Modeling the Impact of Biogeochemical Hotspots and Hot Moments on Subsurface Carbon Fluxes from a Flood Plain Site

Abstract:

Biogeochemical hotspots and hot moments are known to account for a high percentage of carbon and nutrient cycling within flood plain environments. To quantify the impact of these hotspots and hot moments on the carbon cycle, a 2D reactive transport model was developed for the saturated-unsaturated zone of a flood plain site in Rifle, CO. Previous studies have identified naturally reduced zones (NRZs) in the saturated zone of the Rifle site to be hotspots and important regions for subsurface biogeochemical cycling. Wavelet analysis of geochemical concentrations at the site suggested that hydrologic and temperature variations are hot moments and exert an important control on biogeochemical conditions in the Rifle aquifer.

Here, we describe the development of a reactive transport model that couples hydrologic and biogeochemical processes to microbial functional distributions inferred from site-specific ‘omic’ data. The model includes microbial contributions from heterotrophic and chemolithoautotrophic processes. We use Monod based formulations to represent biomass formation and consider energy partitioning between catabolic and anabolic processes. We use this model to explore community emergence at the Rifle site and further constrain the extent and rates of nutrient uptake as well as abiotic and biotic reactions using stable carbon isotopes.

Results from 2D model simulations with only abiotic reactions predict lower CO₂ partial pressures in the unsaturated zone and severely underpredict (~200%) carbon fluxes to the river compared to simulations with chemolithoautotrophic pathways. δ¹³C-CO₂ profiles also point to biotic sources for the locally observed high CO₂ concentrations above NRZs. Results further indicate that groundwater carbon fluxes from the Rifle site to the river are underestimated by almost 180% (to 3.3 g m^-2 d^-1) when temperature fluctuations are ignored in the simulations. Preliminary results demonstrate the emergence of denitrifiers at specific depths in the unsaturated zone. This modeling study suggests that spatially (e.g., microbial and redox zonation) and temporally discrete conditions (e.g., temperature fluctuations) can significantly contribute to carbon cycling in flood plains and hotspots and hot moments need to be appropriately represented in model simulations.