The well owner is responsible for testing and treating their water. Many well owners appreciate having the flexibility to treat their water as they want.
Most city water supplies are sourced from a surface water body. Surface waters are exposed to the elements, which means they can be polluted by deposits from the air, rainfall, and surface runoff.
Plus, city water is also at risk of picking up contaminants from the underground pipe system as it travels to your home, and there’s always a risk of microbial contamination.