Nitrate removal from groundwater and denitrification rates in a porous treatment wall amended with sawdust

Abstract

Porous treatment walls are increasingly used for remediating contaminated groundwater. These walls are constructed below the water table and perpendicular to the groundwater flow. Successful nitrate removal from groundwater has been demonstrated in porous walls amended with sawdust but the mechanism responsible has not been identified. The objective was to determine whether denitrification rates in such a wall were high enough to account for observed nitrate removal. During a year-long field trial, the rate of nitrate removal from groundwater was measured as it passed through a 1.5 m wide wall. Concurrently, denitrification rates were measured in samples taken from the wall using an acetylene-inhibition technique. Denitrification rates (0.6–18.1 ng cm −3 h −1) were generally high enough to account for the nitrate losses in groundwater (0.8–12.8 ng N cm −3 h −1), except on one occasion, when nitrate loss in groundwater was greater than 50 ng N cm −3 h −1. When the water table dropped below the wall, nitrate inputs were decreased, and there were concurrent declines in denitrification rates. Rates subsequently increased once the water table rose. Laboratory incubations also demonstrated that denitrification was highly responsive to nitrate inputs. Denitrification rates increased by an order of magnitude within 7 h of nitrate addition. This treatment wall has removed nitrate from groundwater for more than 2.5 years and denitrification rates were high enough to account for nitrate removal.