GROUNDWATER AS A POTENTIAL SOURCE OF METHANE AND CARBON DIOXIDE TO A THIRD-ORDER STREAM IN THE SOUTH CAROLINA PIEDMONT

Rivers and streams are responsible for approximately half of methane (CH₄) emissions to the atmosphere from freshwater ecosystems globally. Nearly all streams and rivers are supersaturated with respect to atmospheric CH₄, though few studies have focused on temperate watersheds. Although CH₄ budgets for fluvial systems have been studied, the relative contributions of CH₄ generation within the stream channel versus groundwater inflow are not fully understood. The purpose of this study was to assess the potential for groundwater to function as a source of methane and carbon dioxide to Little Creek, a third-order stream in the Piedmont region of South Carolina. Seven shallow groundwater wells were installed in three transects perpendicular to the stream channel. Wells ranged from 2.1 m to 20.3 m distance from the stream channel, and well depths ranged from 0.7 m to 2.7 m. A total of 109 water samples were collected from the wells and from three locations along the stream itself during base flow on 6 dates during June-August 2024. Both stream water and groundwater had low ionic strengths (~10-4 mol/L) with dilute mixed cation-bicarbonate compositions. Stream pCH₄ ranged from 504 to 1,628 µatm/L. Groundwater pCH₄ ranged from 8 to 14,863 µatm/L and was nearly 5 times greater than stream pCH₄ on average. Stream pCO₂ ranged from 1,852 to 5,061 µatm/L. Groundwater pCO₂ ranged from 3,204 to 44,717 µatm/L and was over 7 times greater than stream pCO₂ on average. Both gasses were always supersaturated with respect to the atmosphere. CO₂ was the dominant carbonate species in groundwater, and HCO₃^- was the dominant carbonate species in stream water. Concentrations of both gases were more variable both spatially and temporally in groundwater than in stream water, but pCH₄ and pCO₂ were not correlated in either groundwater or stream water. pCH₄ was weakly to moderately correlated with dissolved organic carbon concentrations and weakly to moderately inversely correlated with dissolved oxygen and sulfate concentrations. These relationships indicate that groundwater CH₄ originated in saturated riparian soils with reducing conditions and available, but spatially variable, organic matter for methanogenesis. Overall, the results suggest groundwater is a potentially significant source of CH₄ in Piedmont streams.