anaerobic groundwater conditions and the depletion of dissolved oxygen. In agreement with Pourbaix (1963), the redox potential decreases with pH. Nevertheless, it
decreased in the order of magnitude of 100 mV when the pH only varied in the order
of magnitude of 0.3 unit. The main results indicated that the lower the redox
potential, the higher the chloride content, the higher the conductivity, the lower
the dissolved Fe
2+ content, and the higher the alkalinity. Alkalinity reflects the
buffering capacity of the aquifer. An increase in alkalinity in association with a
stable pH value indicate that the buffering capacity of the aquifer is sufficient to
neutralize the metabolic acidic compounds produced by the degradation of substrates. Here, the elevated alkalinity values, ranging from 480 to 630 mg L
À1 at
neutral pH, are the proof of the biodegradation of organic compounds, e.g., natural
organic matter (NOM) or additional anthropogenic contaminant such as BTEX,
which produce CO 2 that dissolves in HCO 3
À
. This result is therefore an indicator
of a microbial activity, for which the organic matter is the electron donor. The high
alkalinity content in combination with the facts that (1) mild redox potential were
measured and (2) no efficient reduction occurred, also demonstrated that no
methanogenesis was happening (Wiedemeier et al. 1998; Suthersan 2002). The
presence of dissolved Fe(II) demonstrated that sulfate reduction to sulfide
(sulfidogenesis) was not occurring, i.e., there was no precipitation of Fe(II) as FeS.
The natural attenuation can be explained by (1) the presence of an indirect
biologically mediated process and (2) the direct reductive dechlorination of COCs,
which act as electron acceptors. The first scenario involves a biologically mediated
abiotic degradation (Scherer et al. 2014). Indeed, the oxidation of organic matter can
also be associated to the reduction of other electron acceptors. Here, the presence of
dissolved Fe
2+ ranging from 2 to 10 mg L
À1 is an indicator of the presence of ironreducing bacteria (IRB) that use Fe(III) oxide-hydroxides as electron acceptors
(Wiedemeier et al. 1998, 1999). Fe(II) species can reduce the aquifer minerals,
e.g., magnetite and clays, and surface-complexed Fe(II) or metastable iron oxidehydroxides can be responsible for the mediated abiotic reductive dechlorination
(Jeong et al. 2011). This hypothesis is in agreement with the decrease in Fe
2+ content
observed in the North-West, in association with the decrease in PCE and TCE
contents and the increase in cis-1,2-DCE and VC contents. The second scenario
requires the presence of specific dehalorespiring bacterial strains that are effective
for the degradation of chlorinated ethylenes. Those microorganisms use carbon
substrate, or H 2 resulting from its fermentation, for COCs reductive dechlorination
in a sequential electron transfer (Stroo et al. 2014). This biological process was
proposed to occur in the upper part (PZs C23 and C33) of the area, where lower
redox potential values were encountered (indicating the increase in H 2 content).
The synergy of the data collected is sufficient to document the occurrence of
natural attenuation and its level of occurrence. The sequential degradation of PCE to
VC was observed inside the small area; however, the complete reduction to ethylene
is a slower process (Middeldorp et al. 1999), which can explain the accumulation of
VC in the upper part (PZs C23 and C33) of the pilot area.
352
R. Rodrigues et al.
decreased in the order of magnitude of 100 mV when the pH only varied in the order
of magnitude of 0.3 unit. The main results indicated that the lower the redox
potential, the higher the chloride content, the higher the conductivity, the lower
the dissolved Fe
2+ content, and the higher the alkalinity. Alkalinity reflects the
buffering capacity of the aquifer. An increase in alkalinity in association with a
stable pH value indicate that the buffering capacity of the aquifer is sufficient to
neutralize the metabolic acidic compounds produced by the degradation of substrates. Here, the elevated alkalinity values, ranging from 480 to 630 mg L
À1 at
neutral pH, are the proof of the biodegradation of organic compounds, e.g., natural
organic matter (NOM) or additional anthropogenic contaminant such as BTEX,
which produce CO 2 that dissolves in HCO 3
À
. This result is therefore an indicator
of a microbial activity, for which the organic matter is the electron donor. The high
alkalinity content in combination with the facts that (1) mild redox potential were
measured and (2) no efficient reduction occurred, also demonstrated that no
methanogenesis was happening (Wiedemeier et al. 1998; Suthersan 2002). The
presence of dissolved Fe(II) demonstrated that sulfate reduction to sulfide
(sulfidogenesis) was not occurring, i.e., there was no precipitation of Fe(II) as FeS.
The natural attenuation can be explained by (1) the presence of an indirect
biologically mediated process and (2) the direct reductive dechlorination of COCs,
which act as electron acceptors. The first scenario involves a biologically mediated
abiotic degradation (Scherer et al. 2014). Indeed, the oxidation of organic matter can
also be associated to the reduction of other electron acceptors. Here, the presence of
dissolved Fe
2+ ranging from 2 to 10 mg L
À1 is an indicator of the presence of ironreducing bacteria (IRB) that use Fe(III) oxide-hydroxides as electron acceptors
(Wiedemeier et al. 1998, 1999). Fe(II) species can reduce the aquifer minerals,
e.g., magnetite and clays, and surface-complexed Fe(II) or metastable iron oxidehydroxides can be responsible for the mediated abiotic reductive dechlorination
(Jeong et al. 2011). This hypothesis is in agreement with the decrease in Fe
2+ content
observed in the North-West, in association with the decrease in PCE and TCE
contents and the increase in cis-1,2-DCE and VC contents. The second scenario
requires the presence of specific dehalorespiring bacterial strains that are effective
for the degradation of chlorinated ethylenes. Those microorganisms use carbon
substrate, or H 2 resulting from its fermentation, for COCs reductive dechlorination
in a sequential electron transfer (Stroo et al. 2014). This biological process was
proposed to occur in the upper part (PZs C23 and C33) of the area, where lower
redox potential values were encountered (indicating the increase in H 2 content).
The synergy of the data collected is sufficient to document the occurrence of
natural attenuation and its level of occurrence. The sequential degradation of PCE to
VC was observed inside the small area; however, the complete reduction to ethylene
is a slower process (Middeldorp et al. 1999), which can explain the accumulation of
VC in the upper part (PZs C23 and C33) of the pilot area.
352
R. Rodrigues et al.