Environmental Engineering Reference
In-Depth Information
The site has an approximate surface area of 50,000 m
2
and is located in an
alluvial plain. Under a roadbase of fill (made up of grounds and pyritic ash
residues), the site includes a layer of sand (4 m under the surface), a muddy layer of
clay (5-6 m thick) and finally a layer of sand and gravel. The subterranean water
enters the higher and unprotected sandy aquifer (the sheet of water is located 1.5 m
under the surface layer of sand and gravel, which is protected by overlying clay -
the nappe of this aquifer can go down to 3.5 m under the surface).
In order to improve underground water quality as required by cleaning criteria
(to Italian drinking water standards) and to allow redevelopment, decontamination
of the site included:
− the excavation of soiled material, follow-up of its sifting and re-use of pure
pyrite ashes (between 30,000 and 40,000 tons) and landfill of non-reusable material;
− construction of an external cover made up of calcareous gravel and topsoil;
− installation of a peripheral barrier containing limestone materials.
The use of limestone was envisaged because of its capacities to react with
residues containing metals to increase the pH of water to form insoluble compounds
(hydroxides in particular) with the majority of metals. The design of this barrier was
examined by installing a pilot barrier through the direction of subterranean
waterflow. The barrier was anchored up to 4.5 m in depth, through the layer of
vegetation and sand to reach a clayey layer up to 4 m in depth. It was 10 m long, 1 m
wide at the bottom and 3 m wide at the surface.
The results obtained showed that the concentrations of three metals of interest
dropped below the legal limits (and in certain cases below the limits of detection)
when the subterranean water crossed the barrier. A cleaning efficacy of 90% was
reached in the case of copper and 90-98% in the case of arsenic. The concentrations
of lead were weak throughout the trial period. The results above refer to a relatively
limited period (six months); no doubt later on the effectiveness of the barrier will
increase. This pilot test made it possible to validate a technological choice that led to
its application in 1999.
15.4. Passive treatment
in situ
15.4.1.
Natural attenuation
Natural attenuation generally starts when the risk posed by contamination is
sufficiently weak. From an industrial viewpoint natural attenuation is tempting,
mainly because cleaning costs less than the other techniques usually used.
