Methane input and evasion in a hardwood forest stream: Effects of subsurface flow from shallow and deep pathways

Streams are typically supersaturated and emit methane to the atmosphere. Much of this gas appears to be imported from groundwater and riparian zones, where anoxia and methane production are common. We investigated how methane evasion and the concentration in groundwater discharge varied between five stream sections with differing rates of groundwater discharge along the east fork of Walker Branch in eastern Tennessee. Evasion and groundwater concentration were determined from measurements of methane in surface water in conjunction with coinjection of conservative solute and volatile gas tracers. Methane in surface water was supersaturated, varying from 0.67 to 1.56 mu g CH4 liter(-1), which translates to concentrations 17.6-41.4 times greater than those at atmospheric equilibrium. Methane evasion rates ranged from 0.4 to 13.2 mg CH4 m(-2) d(-1). Differences in methane concentration and evasion were related to variation in subsurface discharge and concentration in groundwater. All study sections gained flow, although the rate of subsurface discharge into the second study section (section 2) was particularly low. Furthermore, the specific conductance of groundwater flowing into section 2 averaged only 82.3 mu S cm(-1), compared with 110-125 mu S cm(-1) in the other study sections, indicating that groundwater discharge was derived from riparian soils as opposed to deeper flow from fractured bedrock. The mean concentration of 549.2 mu g CH4 liter(-1) in subsurface water flowing into section 2 was notably greater than in the other sections, where average groundwater concentration ranged from 158.9 to 376.2 mu g CH4 liter(-1). Our results suggest that subsurface flow from riparian soils appears to be the major source of methane to streams, although deeper bedrock Bow also supplies methane at a lower concentration to surface waters.