Nitrate removal from groundwater using a denitrification wall amended with sawdust: Field trial

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 mall was constructed by digging a trench (35 m long, 1.5 m deep, and 1.5 m nide) that intercepted groundwater. The excavated soil was mixed with sawdust (30% v/v) as a C source then returned to the french. 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 mall. Total N in the wall declined during the gear demonstrating that N immobilization mas 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 mall 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 mere stable during the last 6 mo. This study demonstrated that denitrification walls can effectively remove NO3 from groundwater thereby protecting receiving waters.