Linking hyporheic flow and nitrogen cycling near the Willamette River - a large river in Oregon, USA

Several approaches were used to characterize ground water/surface water int eractions near the Willamette River - a large (ninth order) river in Oregon , USA. 4 series of potentiometric surface maps demonstrated the presence of highly dynamic hydraulic gradients between rivers and the adjacent aquifer . Hyporheic zone gradients extended on the order of hundreds of meters. Riv er gains and losses at the river stretch scale (tens of kilometers) were co nsistent with fluxes implied by the potentiometric surface maps, and appare ntly reflect regional ground water/surface water interactions. Gains and lo sses of up to 5-10% of streamflow were observed at this scale. On the river reach scale (1-2 km), gains and losses on the order of 5% of streamflow we re interpreted as representing primarily local hyporheic exchange. Isotopic and chemical data collected from shallow hyporheic zone wells demo nstrated interaction between regional ground water and river water. The ori gin of sampled hyporheic zone water ranged from a mixture dominated by regi onal ground water to water containing 100% river water. The common assumpti on that ground and river water mix primarily in the river channel is not ap plicable in this system. Isotopic and chemical data also indicated that sig nificant (nearly complete) vegetative nitrate uptake and/or nitrate reducti on occurred in water from 4 of 12 hyporheic zone sites. Ln these cases, it was primarily nitrate transported to the hyporheic zone in regional ground water that was removed from solution. Isotopes of water and nitrate indicat ed that hyporheic zone water sampled at two sites was composed of water ori ginating as river water and demonstrated that significant vegetative nitrat e uptake and nitrate reduction occurred along these hyporheic zone flowpath s. Thus, the hyporheic zone may, in some instances, serve to remove nitrate from river water. Additional investigations with chemical tools and microb ial enzyme assays were conducted at one hyporheic site. A strong vertical r edox gradient was observed, with nitrate-limited denitrification potential in deeper sediment and both nitrification and denitrification potential in shallower sediment. Since nitrogen cycling is strongly affected by redox co nditions, nitrogen cycling in the hyporheic zone of this large-river system likely is affected by dynamics of ground water/surface water interactions that control fluxes of nitrogen and other redox species to hyporheic zone s ediment. Published by Elsevier Science B. V.