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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Nitrate Removal in Riparian Wetlands

Interactions between Surface Flow and Soils

National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand

^* Corresponding author (k.rutherford{at}niwa.co.nz).

Received for publication May 14, 2003. Riparian wetlands containing springs are thought to be ineffective at removing nitrate because contact times between the upwelled ground water and the underlying microbially active soils are short. Tracer experiments using lithium bromide (LiBr) and nitrate (NO₃–N) injected at the surface were used to quantify residence times and NO₃–N removal in a riparian swale characteristic of New Zealand hill-country pasture. An experimental enclosure was used with collecting trays at the downstream end to measure flow and concentration, shallow wells to measure subsurface concentrations, and an array of logging conductivity probes to monitor tracer continuously. The majority of added tracer reached the outlet more slowly than could be explained by surface flow, but more quickly than could be explained by Darcy seepage flow. There was evidence from the wells of tracer diffusing vertically to a depth of at least 5 cm into the surface soil layer, which was permanently saturated and highly porous. During dry weather 24 ± 9% of added NO₃–N was removed over a distance of 1.5 m largely by denitrification. The net uptake length coefficient for this wetland (K = 0.08 ± 0.03 m^–1) is slightly higher than the range (K = 0.01–0.07 m^–1) measured in a small stream channel infested with macrophytes. Nitrate removal is expected to decrease with increasing flow. Seepage flow is estimated to have removed only 7 ± 4% of the added NO₃–N and we hypothesize that vertical diffusion substantially increases NO₃–N removal in this type of wetland. Riparian wetlands with springs and surface flows should not be dismissed as having low NO₃–N removal potential without checking whether there is significant vertical mixing.

Abbreviations: DEA, denitrification enzyme activity • SG, specific gravity

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