GLOSSARY OF TERMS AND ACRONYMS A B C D E F G H I J K L M N O P Q R S T U V W X Y Z U Underground storage tank system (UST) An underground storage tank system (UST) is a tank and any underground piping connected to the tank that has at least 10 percent of its combined volume underground. Under the Resource Conservation and Recovery Act (RCRA), EPA has established regulatory programs to prevent, detect, and clean up releases from USTs containing petroleum or hazardous substances. Unsaturated zone The area above the water table where soil pores are not fully saturated, although some water may be present.