Nitrate retention in a sand plains stream and the importance of groundwater discharge
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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.