Environmental Engineering Reference
In-Depth Information
directly related to the emplaced treatment zone which may be emplaced by
one of a number of means as described previously, including trench exca-
vation (by conventional equipment of newly developed single-pass trench-
ing), large diameter filled borings, injection of micron-sized material, or
injection of chemical substrates that geochemically alter native subsurface
materials to afford the required treatment conditions. Whichever way the
treatment matrix is emplaced, the PRB system typically is intended to per-
form using hydraulically passive means; that is, the PRB is designed to
allow groundwater and the target chemicals to flow through the PRB with-
out mechanical assistance. Modern sustainable hydraulic enhancements
may include the use of solar or wind-driven, low-rate groundwater pumps,
or passive-siphon action to further control and route affected groundwa-
ter through a PRB. Aside from monitored natural attenuation or intrinsic
remediation strategies, there may be no other primary groundwater reme-
diation method that conveys the concept of a green and sustainable system
as well as the PRB.
2.3 PRB Developments
A third relatively new design component focuses on the sustainability of
the system. Considering that the firm formally recognized that a PRB was
installed in late 1994 and continues to function more than 17 years after
installation without any energy induced or water removed (except for moni-
toring activities), the PRB must naturally be considered one of the more sus-
tainable groundwater remedies. Today, with the advent of calculations to
assess the sustainability of treatment materials and construction methods,
evaluating the sustainability of the PRB can take on a different meaning.
However, compared to every active groundwater remedy, and most in situ
remedies, perhaps with the exception of MNA and some bioremediation
techniques, the PRB clearly is a leading sustainable remedy.
The following sections provide a discussion of developments in PRB
technology. Again, an exhaustive review is not provided; however, several
important considerations are provided herein.
2.3.1 Chemical Treatment
PRBs historically have been used to treat dissolved organic contaminants in
groundwater. This includes the common halogenated compounds including
tetrachloro- or perchloroethylene (PCE) and trichloroethylene (TCE), and
metals including chromium and arsenic, which are treated using granular
ZVI (chloro-reduction for the aliphatic compounds and geochemical reduc-
tion for the metals). New developments have shown that ZVI also is effective
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