RELATIONSHIPS BETWEEN METHANE CONCENTRATIONS AND WATERSHED LAND COVER IN HEADWATER STREAMS IN THE SOUTH CAROLINA PIEDMONT

Globally, streams and rivers are important sources of methane to the atmosphere. However, the extent to which watershed land cover influences methane concentrations in streams and rivers is poorly understood. Previous research has suggested dense urban land cover in large cities is associated with increased methane concentrations in rivers. Our objective was to determine whether stream methane concentrations are correlated with watershed urbanization in the South Carolina Piedmont, a region experiencing high rates of urban land cover expansion. Grab samples were collected from 28 locations in first and second order streams in and near Greenville, SC, during summer (June-July 2024) baseflow conditions. Sample location elevations ranged from 239 to 306 m above sea level. Watershed land cover ranged from > 95% total developed to > 95% forest to > 50% agricultural (primarily pasture). Urban streams were located mostly within and near Greenville, a small city with a metropolitan area of ~670 km², a population of about 570,000, and a population density of 570 people/km². Methane concentrations in the streams ranged from 60 to 5770 𝜇atm/L, with all locations being oversaturated with respect to the atmosphere. Methane concentrations were inversely correlated with dissolved oxygen and directly correlated with turbidity. However, methane concentrations were not significantly correlated with any class of land cover. These results indicate land cover has little effect on methane concentrations in headwater streams in the Piedmont region of South Carolina, in contrast to studies of urbanized watersheds in other regions. These results also are consistent with previous research in the South Carolina Piedmont which suggested methane concentrations are largely controlled by stream and watershed geomorphology. One possible explanation is that the Greenville region does not have a high enough population density or enough dense urban land cover to significantly affect methane generation in small watersheds, as many cities have a much greater area of dense urban land cover and population densities > 3000 people/km². Alternatively, land cover may not be an important control over methane concentrations relative to geomorphologic controls and riparian soil conditions.