Permeable Reactive Barriers for Treatment of Dissolved Metals

Principal Investigator - David Blowes, Professor, University of Waterloo (2001-2004)

Challenge

Permeable Reactive Barriers (PRBs) have been recognized as a cost-effective groundwater remediation technology. Within the past decade, PRB systems have evolved rapidly from bench-scale studies to full-scale applications. Currently, the use of zero-valent iron is a widely accepted treatment for select metals. Yet, treatments of a number of additional dissolved metals have not received widespread application beyond the field scale, despite evidence from laboratory-scale studies indicating the treatment potential of a wider range of dissolved metals. In order to assess the potential for widespread application of PRB technology, further research is required into the economic costs and benefits of PRB installations, as well as the demonstration of PRB’s long-term performance.

Project

The research team proposed to develop and apply a rigorous laboratory protocol to screen and compare different reactive materials for treating contaminated groundwater. These reactive materials were then evaluated on a field scale and ranked according to relative performance. The experimental results indicated that reactive mixtures containing combinations of organic carbon were more reactive than individual carbon sources, and that poultry manure was an effective additive.

Using numerical modelling approaches, the laboratory results were scaled to predict the performance and longevity of field-scale installations. Three field sites were characterized to assess the influence of the mixture components on sulfate-reduction and metal removal rates. Environmental isotope analyses indicated that microbially-mediated processes were active in all barriers. Results from the bacterial, microbial, and mineralogical characterization studies provided input parameters and constraints for reactive solute transport modelling. The model was used to simulate the performance of reactive barriers characterized during the field studies, incorporating site data.

To demonstrate the long-term performance of PRB systems, detailed sampling of three systems was conducted, which provided a temporal record of the rate and extent of geochemical reactions occurring within the barriers. At one site measurements were made quarterly to provide continuity in field data.

Finally, a cost benefit analysis model was developed to evaluate and compare the potential costs associated with implementation of a reactive barrier system versus a conventional pump and treat system. This model is based on an evaluation of costs obtained from individual site holders, and should be widely applicable.

Outputs

• Participation in the ITRC network and in the Remedial Technology Demonstration Forum, assisting with the preparation of ITRC guidance documents (available at www.itrcweb.org) and the ITRC training course. Preparation for the Training Course occurred through 2004. The first course offering was held on April 26, 2005, with approximately 180 registrants.
• The project itself contributed to the publication of 5 peer-reviewed journal articles, 14 papers in conference proceedings, and 35 conference presentations.
• Project results were presented at CWN meetings, as well as national and international scientific meetings and symposiums. In terms of HQP, the project contributed to the training of 9 MSc students, 8 PhD students, two Post Doctoral Fellows and one visiting scientist.
• An experimental protocol was developed to assess the effectiveness of reactive mixture components. Modifications were made to the reactive solute transport model to include the effect of secondary mineral precipitation on PRB permeability and performance.

Outcomes

• Increased knowledge in relation to the widespread application of PRB technology.
• Having developed a widely applicable economic evaluation protocol for conducting cost analysis, the project provides validation for the adoption of PRB technology. As well, monitoring existing PRB systems supplied evidence as to the long-term performance of PRB systems, one of greatest uncertainties currently limiting its widespread application.
• Researchers actively participated with regulators on the evaluation and acceptance of this technology. The project results have contributed to decisions of two of the industrial sponsors, Placer Dome and Falconbridge, to proceed with the construction of large-scale reactive barrier installations. In addition, the US EPA constructed a large-scale reactive barrier system in collaboration with the University of Waterloo.