Groundwater driven carbon fluxes in a restored coastal saltmarsh wetland: Implications for coastal wetland restoration

• Hydrological export of the five main carbon components from a restored saltmarsh wetland were measured. • Groundwater-driven fluxes of alkalinity, DIC, DOC, CO 2 , and CH 4 were estimated using radon. • The contribution of groundwater to surface water carbon export was determined. • SW and GW carbon exports from the restored wetland were lower than those reported for natural wetlands. Coastal wetlands play a crucial role in the global carbon cycle, yet they have been extensively degraded over the past decade. Though restoration efforts are underway worldwide, there is limited understanding of the role groundwater plays in transporting dissolved carbon within restored wetlands. Here, we address this knowledge gap by investigating water and carbon fluxes in the restored Tomago Wetlands in coastal NSW, Australia. We aim to 1) quantify surface water exports of the main carbon components, 2) estimate greenhouse gas emissions from the aquatic parts of the wetland, and 3) determine the contribution of groundwater discharge to the surface water carbon export and greenhouse gas emissions from the wetland. Sampling took place following a wet period and a freshening event. A radon mass balance model estimated groundwater discharge contributing 10 % to the surface water flow in the restored wetland. The wetland was a source of four of the five main carbon components including DOC, DIC, alkalinity, and CO 2 , with most of the exported carbon being in the form of alkalinity. Surface water DOC, DIC, alkalinity, and CO 2 exports were, 1.3 ± 0.5, 35.5 ± 13.1, 39.4 ± 14.6 and 6.5 ± 2.1 mmol/m 2 /d, respectively. The average water to atmosphere flux of CO 2 and CH 4 were 104 ± 209 (mmol/m 2 /d) and 21 ± 42 (µmol/m 2 /d). Groundwater-driven carbon fluxes contributed 100 % of the surface water DOC, 30 % of the DIC, 15 % of the alkalinity exports, and 5 % of the overall CO 2 losses (lateral aqueous export + atmospheric emissions) from the wetland, with minor contributions to CH 4 . Carbon exports from both the wetland and groundwater observed in this restored wetland were found to be lower than those reported in the literature for natural or mature wetlands. This would have important implications when developing carbon accounting methods. Overall, our findings highlight the importance of groundwater in carbon transport within restored wetlands and emphasise the need for inclusion of shallow groundwater dynamics in carbon budgets and inventories of coastal wetlands that have or will undergo restoration.