In-situ immobilization of arsenic in the subsurface on an anthropogenic contaminated site - In-Situ Remediation of Arsenic-Contaminated Sites - page 153

Figure 7.10. Mean values of Fe concentration per cycle in well 1 (triangles) and well 2 (circles); error bars
denote the minimum and maximum of values in a cycle.

7.5 CONCLUSIONS

We presented in this chapter an in-situ method for the immobilization of arsenic from chemical
warfare agents. The method is based on subterranean deferrification and demanganesation with
an artificial infiltration of bivalent Fe. We showed that inorganic as well as chemical warfare
agent based (mainly organic) arsenic compounds could be immobilized in-situ . During the pilot
study which lasted almost two years, the initial arsenic concentration of around 1.65 mg L − 1 in

the groundwater has been reduced to less than 0.3 mg L − 1 and in total more than 2.3 kg As have

been immobilized.
The main advantage of the method is that no contaminated sludge or adsorption material has
to be disposed off. One of the greatest advantages of the method is that all operating cycles use
the same pore space. Therefore, the contaminated groundwater has to flow through the reactive
domain before it reaches the well and the infiltrated Fe has to have contact with the infiltrated
oxygen. Although this describes also the biggest challenges when applying the method: the iron
hydroxide has to precipitate in the pores, and there is a risk that iron hydroxide precipitates
even at the screen or in the well. This would induce clogging and has to be prevented. We did
so by stripping the water with nitrogen. Another possibility is to decrease the pH-value of the
water before infiltration. As a result, we did not observe any hydraulic response in means of
lower infiltration capacities during our study although a camera survey showed that a soft film
covered the well screens after one year of operation. Finally, in the efficiency test of the wells we
determined that the maximum pump rate was not decreased significantly in the wells.
The hydrogeochemical environment of the application site has to be evaluated before applying
this method. High concentrations of competitors for sorption spots may decrease the efficiency
of the method and highly reducing environment may lead to the iron being remobilized and
thus the As, as well. However, we have not found any evidence of remobilizing iron during the
remobilizing experiment at the field site.