DCC1.1 “H” and “B.” These results are in good agreement with those obtained at
laboratory scale. They also highlight the applicability of dithionite treatment, whose
injection is facilitated in groundwater by the use of a soluble reductant.
However, the lower abatement for TCE and cis-1,2-DCE and the accumulation of
1,1-DCE and VC observed in DCC1.2 clearly shows a limitation in the treatment due
to an insufficient dithionite quantity (i.e., COCs reduction is effective when
dithionite is abundant). This phenomenon can first be explained by the dilution of
dithionite by the widening of its plume, since the speed of water table was higher
than 40 cm d
À1 . It is also explained by the oxidation of dithionite due to the presence
of trace elements, organic matter, clays, and other contaminants in the underground.
This hypothesis was confirmed by the strong reducing conditions that remained in
DCC1.1 after the treatment (July 8), in agreement with the redox potential (À650
mV/Ag-AgCl) and the high reductive percentage, and the insignificant impact of the
treatment in DCC1.3, because dithionite did not reach this cPZ.
The later phenomenon agrees with the fact that only VC decreases in DCC1.3,
essentially between June 2 and June 22. This highlights a temporary bio-oxidative
process in agreement with the lack of the reductive chemical treatment and the
probable presence of very low dissolved oxygen content in the groundwater (due to
sampling and measurements). “Mild” oxidizing conditions (À175 mV/AgAgCl)
straitened that hypothesis, while neither pH (7.25) nor temperature (12.2
C) did
vary. Moreover, aerobic oxidation mainly concerns VC (Hartmans and De Bont
1992; Verce et al. 2000, 2001; Coleman et al. 2002; Danko et al. 2004; Elango et al.
2006; Gossett 2010; Mattes et al. 2010).
This highly efficient metabolic process allows the involved microorganisms to
integrate these molecules into their metabolism to lead either to their use as a carbon
source for the synthesis of more complex molecules (anabolism), or to their use as a
source of energy degrading them to CO 2 (catabolism) (Mattes et al. 2010). Nevertheless, because (1) the removal of VC was not efficient (20%) and (2) TCE
dechlorination pursued in agreement with the accumulation of the degradation
products, there was ample time for anaerobic reactions to proceed once the oxygen
was consumed. Dehalorespiration process is thus assumed to be responsible of TCE
dechlorination between June 2 and July 8. Nevertheless, the increase in 1,1-DCE
content remains questionable.
6.5.3.3 Line 2: nZVI Alone
New injection techniques were used to inject nZVI particles at four levels in the
injection well DCI2. Injections were sequential (at each level) and continuous during
a day per level between March 19, 2015 and March 24, 2015, as follows:
• Level 1: March 19, 2015: injection of 1 canister of nZVI solution: water flowrate
of 240 L h
À1 and nZVI solution flowrate of 84 mL min
À1 ; i.e., continuously
16.8 mL of nZVI per liter of water
360
R. Rodrigues et al.
laboratory scale. They also highlight the applicability of dithionite treatment, whose
injection is facilitated in groundwater by the use of a soluble reductant.
However, the lower abatement for TCE and cis-1,2-DCE and the accumulation of
1,1-DCE and VC observed in DCC1.2 clearly shows a limitation in the treatment due
to an insufficient dithionite quantity (i.e., COCs reduction is effective when
dithionite is abundant). This phenomenon can first be explained by the dilution of
dithionite by the widening of its plume, since the speed of water table was higher
than 40 cm d
À1 . It is also explained by the oxidation of dithionite due to the presence
of trace elements, organic matter, clays, and other contaminants in the underground.
This hypothesis was confirmed by the strong reducing conditions that remained in
DCC1.1 after the treatment (July 8), in agreement with the redox potential (À650
mV/Ag-AgCl) and the high reductive percentage, and the insignificant impact of the
treatment in DCC1.3, because dithionite did not reach this cPZ.
The later phenomenon agrees with the fact that only VC decreases in DCC1.3,
essentially between June 2 and June 22. This highlights a temporary bio-oxidative
process in agreement with the lack of the reductive chemical treatment and the
probable presence of very low dissolved oxygen content in the groundwater (due to
sampling and measurements). “Mild” oxidizing conditions (À175 mV/AgAgCl)
straitened that hypothesis, while neither pH (7.25) nor temperature (12.2
C) did
vary. Moreover, aerobic oxidation mainly concerns VC (Hartmans and De Bont
1992; Verce et al. 2000, 2001; Coleman et al. 2002; Danko et al. 2004; Elango et al.
2006; Gossett 2010; Mattes et al. 2010).
This highly efficient metabolic process allows the involved microorganisms to
integrate these molecules into their metabolism to lead either to their use as a carbon
source for the synthesis of more complex molecules (anabolism), or to their use as a
source of energy degrading them to CO 2 (catabolism) (Mattes et al. 2010). Nevertheless, because (1) the removal of VC was not efficient (20%) and (2) TCE
dechlorination pursued in agreement with the accumulation of the degradation
products, there was ample time for anaerobic reactions to proceed once the oxygen
was consumed. Dehalorespiration process is thus assumed to be responsible of TCE
dechlorination between June 2 and July 8. Nevertheless, the increase in 1,1-DCE
content remains questionable.
6.5.3.3 Line 2: nZVI Alone
New injection techniques were used to inject nZVI particles at four levels in the
injection well DCI2. Injections were sequential (at each level) and continuous during
a day per level between March 19, 2015 and March 24, 2015, as follows:
• Level 1: March 19, 2015: injection of 1 canister of nZVI solution: water flowrate
of 240 L h
À1 and nZVI solution flowrate of 84 mL min
À1 ; i.e., continuously
16.8 mL of nZVI per liter of water
360
R. Rodrigues et al.