30 mil PVC is allowed for landfill liners by U.S. EPA Regulation ~ the same U.S. EPA regulation that requires HDPE to be TWICE as thick as PVC in the same application...!
United States Environmental Protection Agency
2003 CFR Title 40, Volume 22 - Select 258.40 "Design Criteria"
§ 258.40 also located at http://www.ehso.com/SolidWasteRegs.htm
40 CFR Ch. I (7–§ 258.29 1–02 Edition)
Subpart D—Design Criteria
§ 258.40 Design criteria.
(b) ... composite liner means a system consisting of two components; the upper component must consist of a minimum 30-mil flexible membrane liner (FML), and the lower component must consist of at least a two-foot layer of compacted soil with a hydraulic conductivity of no more than 1x10-7 cm/sec. FML components consisting of high density polyethylene (HDPE) shall be at least 60-mil thick.
Here is a representative list of States with regulations using the requirements of EPA 40 CFR - Subpart D - § 258.40. This list is not meant to be all inclusive: