[Code of Federal Regulations] [Title 40, Volume 21] [Revised as of July 1, 2007] From the U.S. Government Printing Office via GPO Access [CITE: 40CFR131.13] [Page 409] TITLE 40--PROTECTION OF ENVIRONMENT CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) PART 131_WATER QUALITY STANDARDS--Table of Contents Subpart B_Establishment of Water Quality Standards Sec. 131.13 General policies. States may, at their discretion, include in their State standards, policies generally affecting their application and implementation, such as mixing zones, low flows and variances. Such policies are subject to EPA review and approval.