Sequential reactive barrier remediation of ammonium-contaminated groundwater using polymer mats

Large-scale column experiments and a pilot-scale field trial were undertaken to evaluate the potential of in situ polymer mats (installed in series) as permeable reactive barriers within a "treatment wall" remediation system to induce sequential bioremediation of ammonium-contaminated groundwater. The first upgradient polymer mat was used to deliver oxygen via diffusion to induce bacterial nitrification of the ammonium to nitrite/nitrate as the groundwater moved past, and the downgradient polymer mat was used to deliver ethanol, to induce bacterial denitrification of the nitrite/nitrate to produce nitrogen gas. In these large-scale column experiments, nitrification rates increased and stabilized over the six-month experiment, with stable nitrification half-lives in the range 0.06-0.25 days. Based on the mass of ammonium converted and ox, en aqueous solubility data, nitrification most likely occurred in a biologically active zone at the polymer wall/aqueous interface. When ethanol was delivered via the downgradient polymer mat, denitrification half-lives in the range 0.12-0.34 days were observed. Nitrification/denitrification rates were maintained for groundwater flow rates up to 300 in year(-1), suggesting oxygen and ethanol delivery rates via the polymer mats were sufficient not to limit nitrification or denitrification. In the pilot-scale field trial, a 7-m deep treatment wall was constructed with 10-m wide impermeable wings either side of a 0.75-m wide permeable reactive zone "flow-through box" containing an oxygen and an ethanol delivery polymer mat. Sequential remediation of ammonium was observed with a >90% reduction in total N. The location and rate of nitrification (half-life of 1.2 days) suggested nitrification may not have occurred at a discrete biologically active zone. An average denitrification half-life of 0.4 days was similar to laboratory column results.