Injection of guar gum micro-sized zero-valent iron via direct push – field study

Within the FP7 AQUAREHAB project (GA 226565), injectable micro-sized zerovalent iron particles (mZVI) were 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), but need to be stabilised during injection to prevent sedimentation in storage vessels and injection tubes. Earlier we reported on a field scale injection of guar gum stabilized mZVI that was performed in November 2011 as a joined endeavour of several AQUAREHAB partners. More recently, April 2013, an additional larger scale pilot test was performed at another CAH-contaminated site in Belgium, where findings of the first test and numerous lab scale tests were used to work out an improved approach. At a PCE contaminated test site, 250 kg of mZVI was injected as 5 m³ of 0.45% guar gum stabilized suspension via direct push between 2 and 8 m bgs. The mZVI was injected in a test area of 3 by 6 m via 6 high pressure injections (MIP-IN technology) targeting a radius of influence of 0.5 m. Multilevel monitoring wells were installed at different distances around the injection point, along with temperature sensors and devices specifically developed to detect magnetic particles via measurement of electromagnetic susceptibility. Groundwater samples were taken before the injection and during several months after the injection. After the injection, also several undisturbed soil core samples were collected to evaluate the presence of mZVI (via susceptibility measurements & chemical analyses) and guar gum. The aim of the field test was: (1) to evaluate the feasibility to prepare guar gum stabilised slurry at a larger scale, (2) to evaluate the injectability of the within AQUAREHAB developed mZVI, (3) to determine the radius of influence of the injection, (4) to document the impact of the mZVI on the pollutant concentrations, and (5) to study the fate and added value of the guar gum after the injection. This paper presents the results of the followed approach, as well as some practical considerations related to lessons learned.