* U.S. EPA, Office of Research and Development, National Risk Management Research
Laboratory, Subsurface Protection and Remediation Division, Ada, OK.
Long-term Performance of
Permeable Reactive Barriers
Using Zero-valent Iron:
An Evaluation at Two Sites
Richard T. Wilkin*, Robert W. Puls*, and Guy W. Sewell*
The permeable reactive barrier (PRB) technology is an in-situ approach for remediating groundwater
contamination that combines subsurface fluid flow management with a passive chemical treatment
Removal of contaminants from a groundwater plume is achieved by altering chemical
conditions in the plume as it moves through the reactive barrier.
Because the reactive barrier
approach is a passive treatment, a large plume can be treated in a cost-effective manner relative to
traditional pump-and-treat systems.
There have now been more than forty implementations of the
technology in the past six years, which have proven that passive reactive barriers can be cost-
effective and efficient approaches to remediate a variety of compounds of environmental concern.
However, in all of the installations to date comparatively few data have been collected and reported
on the long-term performance of these in-situ systems, especially with respect to the buildup of
surface precipitates or biofouling (O’Hannesin and Gillham, 1998; McMahon et al., 1999; Puls et al.,
1999; Vogan, 1999; Phillips et al., 2000; Liang et al., 2000).
A detailed analysis of the rate of surface precipitate buildup in these types of passive, in-situ systems
is critical to understanding how long these systems will remain effective and what methods may be
employed to extend their lifetime or to improve their performance.
Different types of minerals and
surface coatings have been observed to form under different geochemical conditions that are
dictated by aquifer chemistry and the composition of the permeable reaction zone (Powell et al.,
1995; Mackenzie et al., 1999; Liang et al., 2000).
Microbiological impacts are also important to
understand in order to better predict how long these systems will remain effective in the subsurface
(Scherer et al., 2000).
The presence of a large reservoir of iron coupled with plentiful substrate
availability supports the metabolic activity of iron-reducing, sulfate-reducing, and/or methanogenic
This enhanced microbial activity may beneficially influence zero-valent iron reductive
dehalogenation reactions through favorable impacts to the iron surface or through direct microbial
transformations of the target compounds.
However, this enhancement may come at the expense of
faster corrosion leading to faster precipitate buildup and potential biofouling of the permeable