Wetlands play an important role in the global carbon cycle as they can be sources or sinks for greenhouse gases.Groundwater discharge into wetlands can affect the water chemistry and act as a source of dissolved greenhouse gases,including CO_(2)and CH_(4).In this study,surface water quality parameters and CO_(2)and CH_(4)concentrations were evaluated in a tidal wetland(Hunter Wetlands National Park,Australia)using time series measurements.Radon(^(222)Rn),a natural ground water tracer,was used to investigate the role of groundwater as a pathway for transporting dissolved CO_(2)and CH_(4)into the wetland.In addition,water-to-air CO_(2)and CH_(4)fluxes from the wetland were also estimated.The results showed a high concentration of radon in wetland surface water,indicating the occurrence of ground water discharge.Radon concentration had a strong negative relationship with water depth with a determination coefficient(R^(2))of 0.7,indicating that tidal pumping was the main driver of groundwater discharge to the wetland.Radon concentration also showed a positive relationship with CO_(2)and CH_(4)concentrations(R^(2)=0.4 and 0.5,respectively),while the time series data revealed that radon,CO_(2),and CH_(4)concentrations peaked concurrently during low tides.This implied that groundwater discharge was a source of CO_(2)and CH_(4)to the wetland.The wetland had an average water-to-air CO_(2)flux of 99.1 mmol/(m^(2)·d),twice higher than the global average CO_(2)flux from wetlands.The average CH_(4)flux from the wetland was estimated to be 0.3 mmol/(m^(2)d),which is at the higher end of the global CH_(4)flux range for wetlands.The results showed that groundwater discharge could be an important,yet unaccounted source of CO_(2)and CH_(4)to tidal wetlands.This work has implications for tidal wetland carbon budgets and emphasizes the role of groundwater as a subsurface pathway for carbon transport.
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