Aqueous-phase catalyzed reduction of organic contaminants via zerovalent copper nanoparticles (nCu), coupled with borohydride (hydrogen donor), has shown promising results. So far, the research on nCu as a remedial treatment has focused mainly on contaminant removal efficiencies and degradation mechanisms. Our study has examined the effects of Cu/Cu ratio, surface poisoning (presence of chloride, sulfides, humic acid (HA)), and regeneration of Cu sites on catalytic dechlorination of aqueous-phase 1,2-dichloroethane (1,2-DCA) via nCu-borohydride. Scanning electron microscopy confirmed the nano size and quasi-spherical shape of nCu particles. X-ray diffraction confirmed the presence of Cu and CuO and x-ray photoelectron spectroscopy also provided the Cu/Cu ratios. Reactivity experiments showed that nCu was incapable of utilizing H from borohydride left over during nCu synthesis and, hence, additional borohydride was essential for 1,2-DCA dechlorination. Washing the nCu particles improved their Cu/Cu ratio (1.27) and 92% 1,2-DCA was removed in 7 h with k = 0.345 h as compared to only 44% by unwashed nCu (0.158 h) with Cu/Cu ratio of 0.59, in the presence of borohydride. The presence of chloride (1000-2000 mg L), sulfides (0.4-4 mg L), and HA (10-30 mg L) suppressed 1,2-DCA dechlorination; which was improved by additional borohydride probably via regeneration of Cu sites. Coating the particles decreased their catalytic dechlorination efficiency. 85-90% of the removed 1,2-DCA was recovered as chloride. Chloroethane and ethane were main dechlorination products indicating hydrogenolysis as the major pathway. Our results imply that synthesis parameters and groundwater solutes control nCu catalytic activity by altering its physico-chemical properties. Thus, these factors should be considered to develop an efficient remedial design for practical applications of nCu-borohydride.