Embodiments of the invention relate generally to methods and systems for treating aqueous systems associated with industrial wastewater applications and gas and crude oil drilling, pumping and production to reduce or eliminate contamination. In one embodiment, a method includes: an aqueous volume having an initial oxidant demand, an oxygen-containing oxidant and at least one chlorine oxide at substoichiometric amounts in an amount sufficient to eliminate the oxidant demand. The system includes: an aqueous treatment system comprising a containment system; at least one apparatus for introducing an oxygen-containing oxidant; and at least one apparatus for introducing at least one chlorine oxide into said containment system at controlled, sub stoichiometric quantities.