Measurement of denitrification in rivers: an integrated, whole reach approach

Rivers are believed to play an important role in nitrogen removal via denitrification. Unfortunately, there are few data quantifying these processes in situ, primarily due to methodological constraints. We have developed a new approach for estimating denitrification in rivers at the whole reach scale and have applied this approach to three small rivers, the Millstone River in central New Jersey, and the Iroquois River and Sugar Creek in northwest Indiana–northeast Illinois (USA). The approach is based on measuring the change in dissolved N₂ concentration as a parcel of water moves downstream. Two volatile, non-reactive tracers (propane and isobutane) were co-injected, and the rate of change in the ratio of these gases was used to calculate a first-order transfer rate of N₂ (K_N2) to correct for loss of the gas to the atmosphere. Nitrogen removal via denitrification ranged between 0.27 ± 1.21 mmol N m^–2 h^–1 in Sugar Creek during May 2000 and 15.81 ± 2.51 mmol N m^–2 h^–1 in the Millstone River during March 2001. This approach could permit testing of factors that are believed to control denitrification at the reach scale, such as nitrate concentration, discharge, temperature, and water residence time, and could provide a clearer picture of nitrogen transformations in rivers.