The evasion of CO2 from surface waters to the atmosphere represents a significant flux within the terrestrial carbon cycle. However, due to the intensive nature of quantifying evasion rates across relevant spatial and temporal scales, the dynamics controlling these rates remain largely uncharacterized. Headwater streams, in particular, represent a major uncertainty in global CO2 evasion budgets and may contribute a substantial and underestimated portion of global fluxes due to higher in-stream turbulence and elevated pCO2 that is more reflective of subsurface environments. This project will develop a dataset of observations of ground- and stream water geochemistry at existing stream and groundwater monitoring sites in the Harvard Forest, in order to develop and apply reactive transport models of carbon transport and transformation in connected surface waters. The goal of this project, through the evaluation of testable hypotheses, will be to quantify the relative source/sink contributions to stream CO2 from groundwater inputs, water column metabolism, and respiration within the hyporheic zone. This will be accomplished using a combination of:
(1) Installing stream sensors (dissolved oxygen, pH, pCO2) coinciding with existing stream hydrologic sensor monitoring sites currently maintained by Dr. David Boutt’s group
(2) Regular surface and groundwater sample collection across the Nelson Brook and Arthur stream networks. Groundwater samples will be taking from existing monitoring wells maintained by Dr. David Boutt’s group
(3) Future tracer release experiments involving injection of a reactive tracer dye (resazurin) at non-toxic ~102 ppb concentrations along with NaCl and inert Ar gas to track in-situ stream corridor respiration, transport rates, and gas transfer velocities
(4) Reactive transport modeling of stream carbon transport and transformation based on collected data.