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Nitrate removal in riparian wetlands: interactions between surface flow and soils
1National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand. k.rutherford@niwa.co.nz
Journal of Environmental Quality
|July 1, 2004
Summary
Riparian wetlands with springs can effectively remove nitrate (NO3-N) through denitrification. Vertical mixing of groundwater into saturated soils enhances this removal process, challenging previous assumptions about their limited effectiveness.
Area of Science:
- Environmental Science
- Hydrology
- Soil Science
Background:
- Riparian wetlands with springs are often considered inefficient for nitrate removal due to short groundwater-soil contact times.
- This perception overlooks the potential for enhanced biogeochemical processes in saturated soil environments.
Purpose of the Study:
- To quantify residence times and nitrate (NO3-N) removal in a New Zealand hill-country riparian wetland.
- To investigate the role of vertical mixing and soil properties in nitrate transformation.
Main Methods:
- Tracer experiments using lithium bromide (LiBr) and nitrate (NO3-N) injected at the surface.
- Monitoring of surface and subsurface flow, tracer concentrations using collecting trays and shallow wells.
- Continuous tracer monitoring with logging conductivity probes.
Main Results:
- Tracer breakthrough curves indicated flow paths slower than surface flow but faster than deep seepage.
- Vertical diffusion of tracers into saturated soils (≥5 cm depth) was observed.
- 24 ± 9% of added nitrate was removed via denitrification over 1.5 m during dry conditions, with a net uptake length coefficient (K) of 0.08 ± 0.03 m⁻¹.
- Seepage flow accounted for only 7 ± 4% of nitrate removal.
Conclusions:
- Vertical diffusion and mixing in saturated riparian soils significantly enhance nitrate removal potential.
- Riparian wetlands with springs may have higher nitrate removal capacities than previously assumed.
- Further investigation into vertical mixing is crucial for assessing wetland denitrification efficiency.
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