In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 26

solutions were injected using either direct-push injection (direct injection) with a spacing between
the wells of 4.5 m, or by injection wells for the introduction of larger volumes. The design of the
injection system is the essential criterion for the success of the technology.
Another alternative available technology is in-situ chemical bonding. The purpose of this
technology (via injection of appropriate reagents in the subsoil) is to favor the formation of
insoluble solid phases and drastically reduce their solubility. For example, soils with relatively
soluble As compounds (as in the case of application of the herbicide As 2 O 3 ) can be treated
by introducing reagents, such as FeSO 4 and KMnO 4 , which form insoluble As forms. These
reagents drastically reduce the ability to leach As. Several studies examined the application of
various solutions and concluded that the most efficient reagent was ferrous sulfate (Yang et al .,
2007). Seidel et al . (2005) studied the binding of As in the tailings through the formation of iron
precipitates within a tailings dam. Iron precipitates were formed in situ by the aerobic treatment
of ferrous sulfate solutions that were injected into the site.

1.10 COMBINATION OF ELECTROKINETICS AND PRB

Another approach that has been tested experimentally (Yuan and Chiang, 2007) is the combination
of electrokinetics with a PRB. These researchers tested at laboratory scale the insertion of a PRB
within a electrokinetic cell. The best results were obtained for a barrier built with FeOOH, using
a gradient voltage of 2 V cm − 1 . Electromigration predominates over the electroosmotic flow and
As is removed by adsorption and precipitation over iron hydroxides, being found in a passivating
layer that wraps the granules of hydroxides.

1.11 CONCLUDING REMARKS

In-situ technologies are suitable for groundwater treatment due to several advantages. They are
environmental friendly and less expensive compared with conventional technologies (excavation,
transport and ex-situ treatment of the contaminated soil). It can be applied in places that cannot
be reached by excavation, rendering less polluted wastes.
However, some problems arise at present due to the novelty of these technologies. For example,
handling, mixing and injection of the suspension are more expensive than handling of a solu-
tion, the injection covers a limited radius and more sophisticated injection equipment is needed,
especially for nZVI.
However, the optimal method will come from the characteristics of the site. Generally, in-situ
treatments are optimum where there is only one contaminant at low concentrations and where
highly permeable aquifers have to be treated.
It is possible that in-situ treatments are not cost-effective for large dispersed plumes. There are
also constraints coming for regulations and laws of environmental agencies that may differ from
country to country or even from state to state.
Difficulty in getting approval for field tests by state agencies, to find funding for pilot projects,
lack of information about in-situ remediation by consultants and potential clients, reluctant to use
new technologies because of their inherent risk, lack of long term experience with the technology,
etc. are other of the possible disadvantages.
In conclusion, much more research and development have to be devoted to improve the
applicability of these in-situ techniques which appear to be rather useful to solve pollution
problems hard to be solved by conventional techniques.

REFERENCES

Alvarez, P.: Chemistry and microbiology of permeable reactive barriers for in situ groundwater cleanup.
Crit. Rev. Microbiol . 26 (2000), pp. 221-264.