Can willow roots oxygenate leachate in vegetation filter beds?¬ a mass spectrometer investigation in Wales

Abstract

Membrane inlet mass spectrometry (MIMS) was used to monitor dissolved gas concentrations
in-situ in laboratory microcosms and in a constructed willow vegetation filter bed in Wales
used to treat leachate. A mini-rhizotron digital camera system was developed for use in
combination with MIMS studies of gas dynamics. The major results of the studies are:-
• Diurnal cycles in the concentrations of 02, CO2 and C� were shown to occur with a
rooted willow cutting in a stirred microcosm.
• Willow beds show considerable sub-surface spatial diversity. Generally, oxygen decreases
with depth whereas methane and carbon dioxide increase with depth. Small pockets of
gases were seen. Oxygen occurred throughout the profile.
• Microbial processes follow diurnal cycles. Oxygen is released from willow roots during
daylight. Methane and carbon dioxide accumulate during the dark.
• The high degree of mixing distributes oxygenated water throughout the bed. Stopping the
flow of leachate allowed conditions to become anaerobic, except in local micro-sites.
These techniques appear to offer considerable potential for direct measurement of sub-surface
environments in constructed wetlands, particularly in respect of the microbial processes
occurring in the bed, and the oxygenation capacity of willow vegetation filters.