What kind of substances do USTs typically store?

Underground Storage Tanks (USTs) are primarily designed to store liquids, which can include both hazardous and non-hazardous materials. This versatility is essential as USTs are commonly used to hold fuels such as gasoline, diesel, and various chemicals, which are typically in liquid form.

The inclusion of both hazardous and non-hazardous substances is significant because it acknowledges the vast range of materials stored in these tanks, from petroleum products to certain chemicals used in industrial processes. Recognizing this aspect is vital for understanding the regulation and environmental impact of USTs, as hazardous materials may require stricter compliance measures to protect groundwater and soil from contamination.

In contrast, the other options provided do not accurately represent the typical operation and purpose of USTs. They are not designed for solid or semi-solid materials, which is why specializing only in those forms of storage would be incorrect. Water, while it can be stored in USTs, does not encompass the full scope of their usage, especially since many tanks are used for fuel storage. Thus, the range of liquids stored in USTs makes the choice that includes both hazardous and non-hazardous liquids the most comprehensive and accurate representation of what USTs typically store.