6.5.2.5 Synthesis on the Natural Attenuation and State of the Two Areas
Before Treatment
The objective of the preliminary study regarding the large and the small areas was to
improve our understanding of the mechanisms and kinetics of natural attenuation
occurring in the groundwater of this aged polluted site. The pollution in the sandy
aquifer (Cuisian period) stands today as a majority of COCs, mainly chlorinated
ethylenes. The data basis composed of geochemical analyses from this site since
1999 was assessed.
The nature, abundance, and relative distribution of COCs and their metabolites
were measured and their spatiotemporal evolution were investigated and interpolated. Temperature, pH, redox potential, conductivity, content in chloride, bicarbonate, dissolved iron, sulfate, phosphate, and sulfide were considered, interpolated, and
mapped. Thus, this study has shown the slow natural attenuation process, its
mechanisms (reduction pathways) and limiting steps (nature and abundance of
metabolites). Chloride ion was one of the main end products (evolution from
15 mg L
À1 in a near not impacted zone to 340 mg L
À1 ), as well as bicarbonate
(150 to 650 mg L
À1 ) resulting from CO 2 dissolution after its production by the
biodegradation of organic compounds.
Within the future pilot zone, the total COCs content at this location 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 trans-1,2-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 presence 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 demonstrating the
natural attenuation of chlorinated solvents through dehalorespiring bacteria and/or a
biologically mediated abiotic dechlorination by Fe(II) species.
6.5.3 Treatment on the Demonstration Site
6.5.3.1 Treatment Principle and Calculation of Reagent Concentrations
to be Injected
The pilot demonstration aimed at investigating three different chemical treatments:
(1) line 1 ¼ treatment with dithionite alone, (2) line 2 ¼ treatment with nZVI alone,
and (3) line 3 ¼ treatment with dithionite + nZVI. The objective was to investigate,
verify, and consolidate the obtained laboratory results at a larger scale in in situ
conditions. The three treatment injections were thus sufficiently spaced in space
(13.5 m) and time (sequential treatment) to avoid any interactions between the
reactive zones.
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
353
Before Treatment
The objective of the preliminary study regarding the large and the small areas was to
improve our understanding of the mechanisms and kinetics of natural attenuation
occurring in the groundwater of this aged polluted site. The pollution in the sandy
aquifer (Cuisian period) stands today as a majority of COCs, mainly chlorinated
ethylenes. The data basis composed of geochemical analyses from this site since
1999 was assessed.
The nature, abundance, and relative distribution of COCs and their metabolites
were measured and their spatiotemporal evolution were investigated and interpolated. Temperature, pH, redox potential, conductivity, content in chloride, bicarbonate, dissolved iron, sulfate, phosphate, and sulfide were considered, interpolated, and
mapped. Thus, this study has shown the slow natural attenuation process, its
mechanisms (reduction pathways) and limiting steps (nature and abundance of
metabolites). Chloride ion was one of the main end products (evolution from
15 mg L
À1 in a near not impacted zone to 340 mg L
À1 ), as well as bicarbonate
(150 to 650 mg L
À1 ) resulting from CO 2 dissolution after its production by the
biodegradation of organic compounds.
Within the future pilot zone, the total COCs content at this location 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 trans-1,2-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 presence 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 demonstrating the
natural attenuation of chlorinated solvents through dehalorespiring bacteria and/or a
biologically mediated abiotic dechlorination by Fe(II) species.
6.5.3 Treatment on the Demonstration Site
6.5.3.1 Treatment Principle and Calculation of Reagent Concentrations
to be Injected
The pilot demonstration aimed at investigating three different chemical treatments:
(1) line 1 ¼ treatment with dithionite alone, (2) line 2 ¼ treatment with nZVI alone,
and (3) line 3 ¼ treatment with dithionite + nZVI. The objective was to investigate,
verify, and consolidate the obtained laboratory results at a larger scale in in situ
conditions. The three treatment injections were thus sufficiently spaced in space
(13.5 m) and time (sequential treatment) to avoid any interactions between the
reactive zones.
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
353