Groundwater discharge drives water quality and greenhouse gas emissions in a tidal wetland

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 groundwater 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 groundwater 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.