Environmental Engineering Reference
In-Depth Information
DCE → acetylene was excluded. It is has been observed that semiautomatic
calibration is a useful mechanism to verify and/or identify reaction pathways.
A multicomponent reactive transport model was used by Yabusaki et al.
(2001) to simulate mobile and nonmobile components undergoing equi-
librium and kinetic reactions including TCE degradation, parallel Fe dis-
solution reactions, precipitation of secondary minerals, and complexation
reactions. The system was modeled using 36 chemical components and 8
minerals for a systematic coupling of transport and reactions for multiple
chemical components.
Sass et al. (2001) conducted a geochemical assessment at three PRB sites to
evaluate performance and longevity issues. The assessment was carried out
at former NAS Moffett Field (California), Dover AFB (Delaware), and former
Lowry AFB (Colorado) because they all exhibited different PRB designs,
hydrogeological conditions and groundwater chemistry. Geochemical mod-
eling with PHREEQC and Geochemist's Workbench was used to simulate
Fe reactivity in each of the three groundwater types and to assist in under-
standing precipitation sequences. Geochemical modeling results show that
siderite (FeCO
3
) and marcasite (FeS
2
) are the initial phases to precipitate. As
the reaction between iron and groundwater progresses, marcasite becomes
unstable with respect to mackinawite (FeS). Authors felt that it is difficult to
make a quantitative assessment of the longevity of the iron in the barriers.
4.3 Numerical Models
A number of numerical models have been used for the modeling of PRBs at
various sites. Table 4.1 provides a summary of these models showing that
they vary in input parameters.
4.4 Case Study
To demonstrate the modeling of a PRB, a case study demonstrating reme-
diation of TCE at a long-term contaminated site in South Australia is sum-
marized in the following section. At this site, PRB technology was used in
combination with the pump-and-treat active remediation strategy at the
source zone. The PRB technology works as a safeguard to prevent the con-
taminant plume from migrating off-site. The PRB technology implemented
on this site consists of a network of injection and extraction wells. It is envis-
aged that the most appropriate option for the long-term management of the
groundwater contamination at the site is some form of “funnel-and-gate”-
type approach, taking advantage of the low permeability of the aquifer.
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