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.