Environmental Engineering Reference
In-Depth Information
to degrade EDC has been historically recognized as a limitation of reactive
iron barriers.
Nevertheless, results were very encouraging. CTC was found to degrade
very rapidly—a half-life of <2 h—in the first 0.4 m of the barrier, and did
not stall with formation of DCM, another aliphatic CHC that is not readily
degraded by ZVI. The PCE was found to have a higher half-life (12 h to 2
days); so, a higher relative concentration of PCE would cause percent MR
results to be lower and some degradation products (such as TCE,
cis
-1,2-di-
chloroethene, and vinyl chloride [VC]) to emerge from the barrier.
From Table 12.1 (which includes 19-month data), it can be noted that influ-
ent concentrations of volatile CHCs and TOC varied markedly—with time
and depth. ETI observed that the PCE half-life decreased (i.e., degradation
was faster) with depth, and that influent TOC concentrations were lowest in
the 7 m bgs monitoring port. However, a relationship between PCE half-life
and influent TOC concentration could not be clearly concluded over time at
the 5 and 6 m bgs ports.
There was no notable correlation between the influent organic concen-
trations and percent MR; percent MR was more strongly influenced by the
relative concentration of PCE in the influent and, subsequently, its half-life
and the half-life of its degradation products. (Note that, due to the apparent
effects of upwelling in the 1.5-m ports at 7 m bgs as mentioned above, data
from the 1.5-m ports were disregarded when calculating MR; data from the
1.2-m ports were used instead.)
It is worth noting that the percent MR and PCE half-life values for month
19 were some of the best results achieved up to that time at 5 and 6 m bgs.
This was not the case at 7 m bgs. Although TOC concentrations were not
measured in the month 19 sampling event, it can be seen that at 7 m bgs,
the influent concentration of CHCs was the lowest to date. The concentra-
tion data for the individual CHCs entering the barrier are not presented,
but at that time, it was noted that the relative concentrations of CHCs poorly
degraded by ZVI were higher than normal. Notwithstanding the incon-
sistencies in temporal trends for mass removal and half-lives at the three
monitoring depths, the fact that the results showed no significant signs of
deterioration over time was encouraging, suggesting that long-term CHC
degradation could be achieved with the reactive iron barrier.
Consistently during 19 months of sampling, most of the MR of CHCs was
found to occur within the first 0.4 m of the iron barrier. In particular, CTC
was found to be completely degraded within the first 0.4 m. Sampling of
the multilevel piezometers that installed 0.2 m into the iron barrier prior to
the month 19 sampling event confirmed this: at 5 and 6 m bgs >90% of the
CTC and >50% of the PCE were degraded within 0.2 m. However, at 7 m
bgs, apparently anomalous data prompted ETI to note (in unpublished cor-
respondence to Orica) “these new data from the 7 m interval reinforce our
suspicion that system hydraulics (e.g. vertical gradients) may be influencing
the results at this depth.”
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