dechlorination products—according to their classification by the International
Agency for Research on Cancer (IARC):
• Group 1: carcinogenic to humans
• Group 2A: probably carcinogenic to humans
• Group 2B: possibly carcinogenic to humans
• Group 3: not classifiable as to carcinogenicity in humans
In addition, the Stockholm Convention on Persistent Organic Pollutants has
classified initially 12 COCs (aldrin C 12 H 8 Cl 6 , chlordane C 10 H 6 Cl 8 , dieldrin
C 12 H 8 Cl 6 O, endrin C 12 H 8 Cl 6 O, heptachlor C 10 H 5 Cl 7 , hexachlorobenzene C 6 Cl 6 ,
mirex C 10 Cl 12 , toxaphene C 10 H 8 Cl 8 , polychlorinated biphenyls C 12 H 10Àx Cl x ,
dichlorodiphenyltrichloroethane C 14 H 9 Cl 5 , polychlorinated dibenzo-p-dioxins, and
polychlorinated dibenzofurans) in order to eliminate or restrict their production and
use. New compounds are frequently proposed and added to the different annexes of
the convention, such as hexachlorocyclohexane isomers (C 6 H 6 Cl 6 ), chlordecone
(C 10 Cl 10 O), and pentachlorobenzene (C 6 HCl 5 ) in 2009 or hexachlorobutadiene
(C 4 Cl 6 ) in 2015.
The risks and impact of DNAPL pollution are especially significant as different
chlorinated compounds are involved (Kueper et al. 2003). Moreover, reductive
dechlorination of COCs can lead to the formation of more harmful compounds,
especially VC and TCE, which are both PCE by-products. Therefore, detailed
knowledge regarding the dechlorination mechanisms is necessary to prevent a
possible accumulation of more toxic by-products.
As the presence of COCs in the environment represents a major concern because
of their toxicity, understanding the fate and transport of these compounds in groundwater is crucial.
6.2.3 Transport and Fate Processes in Groundwater
Knowledge of transport phenomena is one of the steps of the overall chemical
remediation mechanism, as the transport of pollutant to the chemical reagent can
be the limited step. Different transport and fate phenomena are used to characterize
the mobility of a contaminant in the groundwater: advection, diffusion, dissolution,
volatilization, and adsorption. Dilution may also occur by natural infiltration or
injection of uncontaminated water in groundwater, but it is generally not a significant
factor (Alvarez and Illman 2005a).
In addition to physical and chemical properties, and geological characteristics
such as wettability and permeability, transport processes have a high influence on
COCs migration pathways. Dissolution and volatilization are responsible for the
respective formation of dissolved and vapor phase plumes, leading to a long-term
pollution and aging/weathering of the pollution. Thus, source zones are progressively enriched in the less volatile and soluble compounds, which will form plumes
292
R. Rodrigues et al.
Agency for Research on Cancer (IARC):
• Group 1: carcinogenic to humans
• Group 2A: probably carcinogenic to humans
• Group 2B: possibly carcinogenic to humans
• Group 3: not classifiable as to carcinogenicity in humans
In addition, the Stockholm Convention on Persistent Organic Pollutants has
classified initially 12 COCs (aldrin C 12 H 8 Cl 6 , chlordane C 10 H 6 Cl 8 , dieldrin
C 12 H 8 Cl 6 O, endrin C 12 H 8 Cl 6 O, heptachlor C 10 H 5 Cl 7 , hexachlorobenzene C 6 Cl 6 ,
mirex C 10 Cl 12 , toxaphene C 10 H 8 Cl 8 , polychlorinated biphenyls C 12 H 10Àx Cl x ,
dichlorodiphenyltrichloroethane C 14 H 9 Cl 5 , polychlorinated dibenzo-p-dioxins, and
polychlorinated dibenzofurans) in order to eliminate or restrict their production and
use. New compounds are frequently proposed and added to the different annexes of
the convention, such as hexachlorocyclohexane isomers (C 6 H 6 Cl 6 ), chlordecone
(C 10 Cl 10 O), and pentachlorobenzene (C 6 HCl 5 ) in 2009 or hexachlorobutadiene
(C 4 Cl 6 ) in 2015.
The risks and impact of DNAPL pollution are especially significant as different
chlorinated compounds are involved (Kueper et al. 2003). Moreover, reductive
dechlorination of COCs can lead to the formation of more harmful compounds,
especially VC and TCE, which are both PCE by-products. Therefore, detailed
knowledge regarding the dechlorination mechanisms is necessary to prevent a
possible accumulation of more toxic by-products.
As the presence of COCs in the environment represents a major concern because
of their toxicity, understanding the fate and transport of these compounds in groundwater is crucial.
6.2.3 Transport and Fate Processes in Groundwater
Knowledge of transport phenomena is one of the steps of the overall chemical
remediation mechanism, as the transport of pollutant to the chemical reagent can
be the limited step. Different transport and fate phenomena are used to characterize
the mobility of a contaminant in the groundwater: advection, diffusion, dissolution,
volatilization, and adsorption. Dilution may also occur by natural infiltration or
injection of uncontaminated water in groundwater, but it is generally not a significant
factor (Alvarez and Illman 2005a).
In addition to physical and chemical properties, and geological characteristics
such as wettability and permeability, transport processes have a high influence on
COCs migration pathways. Dissolution and volatilization are responsible for the
respective formation of dissolved and vapor phase plumes, leading to a long-term
pollution and aging/weathering of the pollution. Thus, source zones are progressively enriched in the less volatile and soluble compounds, which will form plumes
292
R. Rodrigues et al.