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Underground contamination with hazardous wastes has been
one of the largest environmental problems in several
countries, with many sites presenting groundwater
contamination. There are several technologies to control
and remediate groundwater contamination and the technology
more commonly used is the so called pump-and-treat. In
spite of the potential for its use in the control of the
migration of a plume, pump-and-treat possesses several
physical and chemical restrictions that limit its
effectiveness as a strategy of aquifer decontamination on
long duration remediation works, especially if used
isolated or when the aquifer is contaminated with NAPLs
(Non Aqueous Phase Liquids). Additional and associated
techniques to the control system of the plume migration are
often recommended according the slow process of natural
dissolution of these liquids in groundwater. Because these
limitations, several technologies are being investigated.
One of the most promising is the in situ treatment of the
contaminant in reactive barriers, especially in aquifers
contaminated with chlorinated aliphatic organic compounds
(chlorinated solvents). The design involves the
construction of a barrier with reactive and porous
materials (reactors) that are placed in the aquifer to
intercept the contaminant plume. As water passively flows
through the reactor, the contaminants are degraded,
therefore minimising or avoiding the aquifer contamination
downstream the barrier. The degradation process involves
the reductive dechlorination of the compounds in the
presence of zero-valent metals (e.g. metallic iron).
The general objective of this research is to make an
evaluation of the use of reactive barriers in the
emediation of aquifers that are contaminated with
chlorinated solvents, and is emphasised its applicability
and limitations. The specific objective of the research is
to show that the treatment of aquifers should be made of
sequential way, that is, it should involve different
technologies of remediation according to the
characteristics of the contaminants and their by-products.
Several permeability and mass transport experiments were
made for determination of the hydraulic conductivity and
the parameters of mass transport in different mixtures of
powder of iron and quartz. Batch and column tests were also
made with different contaminants in the presence of iron
powder and the results allow evaluating the degradation
rates of these compounds and their by-products. The
conclusion is that the reductive dechlorination with
metallic iron can have different results depending on the
type of contaminant. Some contaminants can not present any
degradation or they have so high half-life times that the
use of this technology in reactive barrier design is out of
practice. Finally the formation of chlorinated by-products,
which are more toxic than the own original contaminant and
not degraded by the same technology, emphasises that the
treatment of aquifers should be sequential.