Environmental Engineering Reference
In-Depth Information
in treating energetic compounds including nitroamines and trinitrotoluene
(TNT). Furthermore, ZVI is now recognized as an effective bioremediation-
enhancing agent due to the ZVI corrosion reaction in water that provides a
sustained (in years) flux of dissolved hydrogen to the aqueous system. PRBs
have also been installed to include such new materials as zeolites to provide
an ion-exchange treatment of radioactive strontium-90 (Warner et al., 2012),
and biological materials to create biowalls (Bellis et al., 2010). Early PRBs
sometimes utilized granular-activated carbon (GAC) within a PRB treatment
cell to provide sorption of target organic compounds. More often used at
installations in Europe than in North America, fewer systems using GAC
appear to have been designed in recent years.
Biowalls using solid organic materials (mulch) have been greatly applied
over the past 10 years to stimulate anaerobic degradation of chlorinated
hydrocarbons as well as energetic and munitions compounds. These sys-
tems also have been employed to nitrate-impacted groundwater. The greater
understanding of biogeochemical transformation processes in biowalls that
are responsible for the resulting abiotic dechlorination of solvents. PRBs com-
posed of “organic material” have evolved with respect to arsenic treatment,
and new media including organic carbon-rich combinations, emulsified ZVI,
and organophyllic clays have been applied as reactive treatment media.
2.3.2 Unintended Performance Issues
PRB performance, particularly when performance is seen as underachieving,
is not often presented in technical literature. While most PRBs are assumed
to have performed adequately, an inadequate performance can occur and
typically results from inadequate hydraulic design. Experience has shown
that an insufficient hydraulic design has its genesis in incomplete site char-
acterization. Even if characterization is satisfactory, overly complex hydro-
geologic conditions may preclude a cost-effective application of PRBs. Such
characteristics may include high rates of groundwater flow, high permea-
bility, extreme aquifer heterogeneity, undiscovered preferential flow paths,
or excessive depth to groundwater. The nature and extent of the contami-
nant distribution must be well characterized to design an effective PRB and
should consider the nature and anticipated persistence of the contaminant
source. The vertical extent of contamination is particularly important. The
contaminant discharge (mass flux) through the PRB should be sufficiently
characterized so that the upgradient concentrations can be accommodated
by the PRB design. It is also imperative to understand the plume shape and
direction variability over time.
While the oldest PRB remains functional after 17 years, it is well under-
stood that these systems age due to both the exhaustion of the treatment
media, and the important effect of inorganic constituents that may lead to
mineral precipitates in pore spaces. Calcium carbonate, iron carbonate, iron
hydroxide, and iron sulfide precipitates may form in the media with pH
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