Methane and Carbon Dioxide Relationships in Subsurface Waters : Exploring broad patterns, drivers, and constraints in groundwater and hyporheic zones

Sammanfattning

Freshwater systems are globally important sources of methane (CH4) and carbon dioxide (CO2), yet the contribution and dynamics of these greenhouse gases in groundwater and hyporheic zones remain poorly understood. Subsurface environments are often anoxic and rich in organic matter, suggesting they may support high CH4 production, but the balance between CH4 and CO2 can vary widely depending on environmental conditions. However, the relative importance of factors that promote or restrict groundwater CH4 within and across regions is unknown. Here, I compiled a database of CH4 and CO2 concentrations from groundwater and hyporheic zone samples across 18 studies. I investigated how gas concentrations and their ratio (CH4:CO2) vary with temperature, dissolved organic carbon (DOC), and terminal electron acceptors (TEAs: DO, NO3, SO4), and compared subsurface data to riverine data from the GRiMeDB database. CH4 concentrations and CH4:CO2 ratios were positively correlated with DOC and suppressed by TEAs, while temperature – often assumed to be a key driver – showed inconsistent effects on CH4, suggesting that local biogeochemical and hydrological factors may override thermal controls. Compared to rivers, groundwater exhibited significantly higher concentrations of both gases and more variable CH4:CO2 ratios, indicating greater potential for CH4 accumulation in subsurface environments. These findings emphasize the importance of lateral and vertical subsurface sediment as sources of CH4 and CO2 to surface waters and ultimately the atmosphere. Future research should expand geographic representation, include additional environmental gradients, and integrate subsurface data with surface water studies to better understand carbon transformations across the aquatic continuum.