Conceptual analysis of zero-valent iron fracture reactive barriers for remediating a TCE plume in a chalk aquifer
Zuansi Cai, David N. Lerner, Robert G. McLaren and Ryan D. Wilson
Abstract
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 modelling simulations to assess the treatment performance of a 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 3-D discrete fracture network model that was developed using data from a number of chalk sites. TCE 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 modelling 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 a 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 a 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.
Summary
The goal of this modelling study is to evaluate the feasibility of the Fe0 FRB as a remediation technology for chlorinated solvent plumes in fractured aquifers. The approach is to conduct a parametric study of the importance of a range of factors such as the proportion of Fe0 fill, the amount of fracture enlargement, the size of the FRB relative to source size, and the reactivity of the Fe0 particles by numerical modelling. We have constructed a hypothetical site, which is based on representative data from a number of chalk sites in England with similar hydrogeological properties to the Sawston site. The results lead to some preliminary conclusions on the optimal design of a Fe0 FRB remediation system.
Citation
Zuansi Cai, David N. Lerner, Robert G. McLaren and Ryan D. Wilson (2007)Conceptual analysis of zero-valent iron fracture reactive barriers for remediating a TCE plume in a chalk aquifer. Water Resources Research, 43, W03436
DOI:10.1029/2006WR004946