Permeable Reactive Barriers in Europe - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 296

and methanogens intensively (CH 4 concentrations up to 5000 μg/L), so that
biofouling and/or accumulation of gaseous methane became an issue (Zolla
et al., 2006, 2009).
Excavation was performed in November 2004 and was carried out by a
crawler crane equipped with a hydraulic grab and supported by guar gum
slurry until the backfill with ZVI. The construction of the 120-m-long and
13-m-deep PRB was performed in 17 panels whose average length was 7 m. The
decision to proceed by panels was a safety measure to avoid trench instability.
ZVI was supplied by “Gotthart Maier Metallpulver GmbH” (Rheinfelden,
Germany) in the quantity of 1700 tons. The material, free from oils and other
impurities was characterized by an iron content higher than 90% in weight
and a carbon content lower than 4% (Di Molfetta and Sethi, 2005; Sethi et al.,
2007).
Monitoring of the PRB, which started in November 2005, aimed to ensure
that the plume was adequately captured and treated, and that downgradient
concentrations of the target contaminants (and any by-product) were below
the cleanup targets.
Monitoring activity includes
• Quarterly measurement of water levels, to indicate seasonal changes
in groundwater flow.
• Chemical monitoring with the determination of groundwater field
parameters (Eh, pH, dissolved oxygen, temperature, and conduc-
tivity), inorganic chemicals, and chlorinated organic compounds.
Sample collection is conducted quarterly to indicate any seasonal
changes in contaminant distribution or geochemistry.
• “Low low purging” and “low low sampling” methods are adopted
to minimize chemical and hydrological disturbances in and around
the well, to yield representative water samples.
Monitoring results gathered between 2005 and 2007 showed that output
concentrations were chiefly below the limit of 30 μg/L of the total carcino-
genic compounds; indeed, carcinogenic chlorinated aliphatic hydrocarbons
(CAHs) were below detection levels in almost every water sample taken
from downgradient wells. Reaction by-products (VC, 1,1-DCE, and 1,2-DCE)
were nearly absent both inside and downgradient of the PRB, verifying that
the barrier was able to perform a complete dehalogenation process (Sethi
et al., 2007).
On the other hand, groundwater sampling found heavy sulfate depletion
and the highest-reported methane concentrations (>5000 μg/L) of a ZVI PRB
site. These were due to intense microbial activity by sulfate reducers and
methanogens, whose proliferation was most likely stimulated by the use of
a biopolymer (i.e., guar gum) applied as a shoring fluid during the excava-
tion of the barrier. Slug tests within the barrier found an apparent hydraulic