of reactive flow on the microstructure and physical properties of sediments subjected to hydraulic forcing. Reactive flow was then simulated in one dimension with the open-source finite element software PFLOTRAN for simplified problems. PFLOTRAN will be used to validate the homogenization models and solve large-scale boundary value problems by assigning a homogenized constitutive law with spatially variable parameters sampled from the field to the sediment. Along with the high spatial and temporal resolution of biogeochemical processes, the developed numerical models will predict how variations in hydrological forcing, competition between microbial metabolic processes, and porosity changes associated with biogeochemical feedback affect carbon and nutrient cycling as well as GHG emissions.