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Nitrate Removal from Groundwater Using a Denitrification Wall Amended with Sawdust: Field Trial

Louis Schipper^* and M. Vojvodi-Vukovi

Landcare Research New Zealand Ltd., Private Bag 3127, Hamilton, NZ.

^* Corresponding author (schipperl{at}landcare.cri.nz).

ABSTRACT

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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