for TCE was observed for 1,1-DCE and VC. In the first period, 1,1-DCE content
decreased by 30–32% and VC content decreased up to 50%. In the second period,
VC remained in the same order of magnitude, whereas 1,1-DCE content increased
by 47–54%.
In DCC3.3, the same behavior was observed, but in a lesser extent.
6.5.3.5 Case Study Overview
Within the pilot zone, the total COCs content was about 30 mg L
À1 . The weight
fractions of COCs were: 0.3–1.0% for PCE; 25–45% for TCE; 0.3–0.4% for trans1,2-DCE; 0.5–15.0% for cis,1,2-DCE; 40–50% for 1,1-DCE; 0.7–30.0% for VC;
0.8–1.3% for 1,1-DCA; 0.2–0.4% for DCM; 3.5–6.0% for TCM; and 0.3–0.4%
for CT.
The existence of mild anaerobic conditions, the production of Fe(II), the existence of a source of fermentable substrates (organic matter as well as BTEX), and the
high concentration of chloride ions are among the qualitative indicators evidencing
natural attenuation of chlorinated solvents through a biologically mediated abiotic
dechlorination by Fe(II) species and dehalorespiring bacteria.
Regarding the occurrence of the reduction processes, it has to be highlighted that
neither the digging of the PZ in 2014 nor the involved reduction process
implemented in 2015 disturbed the natural attenuation.
The results show that insufficient dithionite quantity leads to the accumulation of
1,1-DCE and VC. In contrast, optimal use of dithionite clearly decreased COCs
content up to 100%. These results were in agreement with those obtained at
laboratory scale, highlighting the applicability of dithionite treatment, whose
groundwater injection is facilitated by the use of a soluble reductant.
The use of nZVI particles resulted in higher abatements (up to 60%) in comparison with the data obtained at laboratory scale (up to 10%). This result can be
explained by a symbiosis between the implementation of the reductive process and
the natural attenuation, due to the release of hydrogen. This hypothesis was
supported by the results acquired for alkalinity, which remained in the same order
of magnitude versus time.
The combination of the use of dithionite and nZVI particles may lead to the
decrease of COCs content up to 100% but on average it reached 90%. This result was
essentially attributed to the sequential injection of nZVI and dithionite, which
resulted in dithionite consumption for the rejuvenation of aged nZVI particles.
Consequently, dithionite did not reach the second cPZ. In comparison, the effective
mixture of both reagents resulted in higher reductions at the laboratory scale (almost
100%). The difficulties encountered for the injection of nZVI particles clearly
highlight the advantage of using dithionite solution for in situ reductive
dechlorination.
370
R. Rodrigues et al.
decreased by 30–32% and VC content decreased up to 50%. In the second period,
VC remained in the same order of magnitude, whereas 1,1-DCE content increased
by 47–54%.
In DCC3.3, the same behavior was observed, but in a lesser extent.
6.5.3.5 Case Study Overview
Within the pilot zone, the total COCs content was about 30 mg L
À1 . The weight
fractions of COCs were: 0.3–1.0% for PCE; 25–45% for TCE; 0.3–0.4% for trans1,2-DCE; 0.5–15.0% for cis,1,2-DCE; 40–50% for 1,1-DCE; 0.7–30.0% for VC;
0.8–1.3% for 1,1-DCA; 0.2–0.4% for DCM; 3.5–6.0% for TCM; and 0.3–0.4%
for CT.
The existence of mild anaerobic conditions, the production of Fe(II), the existence of a source of fermentable substrates (organic matter as well as BTEX), and the
high concentration of chloride ions are among the qualitative indicators evidencing
natural attenuation of chlorinated solvents through a biologically mediated abiotic
dechlorination by Fe(II) species and dehalorespiring bacteria.
Regarding the occurrence of the reduction processes, it has to be highlighted that
neither the digging of the PZ in 2014 nor the involved reduction process
implemented in 2015 disturbed the natural attenuation.
The results show that insufficient dithionite quantity leads to the accumulation of
1,1-DCE and VC. In contrast, optimal use of dithionite clearly decreased COCs
content up to 100%. These results were in agreement with those obtained at
laboratory scale, highlighting the applicability of dithionite treatment, whose
groundwater injection is facilitated by the use of a soluble reductant.
The use of nZVI particles resulted in higher abatements (up to 60%) in comparison with the data obtained at laboratory scale (up to 10%). This result can be
explained by a symbiosis between the implementation of the reductive process and
the natural attenuation, due to the release of hydrogen. This hypothesis was
supported by the results acquired for alkalinity, which remained in the same order
of magnitude versus time.
The combination of the use of dithionite and nZVI particles may lead to the
decrease of COCs content up to 100% but on average it reached 90%. This result was
essentially attributed to the sequential injection of nZVI and dithionite, which
resulted in dithionite consumption for the rejuvenation of aged nZVI particles.
Consequently, dithionite did not reach the second cPZ. In comparison, the effective
mixture of both reagents resulted in higher reductions at the laboratory scale (almost
100%). The difficulties encountered for the injection of nZVI particles clearly
highlight the advantage of using dithionite solution for in situ reductive
dechlorination.
370
R. Rodrigues et al.