Which agency is mainly responsible for ensuring safe drinking water in the United States?

The agency primarily responsible for ensuring safe drinking water in the United States is the Environmental Protection Agency (EPA). The EPA oversees the Safe Drinking Water Act, which sets the national standards for drinking water quality. This includes regulations that limit the levels of contaminants in drinking water, thereby protecting public health. The agency also conducts research, provides guidance for states, and ensures compliance with drinking water standards.

While other agencies, such as the Food and Drug Administration (FDA), play roles in food safety and pharmaceuticals, they do not directly regulate drinking water quality in the same manner as the EPA. Similarly, the Centers for Disease Control and Prevention (CDC) focus primarily on public health and disease prevention rather than water quality specifically. The Occupational Safety and Health Administration (OSHA) is concerned with workplace safety, which does not encompass the regulation of drinking water. This clear focus on drinking water quality and safety makes the EPA the correct answer.