Carbon dioxide dynamics driven by groundwater discharge in a coastal floodplain creek

Dissolved carbon dioxide (CO₂) may be highly enriched in groundwater. However, the contribution of groundwater discharge as a source of CO₂ to rivers, estuaries and coastal waters is poorly understood. We performed high resolution measurements of radon (²²²Rn, a natural groundwater tracer) and the partial pressure of CO₂ (pCO₂) in a highly modified tidal creek and estuary (North Creek, Richmond River, New South Wales, Australia) to assess whether CO₂ in surface waters was driven by groundwater discharge. A spatial survey revealed increasing ²²²Rn activities (up to 17.3 dpm L⁻¹) and pCO₂ (up to 11,151 μatm) in the upstream direction. The enrichment occurred in a drained coastal acid sulphate soil wetland upstream of a mangrove forest. Time series experiments (24-h) were performed at two stations upstream and downstream of the pCO₂ enrichment area. Upstream measurements demonstrated a significant correlation between pCO₂ and ²²²Rn while downstream values resulted in a significant inverse relationship between pCO₂ and dissolved oxygen apparently as a result of respiration in nearby mangroves. Measurements taken 2 days after a 245 mm precipitation event revealed the highest recorded ²²²Rn activities (up to 86.1 dpm L⁻¹) and high pCO₂ (up to 11,217 μatm), showing a strong groundwater influence after flooding. These observations imply that groundwater discharge drove CO₂ dynamics at the upstream station while multiple complex processes drove CO₂ at the downstream station. A ²²²Rn mass balance model demonstrated that groundwater discharge accounted for about 76% of surface water in this floodplain creek. The CO₂ evasion rates (799 ± 225 mmol m⁻² d⁻¹) were driven primarily by currents rather than wind. Groundwater-derived CO₂ fluxes into the creek averaged 1622 mmol m⁻² d⁻¹, a value twice as high as atmospheric CO₂ evasion and consistent with carbon uptake within the creek and downstream exports. These results demonstrate that groundwater seepage was a major factor driving CO₂ evasion to the atmosphere from the creek. Groundwater discharge should be accounted for in CO₂ budgets in coastal systems.