Methane dynamics in the Willamette River, Oregon

Weekly monitoring of dissolved methane (CH4) at two sites along an similar to 2-km stretch of the Willamette River (Oregon) between October 2008 and November 2010 revealed persistent supersaturation (24-170X atmospheric equilibrium). The CH4 concentration difference (Delta CH4: 0-200 nmol L-1) between the two sites varied inversely with river flow, which ranged from 125 m(3) s(-1) to 1500 m(3) s(-1) over the time series. At the downstream site, an 'excess' of <= 125% was observed, with groundwater input being the likely CH4 source. Quasi-synoptic studies of spatial trends in summer (2010, 2011) revealed steady CH4 decrease along a 12-km river stretch downstream of the time-series sites. The estimated loss due to air-water exchange for this stretch was similar to 9X greater than the perceived net loss, consistent with regionally widespread groundwater input of CH4. Bi-weekly dissolved nutrient measurements indicated that a distinct nitrate (NO3-) source also existed between the upstream and downstream time-series sites. The excesses of NO3- and CH4 were inversely correlated, with the greatest NO3- supply corresponding to periods of high rainfall in winter and highest river flow. Although groundwater input is also the probable source of NO3-, seasonal seepage of rainwater-saturated soils (shallow groundwater recharge) explains the NO3- input, while hyporheic exchange with a persistent deep aquifer best explains the CH4 input. Improved understanding of groundwater input and exchange dynamics in the Willamette River will clarify the influence of human activities on river biogeochemistry and help to better constrain the magnitude of CH4 and other greenhouse gas fluxes associated with inland waters.