OCCURRENCE AND FATE OF DISSOLVED METHANE WITHIN THE CARBONATE CRITICAL ZONE OF NORTH-CENTRAL, FLORIDA

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.