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

Porous treatment walls are increasingly used for remediating contaminated g roundwater. These walls are constructed below the water table and perpendic ular to the groundwater flow. Successful nitrate removal from groundwater h as been demonstrated in porous walls amended with sawdust but the mechanism responsible has not been identified. The objective was to determine whethe r denitrification rates in such a wall were high enough to account for obse rved nitrate removal. During a year-long held trial, the rate of nitrate re moval from groundwater was measured as it passed through a 1.5 m wide wall. Concurrently, denitrification rates were measured in samples taken from th e 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). W hen the water table dropped below the wall, nitrate inputs were decreased, and there were concurrent declines in denitrification rates. Rates subseque ntly increased once the water table rose. Laboratory incubations also demon strated that denitrification was highly responsive to nitrate inputs. Denit rification rates increased by an order of magnitude within 7 h of nitrate a ddition. This treatment wall has removed nitrate from groundwater for more than 2.5 years and denitrification rates were high enough to account for ni trate removal. (C) 2000 Elsevier Science B.V. All rights reserved.