Pilot Scale Injection of Guar Gum Stabilized Micro-sized Zero-valent iron via Hydro-fracturing

Within the FP7 AQUAREHAB project, injectable micro-sized zero-valent iron particles (mZVI) are being developed and evaluated for in-situ chemical reduction of Chlorinated Aliphatic Hydrocarbons (CAHs). mZVI particles have good reactive properties and a longer life-time than nano-sized particles (nZVI). In contrast to nZVI, mZVI suspensions need to be stabilised to prevent sedimentation in storage vessels and injection tubes. Here we report on a field scale injection of guar gum stabilized mZVI that was performed in November 2011 as a joined endeavor of several AQUAREHAB partners. At the test site (located in Belgium) a TCE and 1,1,1-TCA contamination is present at 4-11 m bgs. The aquifer has a relatively low permeability and the gradient is very small resulting in an estimated groundwater velocity of approximately 10 m per year. Different injection techniques and approaches were considered. Based on the site characteristics, the final pilot test was performed with direct push injection (Geoprobe) at 5 depths (1 location) at high pressure to induce fracturing. Multilevel monitoring wells were installed at different distance around the injection point, along with temperature sensors and devices specifically developed to detect magnetic particles via measurement of electromagnetic susceptibility. In total, 100 kg of mZVI was injected suspended in 1.5 m³ of guar gum (6 g/l) slurry. The objectives of the field test were: - To evaluate the preparation procedure and injectability of guar gum stabilized mZVI - To understand the pressure range developed during the injection - To test the applicability of the newly developed monitoring probes - To determine the vertical and horizontal distribution of mZVI particles in the subsurface after injection - To evaluate the reactivity of mZVI after the injection A short overview of the approach followed is described in this paper.