Denitrifying permeable reactive barriers for groundwater remediation: Barrier design and assessment using resistivity imaging and solute transport modelling

Abstract:

Permeable Reactive Barriers (PRBs) are engineered, passive remediation technologies that enhance mineral precipitation, redox, and sorption reactions along a contaminated groundwater flow path. The content of a PRB depends on the target remediation; example PRB materials include zero-valent iron, zeolites and organic mulches. Once installed, characterisation of the physical integrity of the barrier can be aided by appropriate geophysical techniques. During operation of the barrier, monitoring of the changing nature of the physical and geochemical properties of the barrier may also offer scope for geophysics. The potential value of geophysics for assessing the structure and performance of zero-valent iron PRBs has been well documented. Here we focus on the potential role of geophysics for studying denitrifying PRBs. Many parts of the world we are experiencing increasing concentrations of nitrate in groundwater. Land management controls will likely lead to improvements in future water quality but in some regions action is needed to minimise the migration of high nitrate concentration waters to sensitive receptors. Woodchip denitrifying PRBs enhance in situ natural nitrate attenuation in shallow groundwater systems. Solid carbon is added in trenches through the aquifer to intercept nitrate-contaminated groundwater flow. The carbon stimulates denitrification by bacteria which converts nitrate (NO₃) predominantly to inert di-nitrogen gas (N₂). A critical element in the design of such PRBs is matching hydraulic conductivity of the PRB to the surrounding aquifer media to ensure that water flows through the PRB rather than around it. In this case study we illustrate how electrical geophysics is being used to inform design of denitrifying PRBs targeting treatment of nitrate in gravel aquifers of the Canterbury region, New Zealand. A particular challenge of the setting is the very high advective velocity of local groundwater. In a solute tracer experiment, time-lapse resistivity imaging was used alongside solute transport modelling to characterise flow paths and residence times of groundwater in and around two trial PRBs. We report on results from recent field experiments and offer perspectives on the future role of geophysics to aid the monitoring of the effectiveness of this remediation technique.