A project undertaken at The University of New South Wales, and supervised by Dr Mahmood Sadat-Noori.
Wetlands are known to play a critical role in the global carbon cycle by storing carbon and influencing greenhouse gas (GHG) emissions, such as carbon dioxide (CO₂) and methane (CH₄). However, the role of groundwater in delivering carbon and greenhouse gases to wetlands has been poorly quantified.
To address this knowledge gap, we first conducted a global review of groundwater methane concentrations and fluxes across various environments. The review found that methane concentrations in groundwater vary with climate, environmental setting, and subsurface geology. Notably, the highest average concentrations were observed in temperate estuaries and sandy aquifers. Our global assessment estimated that groundwater methane fluxes could contribute up to 70% of methane released from surface waters in certain environments. In cold regions, such as subarctic zones, methane delivered by groundwater to rivers, lakes, and wetlands may even exceed the amount emitted directly from the water surface. These findings suggest that the influence of groundwater on methane emissions has been significantly underestimated in global carbon budgets and climate models.
Figure 1. Groundwater discharge of CH4.
Building on these insights, we conducted a detailed field study in a restored wetland in the Hunter region of New South Wales (NSW), Australia, to quantify the role of groundwater in carbon transport and GHG emissions. Using water balance calculations and tracer techniques, we measured groundwater inflows and their associated carbon loads. Our analysis revealed that groundwater accounted for 100% of the dissolved organic carbon (DOC), 30% of the dissolved inorganic carbon (DIC), and 15% of the alkalinity exported from the wetland, relative to contributions from surface water. In terms of GHG emissions, groundwater contributed approximately 5% of total CO₂ losses and had only a minor role in CH₄ emissions at the site.
The restored wetland showed lower groundwater-driven fluxes of CO₂ and CH₄ compared to values reported in the literature for natural or mature restored wetlands. This suggests that wetland restoration can help reduce groundwater-driven greenhouse gas fluxes, including methane, offering a promising pathway for climate mitigation through nature-based solutions.
Figure 2. Researchers working on-site.
In summary, this project has demonstrated that groundwater is an often-overlooked source of carbon and greenhouse gases in aquatic systems, especially wetlands, but that its influence in degraded wetlands can be moderated through restoration. These findings will help refine methane emission estimates and support more effective environmental management and climate change mitigation strategies. Further, the findings have important implications for developing carbon accounting methods for wetland restoration. The support from the Foundation has been vital in enabling this innovative research, and the outcomes will continue to inform science and policy in the years ahead.