Modeling the Biogeochemical Response of a Flood Plain Aquifer Impacted By Seasonal Temperature and Water Table Variations

With the overarching goal of understanding the impacts of climate and land use changes on carbon and nutrient cycles, we are developing a reactive transport model that couples hydrologic and biogeochemical processes to microbial functional distributions inferred from site-specific 'omic' data. The objective of the modeling approach is to simulate changes in carbon and nutrient fluxes and aquifer biogeochemistry over longer time periods due to changes in climate and/or land use, while also considering shorter time periods in which water table fluctuations and temperature variations are important. A 2-D reactive transport model has been developed for the unsaturated-saturated zone of the Rifle site, CO, an alluvial aquifer bordering the Colorado River. Modeling efforts focus on the April through September 2013 time frame that corresponds to the spring snow melt event that lead to an approximately 1 meter rise in the water table followed by a gradual lowering over 3 months. Temperature variations of as much as 10ºC are observed at shallow depths (< 1m), while at least some temperature variation (1ºC) occurs as deep as about 7m. A field survey of the microbial populations indicates the presence and activity of chemo(litho)autotrophic bacteria within the saturated zone of the alluvial aquifer. Model simulations are used to quantify the release of carbon dioxide and consumption of oxygen via abiotic pathways and heterotrophic microbial oxidation of reduced species (Fe(II), S(-2)) and minerals (pyrite). Results indicate that the observed oxygen profiles and/or carbon fluxes cannot be matched by considering abiotic reactions alone. The importance of including microbial contributions from chemo(litho)autotrophic processes (e.g., ammonia, sulfur and iron oxidation) is supported by both field observations and model simulations. Important conclusions from the study are to: (1) include microbially-mediated processes and contributions from the unsaturated zone, and (2) account for seasonal temperature changes to accurately represent lateral and vertical delivery of water and nutrients as well as biogeochemical transformations within the Rifle Flood Plain system.