Nitrate movement and removal along a shallow groundwater flow path in a riparian wetland within a sheep-grazed pastoral catchment: Results of a tracer study

Abstract

The movement and removal of nitrate (NO3 -) along a groundwater flow path within a riparian wetland was investigated during a 24-day period in late autumn-early winter, using a lithium bromide (LiBr)-potassium nitrate (KNO3) tracer solution containing 19 200 mg/litre as Br- and 193.8 mg/litre as NO3-N. The tracer solution was added as an instantaneous dose of tracer solution at a depth of 10–20 cm to four injection wells in two 1 m2 plots within a sheep-grazed pastoral catchment at the Whatawhata Agricultural Research Centre near Hamilton, New Zealand. Bromide and NO3-N concentrations were measured periodically in: (1) wetland groundwater samples from piezometers installed at 15 and 30 cm depths and located at 30, 60, and 100 cm down gradient from the injection wells; and (2) surface flow samples. Peak concentrations of 50–250 mg/litre of Br- and 0.2–1.1 mg/litre of NO3-N were reached within 1–2 days after application at most piezometers. Nitrate concentrations decreased thereafter more sharply than did those of Br-, resulting in decreased NO3-N/Br- ratios from Days 2 through 7. More than 99% of groundwater samples collected after the tracer application had NO3-N/Br- less than the value in the tracer solution indicating removal of NO3-N during transport. Mass flux estimates indicated removal of >90% of added NO3-N along the 100 cm flow path from the injection, with essentially all of the NO3 - removed within the first 30 cm of transport. On Days 10 and 24, just after rain events, surface flow from the experimental plots had greatly elevated NO3-N concentrations that were not accompanied by correspondingly elevated Br- concentrations, indicating that NO3-N originating from the surrounding catchment was transported over the wetland surface with little penetration or mixing with wetland groundwater. Despite a significant capacity for NO3-N removal from shallow groundwater equivalent to an annualised value of 50 kg/ha in these wetland study plots, large amounts of NO3-N from the catchment are likely to move over the wetland surface during rain events without adequate soil contact time for efficient denitrification and retention. © 2002, Copyright Taylor & Francis Group, LLC.