Environmental Engineering Reference
In-Depth Information
3.1 Introduction
At many sites, groundwater (GW) remediation is proving to be a much more
difficult and persistent problem. Permeable reactive barriers (PRBs) employ
an innovative technology that offers a passive system for addressing long-
term GW contamination problems. Although PRBs were initially applied to
treat chlorinated hydrocarbon (CHC) plumes, they have also been applied
to treat or capture other inorganic contaminants such as trace metals (chro-
mium, manganese, uranium, zinc, lead, etc.), and anionic contaminants
(sulfate, nitrate, phosphate, arsenate) (Gavaskar et al. 2000). PRBs containing
lime have also been used to raise the pH of GW with acid mine drainage near
mining operations (Johnson and Hallberg 2005).
There are presently over 200 field-scale PRBs in operation throughout the
world (see Chapter 1) and they have been in use since the early 1990s.
The salient features of a PRB system include (ITRC 2011; Gavaskar et al. 2002)
• Underground installation is limited by land use (i.e., large space
requirement and not easy to use in an urban setting)
• The PRB serves as a barrier to contaminants but not to GW flow
(should not alter the GW flow rate and direction)
• Once installed there is little possibility for rectification or maintenance
• Passive remediation can continue for many years even for decades
• Performance should not change under varying GW parameters,
including those of the contaminant(s) of concern
• PRBs should not only remediate the parent contaminant but also its
by-products
• Reactions in the PRBs should not introduce additional contaminants
to the GW
Most field- and pilot-scale PRBs have affected their remediation objectives
but a few are failing to meet their objectives (Richardson and Nicklow 2002).
There are several causes for this including: failure to encompass potential
shifts in hydraulic gradient, incorrect design of thickness of the reactive
media, channeling effects, diminishing reactivity with time, smaller poros-
ity of the media compared to the aquifer, and fast corrosion of the reactive
media (RTDF 2001). Common causes of short-term failures of PRBs include:
inadequate reactive material at some part of the barrier (e.g., heterogeneous
contaminant source), an insufficient depth of barrier to an aquitard, leakage
between the permeable and impermeable barrier joints (funnel-and-gate sys-
tem), smaller porosity of the barrier, and hence channeling.
PRB is a capital intensive technology with limited possibilities for rectifi-
cation after installation, so any form of failure needs to be anticipated and
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