Conceptual analysis of zero-valent iron fracture reactive barriers for remediating a trichloroethylene plume in a chalk aquifer

A novel concept, the Fe0 fracture reactive barrier (Fe0 FRB), is proposed to clean up chlorinated solvent pollution of groundwater in a chalk aquifer. Iron particles, suspended in a viscous biodegradable gel, can be injected into selected fractures to create an extended reactive zone of partly iron-filled fractures. To evaluate the feasibility of Fe0 FRB as a remediation strategy, we conducted numerical modeling simulations to assess the treatment performance of an Fe0 FRB in a hypothetical chalk aquifer. The assessment was carried out using a numerical model for flow and solute transport in a discretely fractured porous medium coupled with an analytical expression representing degradation by iron. The hypothetical chalk aquifer was represented by a three-dimensional discrete fracture network model that was developed using data from a number of chalk sites. Trichloroethylene reactive transport in the Fe0 FRB and mass exchange of solute between fractures and the porous matrix were fully accounted for in the model. This modeling revealed that the success of the remediation technology lies in creating a highly reactive Fe0 FRB without reducing fracture permeability, which could lead to the plume being diverted around the barrier. A parametric study of various design parameters for the Fe0 FRB suggested that high treatment efficiency could be achieved by employing highly reactive nanoscale iron or by using a high proportion of microscale iron fill and fracture enlargement. The model study also provided some preliminary conclusions on sensitive design parameters of an Fe0 FRB such as the proportion of iron fill, the size of the FRB, and the amount of fracture enlargement. A preliminary analysis suggests that an Fe0 FRB containing a small amount of highly reactive nanoscale iron could provide satisfactory treatment for up to 50 years, depending on contaminant mass flux through the barrier.