Whole-system estimation of denitrification in a plains river: a comparison of two methods

Whole-system denitrification in the South Platte River was measured over a 13-month period using an open-channel N-2 method and mass-balance measurements. Concentrations of dissolved N-2 were measured with high precision by membrane-inlet mass spectrometry and estimates of denitrification were based on the mass flux of N-2, after correction for reaeration and groundwater flux. Open-channel estimates of denitrification ranged from 0 to 3.08 g N m(-2) d(-1) and the mean annual rate was 1.62 g N m(-2) d(-1), which corresponds to removal of approximately 34% of the nitrate transported by the river over a distance of 18.5 km. Over the same period of time, estimates of denitrification based on mass-balance measurements ranged from 0.29 to 5.25 g N m(-2) d(-1) and the mean annual rate was 2.11 g N m(-2) d(-1). The two methods revealed similar seasonal patterns of denitrification the highest rates were measured from late April to August and the lowest rates were in winter. Both methods provide whole-system estimates of denitrification in running waters; where reaeration rate coefficients are low and flux of groundwater is well quantified, the open-channel method has fewer sources of uncertainty and is easier to implement.