(a) The rules in this chapter provide secondary containment requirements for underground storage tank (UST) systems that are in accordance with and in addition to the requirements prescribed by Chapter 334 of this title (relating to Underground and Aboveground Storage Tanks) and where applicable, Chapter 213 of this title (relating to Edwards Aquifer).(b) A UST system which is installed, upgraded, or replaced on or after September 1, 2001, shall, at minimum, incorporate a method for secondary containment if that system is located in:(1) the outcrop of a major aquifer composed of limestone and associated carbonate rocks of Cretaceous age or older; and(2) a county that:(A) has a population of at least one million and relies on groundwater for at least 75% of the county's water supply; or(B) has a population of at least 75,000 and is adjacent to a county described by subparagraph (A) of this paragraph.(c) Effective September 1, 2001, the requirements in subsection (b)(1) of this section apply only to the outcrop of the Edwards (Balcones Fault Zone) and Trinity aquifers as defined by the Texas Water Development Board in Bexar and Comal Counties.