Embodiments of the invention provide for treating arsenic contaminants in an in situ environment. In accordance with an embodiment of the invention, a stabilized oxidizing agent can be prepared, for instance a stabilized liquid hydrogen peroxide agent, as can an aqueous chelated iron solution. Both the stabilized oxidizing agent and the aqueous chelated iron solution can be alternately introduced into the in situ environment contaminated with arsenic by way of alternate injection screens into the in situ environment so as to remediate the arsenic contamination of the in situ environment.