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Nitrate (NO3) contamination of groundwater can cause pollution of receiving waters. We examined the mechanisms by which a “denitrification wall” removed NO3 from shallow groundwater. The denitrification wall was constructed by digging a trench (35 m long, 1.5 m deep, and 1.5 m wide) that intercepted groundwater. The excavated soil was mixed with sawdust (30% v/v) as a C source then returned to the trench. We assessed NO3 removal and denitrification in the wall for 1 yr. Incoming concentrations of NO3 in groundwater ranged from 5 to 16 mg of N L−1 but these decreased to <2 mg N L−1 in the denitrification wall. Total N in the wall declined during the year demonstrating that N immobilization was not a large sink for NO3. Denitrifying enzyme activity (DEA) reached a maximum of 906 ng of N g−1 h−1 after 6 mo of operation, indicating that denitrification was an important mechanism for NO3 removal. We calculated a maximum rate of NO3 removal by denitrification of 3.6 g N m−3 d−1. Substrate‐amendment experiments showed that denitrification in the wall was primarily limited by NO3 concentration and not C. During the study there was no significant decrease (P < 0.05) in total C but the availability of the remaining C declined. Despite this decrease, the DEA and microbial biomass were stable during the last 6 mo. This study demonstrated that denitrification walls can effectively remove NO3 from groundwater thereby protecting receiving waters.

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