『Abstract
　We measured net nitrate retention by mass balance in a 700-m upwelling reach of a third-order sand plains stream, Emmons Creek, from January 2007 to November 2008. Surface water and groundwater fluxes of nitrate were determined from continuous records of discharge and from nitrate concentrations based on weekly and biweekly sampling at three surface water stations and in 23 in-stream piezometers, respectively. Surface water nitrate concentration in Emmons Creek was relatively high (mean of 2.25 mg NO3-N l^-1) and exhibited strong seasonal variation. Net nitrate retention averaged 429 mg NO3-N m^-2 d^-1 and about 2％ of nitrate inputs to the reach. Net nitrate retention was highest during the spring and autumn when groundwater discharge was elevated. Groundwater discharge explained 57-65％ of the variation in areal net nitrate retention. Specific discharge and groundwater nitrate concentration varied spatially. Weighting groundwater solute concentrations by specific discharge improved the water balance and resulted in higher estimates of nitrate retention. Our results suggest that groundwater inputs of nitrate can drive nitrate retention in streams with high groundwater discharge.

Keywords: Focused discharge; Groundwater; Mass-balance; Nitrogen; Sediments』

Introduction
Methods
　Site description
　Surface water stations and piezometers
　Nutrient sampling - surface water
　Nutrient sampling - groundwater
　Hydrology - surface water
　Hydrology - groundwater
　Nutrient analysis
　Mass balance model
Results
　Hydrology
　Nutrient concentrations
　Nitrate retention and flux
Discussion
　Hydrology
　Nitrate retention
　Mechanisms
　Stream ecosystem comparison
　Nitrate time series
　Implications for nitrogen removal from ecosystems
Acknowledgements
References