Evidence for VOC Mass Reduction Adjacent to Hydraulically Induced, ZVI-Filled Fractures in Clay

It has long been known that zero valent iron (ZVI) destroys chlorinated ethenes such as TCE by reductive dehalogenation and ZVI has been used for remediation in many granular aquifers in applications as a passive reactive barrier (PRB) to plume migration (O’Hannesin & Gillham, 1998; Vogan et al., 1999). However, at many aged contaminated sites much of the contaminant mass resides in lower permeability zones such as clayey or silty deposits within or at the base of the aquifers controlling remediation timescales. In these situations the challenge in the use of ZVI is to bring it into close contact with the chlorinated solvents stored in these low permeability zones.

A research project is underway to examine in detail the performance of particulate ZVI injected into hydraulically-induced fractures within a contaminated glaciolacustrine clayey deposit as (1) ZVI and glycol (G-ZVI), and (2) an emulsified ZVI (EZVI) mixture. Continuous vertical cores were collected through the treatment zone at 2 and 2.5 years after injections to locate the ZVI amendments in the induced fractures, and detailed sub sampling was conducted for chlorinated volatile organic contaminants (CVOCs) at discrete depths (distances) into the clay matrix away from the reactive material in the hydraulic fractures. The sample spacing was scaled to show the extent of the treatment zone adjacent to the ZVI in the fractures, expecting the treatment would be controlled by diffusion limited transport to the reaction zone in the fracture. Analytical results show evidence of treatment in both the EZVI and the G-ZVI contained fractures with the presence of degradation products and reduced CVOC concentrations in the fracture and surrounding clay matrix.  Reverse concentration gradients extend away from amended fractures 5-10cm after 2.5 years. Reactive transport modeling is used to examine the diffusion, sorption and reaction processes and how these control remediation timeframes.