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. After 14 years, the denitrification wall removed 92% of NO3− input, which ranged from 2.2 to 3.7 mg N L−1. The NO3− input to the wall had decreased since first constructed, which was attributed to a change in upslope irrigation practices on the farm. Denitrifying enzyme activity (DEA) remained high after 14 years and the wall remained NO3− limited. However, total C and microbial biomass C in the wall had decreased by approximately half, while available C remained relatively constant since year 2. By applying a first order decay curve, 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.