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

Denitrification walls are a useful approach for removing nitrate from shallow groundwater, but little is known about the sustainability of nitrate removal, which is dependent on the continued supply of organic carbon to denitrifying bacteria. To address this question, we monitored nitrate removal, denitrification and carbon dynamics in a pilot-scale denitrification wall for 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 decreases 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 relatively constant. While microbial biomass has varied between 350 and 550 microg 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 denitrifying enzyme activity. Despite this decrease, denitrification rates have remained high enough to remove nitrate from groundwater and denitrification was limited by nitrate rather than by carbon. Our data demonstrates that there was sufficient available carbon in this denitrification wall to support denitrification and nitrate removal for at least 5 yr.