Environmental Engineering Reference
In-Depth Information
include examples of concerns related to PRB performance including, but not
limited to: (1) permeability loss due to solids formation and gas buildup (e.g.,
Henderson and Demond, 2011); (2) insufficient hydraulic performance due to
incomplete or inaccurate subsurface characterization (e.g., Henderson and
Demond, 2007); (3) the negative impact of anion competition (e.g., nitrate and
chloride) on the treatment mechanisms important to contaminant reduction
by iron metal (e.g., Moore and Young, 2005); (4) flow reduction upgradient of
a PRB potentially due to upgradient diffusion of hydrogen and/or guar-gum
from the PRB installation (Johnson et al., 2008), and (5) biofouling.
Despite the vast collection of sites, longer-term performance aspects of PRBs
are still a source of uncertainty in planning future applications. Sustained
field data with sufficient detail to enable a relatively thorough evaluation of
longer-term performance were available at very few sites. The primary fac-
tors limiting longevity of ZVI barriers are corrosion of iron and precipitation
on iron surfaces of native inorganic constituents from groundwater. When
excessive, these factors have led to reduction in reactivity of the ZVI, loss of
porosity and permeability, hydraulic mounding, and plume bypass around
the PRB. At some sites, this loss of performance has been fairly severe within
5 years of installation.
The effectiveness and longevity of biowall PRBs primarily depends on
sustaining appropriate levels of bioavailable organic substrate in the biow-
all reactive zone and maintaining the permeability of the biowall trench.
The primary factor limiting longevity in biowalls has been the depletion of
the more easily biodegradable portion of the organic substrate in approxi-
mately 4-5 years after installation. Injection of a slow-release biodegradable
substrate, such as vegetable oil, has been effective in extending the life of a
biowall, although this periodic enhancement increases the life-cycle cost of
the PRB and makes it more of a semipassive system. Longevity expectations
for PRBs are closely tied with the economics of the application. In previous
years, the economic comparison revolved around PRBs and pump-and-treat
systems. While this comparison is still valid—especially where there is a
potential to replace aging pump-and-treat systems with a PRB—fewer new
sites are considering or installing pump-and-treat systems. At newer sites,
technology selection and economic comparison usually revolves around
PRBs and other
in situ
plume treatment or control options. In all these eco-
nomic comparisons, how long the treatment media will last without the need
for active replenishment or replacement is a key consideration.
2.3.4 Sustainability
PRB technology is widely considered a sustainable groundwater remediation
method because: (1) the general intent of a PRB system is to perform under
hydraulically passive means (i.e., no energy or mechanical input for routing
chemically impacted groundwater through the PRB), (2) groundwater is not
removed from the subsurface nor degraded through discharge with lower
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