Environmental Engineering Reference
In-Depth Information
Natural attenuation is a passive approach with
in situ
treatment because it is
based on natural processes to attenuate contamination of the ground in subterranean
water without human intervention. The idea to combine long-term monitoring with
natural attenuation is called “supervised natural attenuation”. It is a concept recently
developed by [BRA 98]. Principal degradations at the origin of this phenomenon are
of biological or abiotic type. The other mechanisms consist of a phase transfer or
transport of the contaminant within a phase.
Supervised natural attenuation can be used as a method of environmental
remediation when the natural processes reduce the mass, toxicity, mobility and/or
volume of the contaminants. These processes are a function of the underground
medium. Natural processes resulting in a reduction or propagation of contaminants
in unsaturated zones include, for example, volatilization, sorption and natural
bioventing. The natural processes involving reduction or propagation of the
contaminants in saturated zones include sorption, diffusion/dispersion, biological
breakdown and dilution. Thus, natural attenuation requires a complete
comprehension of the mechanism of contaminant transport and narrow monitoring
of the grounds and subterranean water to check contaminant movement.
The natural attenuation of organic contaminants is based on the activity of the
indigenous micro-organisms that degrade (or even destroy) the contaminants. On the
other hand, natural attenuation of metals is different because, unlike organic
materials, metals cannot be destroyed. Many metals show a tendency to be reduced,
precipitate or to be adsorbed through various mineral phases in the ground [NOZ 00,
RAH 99]. Moreover, fuller details concerning natural attenuation are given by [BRA
98].
To evaluate the existence and persistence of natural attenuation, three parameters
are usually measured: (i) concentration of the contaminants; (ii) dissolved oxygen
contents (DO); (iii) potential oxydoreduction (redox).
A low level of C (<2 mg/L) in the aureole of contamination and an elevated level
of C downstream of the aureole indicate the existence of a biological breakdown
around the perimeter of the aureole. The level of C also increases when the
concentrations of contaminant decrease. Moreover, the smaller the potential of
oxydoreduction of subterranean water, the more anaerobic the environment. Thus,
the redox values inside the aureole should be lower (anaerobic) than the values
outside the aureole for natural attenuation to occur. The potential redox of
subterranean water varies typically between 800-400 mv.
