Graduate Thesis Or Dissertation
 

The Vadose Zone as a Hyporheic Carbon Source : A Look at Temporal Trends in Hyporheic Zone pCO₂

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  • Stream chemistry studies conducted in the forested Watershed 1 of the HJ Andrews Experimental Forest show a contribution of CO2 from the hyporheic zone. Hyporheic CO₂ concentrations, measured as pCO₂, have a seasonal trend as well as a responsiveness to storm events. Concentrations are highest at the end of the dry season (~14,000 μatm) and lowest during the wet season (~6,000 μatm). Hyporheic pCO₂ has been observed in a well to respond to winter storm events with a sudden decrease followed by a sharp increase in pCO₂. The increase in pCO₂ exceeds pre-storm levels, suggesting an additional contribution of pCO₂ into the hyporheic zone. Concentrations gradually return to pre-storm levels as stream discharge decreases. I hypothesize that surplus CO₂ is flushed into the hyporheic zone from the overlying soil (vadose zone) during storm events. I tested the hypothesis by monitoring soil gas (pCO₂) at equilibrium with soil water, temperature, water table height, and soil moisture content at various depths throughout the m2 soil column of our study site. I modeled well F2 dissolved CO₂ concentrations to compare to observed F2 dissolved CO₂ concentrations to determine the contribution of CO₂ from the vadose zone to hyporheic flow during storm events. The findings from the study suggest that the connectivity from the vadose zone to the hyporheic zone is seasonal, dissolved CO₂ contribution from the vadose zone to the hyporheic zone occurs when the soil is sufficiently saturated to enable complete percolation from the vadose zone to the hyporheic zone, our study column can be described as “dry”, “transitional”, and “wet”, and prior to complete saturation the system is a complex network with unidentified boundaries.
  • KEYWORDS: Hyporheic Zone, Vadose Zone Model, CO2
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