Limited studies have been conducted to evaluate synergistic benefits between disinfectants, although promise has been shown between ultraviolet (UV) light and chlorine-based disinfectants. This research aimed to determine drinking water quality factors that affect potential for enhanced removal of heterotrophic bacteria due to synergy between UV light and free chlorine. An additional goal was to determine the impact of pipe material on the formation of biofilm and the effectiveness of combined disinfection in controlling it. Annular reactors (ARs) containing polycarbonate or cast iron coupons were used to simulate typical drinking water distribution systems. Two experiments were conducted with untreated hard groundwater and groundwater treated with ion exchange to remove hardness, each comparing chorine alone to chlorine with UV pre-treatment. Results show that enhanced removal of heterotrophic plate count (HPC) bacteria existed with hard water when UV preceded chlorine, but synergistic benefits were not consistently observed. Under softened water conditions, no enhanced reduction or synergy was observed. Cast iron ARs supported biofilm HPC bacteria that were unaffected by any disinfection scheme regardless of source water, while bulk HPC bacteria were more resistant to disinfection in polycarbonate than cast iron ARs. Results indicate that disinfection synergy between UV and chlorine is limited in groundwater sources.