According to R.61-92. Part 280.25 (pdf)

Not later than December 22, 2018, all UST systems located within 100 feet of an existing water supply well , a coastal zone critical area (pdf) , or state navigable waters (pdf) must comply with one of the following requirements:

• Secondary containment requirements of Section 280.20(g); or
• Permanent Closure requirements under Subpart G of this Part, including applicable requirements for corrective action under Subpart F.