Environmental Engineering Reference
In-Depth Information
in size (silty gravel-sand mixture referred to as “aquifer 1a,” and medium-
grained sand with a small portion of fine grain referred to as “aquifer 1b”).
There are permeable layers with a varying thickness of 4-7 m. However, the
overall permeability coefficient (kf)
f
) was poor: 5 × 10
−5
m/s. Attempts to apply
soil vapor extraction as well as preliminary experiments for hydraulic treat-
ment of the hot spots failed to meet reasonable remediation targets. These
results demonstrated that remediation could not be completely accomplished
using conventional approaches alone. Furthermore, because of the relatively
large plume, it was considered that protective measures downgradient of the
hot spots were essential to protect the neighboring properties. A feasibil-
ity study including column experiments employing groundwater from the
site along with groundwater modeling revealed that a PRB using ZVI and
shaped as an F&G could be successfully employed. In 1998, a pilot-scale F&G
was established for field testing in the middle of the plume and trialled 6 six
months. Promising contaminant destruction rates of 99% were observed dur-
ing that term. As a consequence, the site owner decided to have a full-scale
PRB system designed and installed, despite the fact that it would be the very
first (privately financed) full-scale PRB in Germany associated with a certain,
increased risk compared to conventional techniques.
The full-scale F&G was installed in late 2000 and was put into service in
February 2001. The Edenkoben gate type was designed for a diverted, vertical
flow inside, whereby the groundwater is passively lifted by a vertical drain
(gravel columns, diverting the groundwater flow by 90° relative to its natural
horizontal flow) and directed through an ZVI bed that is installed just below
ground level (Rochmes, 2000; Birke et al., 2003). The vertical drains thus pro-
vided a complete connection to the deeper, polluted groundwater areas. Six
gates (each 10 m long and 1.25 m wide), constructed as a sheet pile caisson
(open toward its bottom) and packed with granular ZVI (825 tons in total),
reached down to approximately 8 m below ground level. A continuous sheet
pile wall, 400 m long and more than 14 m deep (i.e., at around 14 m depth, it
was pushed into the aquifer base) shaped the funnel that also ran through the
gates, thus separating each gate into two chambers. However, inside the gates,
the sheet pile wall is buried 1 m below the lowest groundwater level antici-
pated (5 m below ground level), hence serving as an overflow weir between
the chambers. In other words, the flow path through the ZVI is intention-
ally doubled, due to the special gate design/construction. The sophisticated
design of the PRB system in Edenkoben is based on two innovative principles:
1. Compared with a conventional gate construction, the width of a gate,
that is, the actual (horizontal) thickness/extension of an iron layer
required for a sufficient dehalogenation was significantly reduced
by diverting the groundwater flow twice vertically through a given
portion of ZVI.
2. Installing the gates near ground level, made access to and the poten-
tial recovery of ZVI significantly easier.
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