Methane dynamics in the Willamette River, Oregon, U.S.A.

Weekly monitoring of dissolved methane (CH4) at two sites along a 2 km-stretch of the Willamette River (Oregon) between October 2008 and November 2010 revealed persistent supersaturation (24-170x atmospheric equilibrium). The concentration difference (ΔCH4: 0 to 200 nM) between the two sites varied inversely with river flow throughout the year, with an ‘excess’ of ≤125% observed at the downstream site. Groundwater input is the likely source of the ‘excess’. Quasi-synoptic studies of spatial trends in summer (2010, 2011) revealed a steady decrease in methane along a 12 km river stretch downstream of the time series sites. Estimated loss due to air-water exchange for this stretch was ~9x greater than perceived net loss, consistent with regionally widespread groundwater input of methane. Bi-weekly dissolved nutrient measurements indicated a distinct nitrate source also exists between the upstream and downstream time series sites. The ‘excess’ for nitrate and methane correlated inversely, with the greatest nitrate supply corresponding to periods of high rainfall in winter and highest river flow. Although groundwater input is also the probable source for nitrate, two different mechanisms are invoked to account for the distinct seasonal patterns. Seasonal seepage of rainwater-saturated soils (shallow groundwater recharge) explains the nitrate input while hyporheic exchange of groundwater from a persistent deep aquifer explains the methane input. Improved understanding of groundwater exchange dynamics in the Willamette River will clarify the influence of human activities on river biogeochemistry and help to better constrain the magnitude of methane and other greenhouse gas fluxes associated with inland waters.