the total concentration of ClE contamination fell from 1305 μmol/L to 72 μmol/L.
Similarly to AW5–57, the sudden reduction in AW5–58 was caused by the combination of dilution and reduction of ClE contamination present in the groundwater.
The reduction ratio 2 and 3 months after the injection, which was 57% and 48%,
respectively, in the case of total ClE concentrations, showed a high reductive
potential. The nZVI application showed very high reductive influence on TCE,
which accounted only for 11% 3 months after the nZVI injection, compared with
its pre-application concentration—89% reduction of TCE was monitored in
AW5–58. A similar course was observed in VC concentrations—the samples of
groundwater from the time of 3 months after the nZVI injection showed 57%
reduction of VC.
Figure 5.6 shows an ongoing process of dechlorination in the most contaminated
boreholes AW5–57 and AW5–58. In borehole AW5–57, the degree of dechlorination was increasing only during the first week after the application. It went up from
48% to 74%. After that, the process attenuated and the final value of the degree
accounted for 51% (Vacková 2018).
In the borehole AW5–58, the rising trend lasted up to 28 days during which
dechlorination went from 46% to 66%. After that, the process attenuated similarly to
AW5–57 and the final value of the degree accounted for 55%.
In the borehole AW5–60, immediately after the injection, there was a drop in the
degree of dechlorination by 8%, which could have resulted from the dilution of
groundwater by the injected suspension of nZVI. The rising trend was delayed and
started 7 days after the injection when it went up from the low of 50%. Two months
after the injection time, it reached the high of 67%. After that, a decreasing trend was
observed.
Overall, the post-application values remained higher than the pre-application,
which reflects that the reduction process had a long duration of 89 days. The course
of the degree of dechlorination indicates an ongoing process of dechlorination.
5.3.2 NZVI-C3
Figure 5.7 illustrates the effects of the NZVI-C3 nZVI application on the redox
potential. It decreased significantly and immediately only in the borehole AW5–60
where the values of E H decreased from 90 mV to À100 mV. In the other boreholes,
the E H values mainly oscillated around the original positive values (Vacková 2018).
Compared with the previous application, the main contaminant in the subsurface
was not TCE but DCE. Furthermore, the total concentration of ClE decreased by
27% after the first application. The results of the second injection of nZVI were, for
easier comparison, plotted on the graphs in the same scale (max value 1000 μmol/L).
Figures 5.8 and 5.9 demonstrate the effect of the NZVI-C3 nZVI application on
the concentration of ClE in the subsurface. The injection caused a short-term
decrease in ClE (mainly of DCE) in every single borehole. Compared with the
first injection in April 2015, the contaminant reduction was more moderate. The
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V. Stejskal and N. Vacková
Similarly to AW5–57, the sudden reduction in AW5–58 was caused by the combination of dilution and reduction of ClE contamination present in the groundwater.
The reduction ratio 2 and 3 months after the injection, which was 57% and 48%,
respectively, in the case of total ClE concentrations, showed a high reductive
potential. The nZVI application showed very high reductive influence on TCE,
which accounted only for 11% 3 months after the nZVI injection, compared with
its pre-application concentration—89% reduction of TCE was monitored in
AW5–58. A similar course was observed in VC concentrations—the samples of
groundwater from the time of 3 months after the nZVI injection showed 57%
reduction of VC.
Figure 5.6 shows an ongoing process of dechlorination in the most contaminated
boreholes AW5–57 and AW5–58. In borehole AW5–57, the degree of dechlorination was increasing only during the first week after the application. It went up from
48% to 74%. After that, the process attenuated and the final value of the degree
accounted for 51% (Vacková 2018).
In the borehole AW5–58, the rising trend lasted up to 28 days during which
dechlorination went from 46% to 66%. After that, the process attenuated similarly to
AW5–57 and the final value of the degree accounted for 55%.
In the borehole AW5–60, immediately after the injection, there was a drop in the
degree of dechlorination by 8%, which could have resulted from the dilution of
groundwater by the injected suspension of nZVI. The rising trend was delayed and
started 7 days after the injection when it went up from the low of 50%. Two months
after the injection time, it reached the high of 67%. After that, a decreasing trend was
observed.
Overall, the post-application values remained higher than the pre-application,
which reflects that the reduction process had a long duration of 89 days. The course
of the degree of dechlorination indicates an ongoing process of dechlorination.
5.3.2 NZVI-C3
Figure 5.7 illustrates the effects of the NZVI-C3 nZVI application on the redox
potential. It decreased significantly and immediately only in the borehole AW5–60
where the values of E H decreased from 90 mV to À100 mV. In the other boreholes,
the E H values mainly oscillated around the original positive values (Vacková 2018).
Compared with the previous application, the main contaminant in the subsurface
was not TCE but DCE. Furthermore, the total concentration of ClE decreased by
27% after the first application. The results of the second injection of nZVI were, for
easier comparison, plotted on the graphs in the same scale (max value 1000 μmol/L).
Figures 5.8 and 5.9 demonstrate the effect of the NZVI-C3 nZVI application on
the concentration of ClE in the subsurface. The injection caused a short-term
decrease in ClE (mainly of DCE) in every single borehole. Compared with the
first injection in April 2015, the contaminant reduction was more moderate. The
94
V. Stejskal and N. Vacková