33 Undergraduate Research III (Posters)

Friday, 21 March 2025
8:00 AM - 12:00 PM
Shenandoah ABC

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

Paper number: 33-2
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 57, No. 2

doi: 10.1130/abs/2025SE-407521

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