Effects of Sediment Porosity and Particulate Organic Carbon on Fe, S and U Cycling in Naturally Reduced Zones (NRZs) of a Contaminated Aquifer

Morris E. Jones (1) , Noémie Janot (2) , John R Bargar (2) , Scott Fendorf (1)

1 ESS - Department of Earth System Science [Stanford]
2 SSRL SLAC - Stanford Synchrotron Radiation Lightsource

Previous studies have illustrated the importance of Naturally Reduced Zones (NRZs) within saturated sediments as a source of reduced organic compounds and hydrogen, providing electron donor for subsurface microbial respiration. NRZ’s are typically characterized by low permeability and elevated concentrations of organic carbon and trace metals. However, both the formation of NRZs and their importance to the overall aquifer carbon remineralization is not fully understood. Within NRZs the hydrolysis of particulate organic carbon (POC) and subsequent fermentation of dissolved organic carbon (DOC) to form low molecular weight dissolved organic carbon (LMW-DOC) provides electron donors necessary for the respiration of Fe, S, and in the case of the Rifle aquifer, U. Rates of POC hydrolysis and subsequent fermentation have been poorly constrained and rates in excess and deficit to the rates of subsurface anaerobic respiratory processes have been suggested. In this study, we simulate the development of NRZ sediments in diffusion-limited aggregates to investigate the physical and chemical conditions required for NRZ formation. Effects of sediment porosity and POC loading on Fe, S, and U cycling on molecular and nanoscale are investigated with synchrotron-based Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) and Fourier Transform Infrared spectroscopy (FTIR) are used to characterize the transformations in POC and DOC. Sediment aggregates are inoculated with the natural microbial biota from the Rifle aquifer and population dynamics are monitored by 16S RNA analysis. Overall, establishment of low permeability NRZs within the aquifer stimulate microbial respiration beyond the diffusion-limited zones and can limit the transport of U through a contaminated aquifer. However, the long-term stability of NRZs and the colocated U is unknown and requires further study.

Domaines: Géochimie

Morris E. Jones, Noémie Janot, John R Bargar, Scott Fendorf. Effects of Sediment Porosity and Particulate Organic Carbon on Fe, S and U Cycling in Naturally Reduced Zones (NRZs) of a Contaminated Aquifer. 23rd Annual V.M. Goldschmidt Conference, Aug 2013, Firenze, Italy. ⟨hal-02537145⟩