Environmental Engineering Reference
In-Depth Information
2.2 General Considerations of the PRB
The PRB concept has been the focus of hundreds if not thousands of technical
articles and publications (e.g., ITRC, 1999, 2005; Naftz et al., 2002; RTDF, 1998;
Roehl et al., 2005, Thiruvenkatachari et al., 2008) as well as Internet web sites
(e.g., the Oregon Graduate Institute [now part of the Oregon Environmental
Health University] database for the use of ZVI as a treatment media within
a PRB—http://cgr.ebs.ogi.edu/ironrefs/). A paper describing the 10-year
performance history of this remedial concept was provided by Warner and
Sorel (2003). This article provides a brief update to what was learned then
and what is understood today. However, this article is not an exhaustive
review of PRB technology. The reader is instead referred to other examples,
including the newly published ITRC guidance document (ITRC, 2011), which
provides several hundred technical references on PRB development and
performance, and the CRC CARE Technical report on PRB Guidance (CRC
CARE Technical report Number 25) (Perlmutter et al., 2013).
Since the concept of either destroying dissolved contaminants or rendering
them immobile or less toxic within a subsurface treatment zone was described
by McMurty and Elton (1985), several hundred formally recognized PRBs, and
perhaps several hundred more similar type remedies have been installed at
a variety of sites including industrial, mining, and retail petroleum facilities.
Simply, the goal of the PRB is to introduce, or enhance, geochemical or biolog-
ical reactions that afford the necessary treatment of the target contaminants
within a subsurface engineered zone. The key and perhaps most important
aspect of the PRB, and its greatest asset, is that the treatment is performed
under hydraulically ambient conditions; that is, affected groundwater flows
and contaminants migrate through the PRB under natural gradient condi-
tions without the addition of pumping or other energy-induced methods.
An important acknowledgment proven for over 20 years is that each PRB
application is unique in all characteristics—design, objectives, construction,
economics, monitoring, and specific performance. One supporting reason
for this is that each remediation site is unique with respect to site geol-
ogy and hydrogeology, contaminant occurrence and distribution, land-use
issues, and regulatory drivers. This uniqueness is both an advantage and
disadvantage for PRB application. As an advantage, because the PRB is not
an “off-the-shelf” remedy, it must be designed to suit the needs of the site
(Shoemaker et al., 1995). This has led to the identification and development
of multiple installation methods (e.g., excavate and fill, injection and jetting,
single pass trenching and replacement, large diameter boring emplacements)
and geometries (e.g., continuous wall, funnel and gates, multidiscrete depth
application, dual walls), and the identification and use of different treatment
media (beyond ZVI) for a variety of chemicals (for and beyond chlorinated
hydrocarbons). For example, PRB treatment materials in addition to ZVI have
ranged from bi-active mulch, to crushed limestone, to zeolite minerals, with
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