Five years of nitrate removal, denitrification and carbon dynamics in a denitrification wall

Denitrification walls are a useful approach for removing nitrate from shall ow groundwater, but little is known about the sustainability of nitrate rem oval, which is dependent on the continued supply of organic carbon to denit rifying bacteria. To address this question, we monitored nitrate removal, d enitrification and carbon dynamics in a pilot-scale denitrification wall fo r 5 yr. The wall continuously removed more than 95% of the incoming nitrate in groundwater, which ranged from 5 to 15 mg N L-1. We did not detect decr eases in total carbon during the 5-yr study. Available carbon declined for the first 200 days after the wall was constructed but has since remained re latively constant. While microbial biomass has varied between 350 and 550 m ug C g(-1) there was no downward trend, suggesting that carbon availability was not limiting the size of the microbial population. However, there was a large decrease in denitrifying population, as indicated by declines in de nitrifying enzyme activity. Despite this decrease, denitrification rates ha ve remained high enough to remove nitrate from groundwater and denitrificat ion was limited by nitrate rather than by carbon. Our data demonstrates tha t there was sufficient available carbon in this denitrification wall to sup port denitrification and nitrate removal for at least 5 yr. (C) 2001 Elsevi er Science Ltd. All rights reserved.