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

Abstract

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