Efficacy of a denitrification wall to treat continuously high nitrate loads

Denitrification walls have been proven as an effective, long-term method for remediating nitrogen in groundwater underneath agricultural lands. Utilizing walls to provide large N load reductions requires targeting a significant portion of agricultural effluent. One approach for more efficient application of walls is to locate them adjacent to zones with high groundwater flow, although treatment efficacy in these conditions is uncertain. In this study, a large wall (168 m3) receiving high N loads was assessed using a well transect array for hydraulic and water quality evaluations and media were collected from within the wall to evaluate enzyme activity with flow distance. Porewater velocity through the wall was rapid (1.7 m day−1) with short detention times (1.7-1.9 days), yet the wall treated 100 ± 28 m3 of groundwater per day, effectively removing 228 ± 155 kg of total N per year. Maximum nitrate-N removal rates per media volume (4.9-5.5 g-N m−3 d−1) were at the upper end of published values. Rapid reduction of potential denitrification rates in media samples from 4.89 g N m−3 d−1 to undetectable within a quarter of the wall length suggests that nitrate-N depletion drove a rapid reduction in denitrifying enzymes. Based on a carbon mass balance, dissolved organic C leaching was initially the largest C export process and the longevity of total bioavailable C was estimated as 23 ± 5.9 years. These results indicate the ability of walls to reduce high N-loads over long timespans.

► Denitrification wall with continuously high nitrate loading and short detention time. ► High nitrate removal rates (4.9-5.5 g-N m−3 day−1) observed and large nitrate load reduction. ► High efficiency indicates they rapidly remove nitrate and are generally oversized. ► Greater nitrate load reductions possible with correct size and scale.