AbstractA study was conducted to enhance the performance of an advanced oxidation process in treating chlorinated ethenes in groundwater at IBM's groundwater treatment system at its Essex Junction, Vermont facility. A model describing the reaction kinetics and mass transfer of a co‐current ozone injection process is presented. This model, in conjunction with experiments, demonstrates that the treatment performance of the ozone treatment process at a given ozone/air concentration and ozone mass flowrate cannot be improved by varying process operating parameters such as number of ozone injectors utilized, use of a static mixer, or variation of groundwater flowrate through each injector. This is because dissolved ozone reaches equilibrium with the injected ozone/air mixture within two seconds of initial contact. Also, the Venturi‐type ozone injection system presently in use destroys nearly half of the injected ozone.Injection of hydrogen peroxide in conjunction with ozone increases the overall tetrachloroethylene (PCE) treatment efficiency by a factor of four (in comparison to ozone alone) at a H2O2/O3 mass ratio of between 1 and 2. Treatment of trichloroethylene (TCE) is enhanced by a factor of two. This enhancement of the oxidative treatment process results in a reduction in solvent mass load to a granular activated carbon (GAC) adsorption system located downstream, thus potentially reducing the usage GAC and regeneration of spent GAC. However, residual hydrogen peroxide and/or hydroxyl free radicals from the oxidation process effluent may interact adversely with certain grades of GAC; the causes of this interaction and methods to attenuate it (i.e., the use of more resistant grades of GAC) are discussed. Overall O3/H2O2/GAC system operating costs can potentially be reduced significantly (up to $ 20K annually). An economic analysis and system operation/cost optimization study are presented.