Environmental Engineering Reference
In-Depth Information
1999, respectively). At the completion of the first 6 months' monitoring, it
was concluded by ETI that results were broadly consistent with the labo-
ratory column trial results and also the geochemical modeling. However,
given the variability of the results, it was recommended that two further
rounds of monitoring be conducted, and, as noted above, that two addi-
tional multilevel monitoring points be installed at a distance of 20 cm into
the iron zone alongside the center monitoring transect to better evaluate the
CTC degradation.
Subsequent sampling rounds were added for 9 (early December 1999), 13
(late March 2000), and 19 months (early October 2000) to further evaluate
the pilot-scale reactive iron barrier's performance. Two iron cores were also
collected at month 17 (see Section 12.8) to examine the condition of the iron,
particularly with regard to mineral precipitation and biological fouling.
These works concluded the formal evaluation of the barrier. A number of ad
hoc supplementary monitoring events were arranged to periodically check
whether conditions or performance might have changed.
12.7 Groundwater Monitoring Results
Data from the first 13 months of performance monitoring have been reported
previously (Duran et al., 2000; Stening et al., 2000).
12.7.1 Hydraulic Analyses
Groundwater velocities through the barrier were found to range from 0.1
to 0.8 m/day. The average velocity was determined to be around 0.6 m/day,
which was greater than the velocity assumed for the design of the pilot-scale
barrier, 0.15 m/day. The results confirmed that the iron was more permeable
than the surrounding sand.
The higher relative permeability of the iron was believed to be the cause of
apparent upwelling at the 1.5-m ports at 7 m bgs (i.e., at the back end of the
iron barrier only 0.5 m above the bottom), evidenced by CHC concentrations
increasing at this point to values similar to influent concentrations. (It has
also been postulated that the irregular concentration profile might have been
at least in part due to two bulk bag liners that had slipped into the bottom of
the excavation during installation of the iron.)
12.7.2 Organic Analyses
The first 13 months of performance monitoring showed significant reduc-
tion of most CHCs compared with total influent concentrations, but, as
expected, little reduction of EDC. As mentioned above, the inability of ZVI
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