Catalytic reduction of nitrate (NO3-) in groundwater over a Cu-Pd catalyst supported on active carbon was investigated in a gas-liquid co-current flow system at 298 K. Although Cu-Pd/active carbon, in which the Cu/Pd molar ratio was more than 0.66, showed high activity, high selectivity for the formation of N-2 and N2O (98%), and high durability for the reduction of 100 ppm NO3- in distilled water, the catalytic performance decreased during the reduction of NO3- in groundwater. The catalyst also irreversibly deactivated during the reaction in groundwater. The organic species in the groundwater caused the decrease in the catalytic performance and the irreversible catalyst deactivation. Ozone-treatment of the groundwater to remove the organic species substantially helped to maintain the catalytic activity and to halt the irreversible deactivation of the catalyst. Chloride ion (Cl-) in the groundwater also caused the decrease in the activity and selectivity, but the effects of Cl were reversible. Sulfate ion (SO42-) and cations, including Mg2+, Ca2+ and K+, had little or no effect on the catalytic performance of Cu-Pd/active carbon, though they were present in the groundwater sample. More than an allowable level of NH3 (NH4+) was formed during the catalytic reduction of NO3- in the groundwater, but was completely removed by the cation-exchange process using Na-mordenite. (C) 2009 Elsevier B.V. All rights reserved.