Which of the following best defines a "hazardous substance" in UST operations?

A "hazardous substance" in UST operations is best defined as a substance defined by the Environmental Protection Agency (EPA) that poses a risk to health or the environment. This definition is critical in the context of underground storage tanks (USTs) because understanding what constitutes a hazardous substance helps in implementing appropriate safety, monitoring, and regulatory measures to prevent contamination and protect public health.

The term typically refers to chemicals and materials that can cause adverse effects if released into the environment, making them significant in the management of USTs. This is essential for ensuring compliance with environmental regulations, which aim to minimize the risk of leaks or spills that could lead to soil and groundwater contamination.

In contrast, the other choices do not accurately define what a hazardous substance entails. For example, stating that a substance is "safe for health and the environment" or "non-toxic" directly contradicts the very essence of what makes a substance hazardous. Furthermore, a substance excluded from environmental regulations would not fall under the category of "hazardous substances" since it would not require the same level of scrutiny or control. Understanding this distinction is vital for UST operators in effectively managing and mitigating risks associated with hazardous materials.