Using Concentrations and Isotopic Compositions of CO2 to Distinguish Microbial Production of CO2 in Unsaturated Zone Sediments in Hydrogeochemical Models

Abstract:

Quantifying levels of microbial activity in unsaturated subsurface environments is an important factor for understanding exchange of greenhouse gases (e.g., CO₂, CH₄, N₂O) with the atmosphere. For CO₂, this is problematic due the variety of processes affecting its sources, fate and transport in the vadose zone (e.g., root respiration, carbonate dissolution and precipitation, exchange with pore water/groundwater and microbial production and consumption). To determine the impacts of all these processes and dependence on variables including temperature, moisture content and pH requires coupled numerical models supported by robust, long-term data sets. We are attempting to do this for a small (~90,000 m²) floodplain adjacent to the Colorado River in the town of Rifle, Colorado. The geology of the site consists of 6-8 m of Quaternary alluvium composed of quartz and feldspar sands with silts, clays, pebbles, and cobbles overlying the less permeable Tertiary Wasatch Formation. The unsaturated zone is 3-4 m thick overlying a perched aquifer above the alluvium-Wasatch contact. Depth-distributed gas samplers and suction lysimeters for pore water sampling were installed at three locations across the site in March of 2013. From April 2013 to present, soil gas samples have been collected on a bi-weekly to monthly interval and analyzed for gas concentrations and isotopic compositions. Through the first year, several significant spatial and temporal trends have emerged from the data including higher soil gas CO₂ (up to 6% near the water table) with higher δ¹³C values during the warmer summer months, possibly signifying higher levels of microbial activity. In the shallower soils, CO₂ concentrations are lower (atmospheric to 2%) with more variable δ¹³C values (ranging from -18‰ in the summer to -24‰ in the winter) likely due to slower exchange with the atmosphere during the winter when surface soils are more saturated due to snow cover and melting. Extensive data has also been collected on the concentrations and isotopic compositions of pore water organic carbon and groundwater DIC across the site in order to understand the effects of these components on carbon transport in the unsaturated zone. These data are being incorporated into a numerical model of the site that includes transport, chemical and biological reactions.