Long-Term Nitrate Removal In a Denitrification Wall.

Denitrification walls are a low-cost approach for removing excess nitrate (NO₃⁻) from shallow groundwater.  Denitrification walls need to be maintenance-free for a number of years to remain cost effective, but little is known about the longevity of these walls.  In this study, a denitrification wall constructed on a New Zealand dairy farm in 1996 was monitored to determine NO₃⁻ removal by the wall 14 years after installation.  The farm has been spray-irrigated with effluent from the nearby dairy factory for over 30 years.  The NO₃⁻ input to the wall had decreased since first constructed from an average of 9 mg N L^-1 to 2.6 mg N L^-1 in year 14, which was attributed to a change in upslope irrigation practices on the farm. After 14 years, the denitrification wall removed 92% of NO₃⁻ input.   Denitrifying enzyme activity (DEA) remained high after 14 years at 695 ng N g^-1 hr^-1 (compared to 740 ng N g^-1 hr^-1 in year 1) and the wall remained NO₃⁻ limited.  However, total C in the wall has decreased by half, from an average of 4.7 g C kg^-1 soil in year 1 to 2.2 g C kg^-1 soil in year 14.  By applying a first order decay curve to the total C data, it was determined that total C in the denitrification wall would not be depleted for 66 years, but it is unclear at what amount of total C that denitrification would become limited.  This long-term study suggested that denitrification walls are cost effective solutions for remediating groundwater NO₃⁻ pollution, as they can be effective for a number of years without any maintenance.