Which disinfectant applied near the point of entry is least likely to provide residual throughout the domestic water system?

Using ultraviolet light as a disinfectant near the point of entry is least likely to provide a residual effect throughout the domestic water system. This is due to its method of action; ultraviolet light disinfects by disrupting the DNA of microorganisms, effectively killing or inactivating them as water passes through the UV chamber. However, it does not remain in the water post-treatment.

In contrast, other disinfectants such as chlorine and chloramine are chemical agents that not only kill pathogens but also leave a residual concentration that continues to inhibit microbial growth throughout the plumbing system. Chlorine can effectively provide a residual effect, ensuring continued disinfection as water moves through the system. Similarly, chloramine, a compound formed by combining chlorine with ammonia, is designed to provide long-lasting disinfection. Ozone, while highly effective as a disinfectant due to its strong oxidizing properties, also does not leave a residual. It breaks down quickly and does not provide protection further along the supply lines.

Overall, while multiple disinfection methods are effective in their own rights, UV light stands out as the method that does not maintain a residual presence in the water system, making it the correct choice.