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

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 uatm) and lowest during the wet season (~6,000 uatm). 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.