GSA Connects 2021 in Portland, Oregon

Paper No. 235-9
Presentation Time: 3:55 PM


OBERHELMAN, Andrew, MARTIN, Jonathan and FLINT, Madison, Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611-2120

Methane (CH4) is a potent greenhouse gas with a 20-year global warming potential 84 times that of CO2 and a common solute in groundwater. The magnitude, range, and atmospheric flux of groundwater CH4 remain poorly understood, particularly for the carbonate critical zone (CCZ) where hydrology permits the rapid exchange of water and solutes between surface and subsurface. To assess CH4 consumption and production in the CCZ, how hydrologic conditions affect spatial and temporal concentration variations, and potential impacts on the global CH4 cycle, CH4 concentrations and 13C isotopes were measured in water samples (n = 144) from 11 springs, a sink-rise stream system, and wells in the karstic Upper Floridan Aquifer (UFA) of North-Central Florida. Sampled waters have a range of interactions with surface drainages, subsurface residence times, and flow paths through the aquifer. Dissolved CH4 concentrations ranged from 0 to 89 μM with the highest median values in well (0.72 μM) and surface (0.34 μM) waters. Spring water concentrations were less than well water indicating CH4 oxidation or evasion in the aquifer. Springs showed increasing median dissolved CH4 in systems with greater surface water interaction and shorter subsurface residence time. Most springs had median CH4 concentrations greater than expected from atmospheric equilibrium (0.01 μM); the exceptions were four springs that discharged water with the oldest apparent ages of decades. At the stream sink-rise system, CH4 exhibited a significant reduction in concentration and 13C enrichment (p<0.02) between the sink and resurgence that cannot be explained by source water mixing or evasion, suggesting CH4 is lost to methanotrophy. A more complex dynamic was observed at a spring that periodically reverses flow and receives CH4-rich stream water during high stage. Concentrations and 13C isotopes of CH4 indicate consumption soon after reversal, followed by a short period of production before return to baseflow concentrations (~0.03 μM) 48 days after reversal. Despite indications of CH4 consumption in groundwater and surface water, many UFA springs are oversaturated with CH4 or show signs of production and thus would represent atmospheric sources. Consequently, CCZs appear to play a role in CH4 cycling, but if the cycle results in a net source or sink remains unclear.