This is an overview:
The National SCADA Test Bed Standards Report outlines the common industry standards and guidelines. From what I know, the requirements from DOE do not dictate the on-site systems to component or protocol levels, but the general communication schemes - Nothing in, nothing out. I imagine that "regulations" vary widely from site to site since each company must certify its compliance, but within how that company manages its security. Among those I've done work for pipeline, refining, and ethanol producing companies, in addition to the "normal" work for food, beverage, chemical, paper, metal finishing, and other types of businesses along with the typical municipalities.
I've had to submit a "secure" internal network plan to the company for approval so that they can certify it "secure" to their agencies. AFAIK the DOE doesn't dictate below that level. I've been on DOE regulated sites which do not allow anything but text email in and out (no html, no attachments, etc.), restrict cellular phones, have no wifi, and similar restrictions on communications. I have provided control systems on these sites with no external true duplex communications. Anything on the site can be wide open, but with no external ethernet, RF or other externally accessible network connections. The SCADA PCs look like a "normal" one, HMIs have all the same features and animations and functions, but it's all internal to the site. The SCADA PC is on the site with no outside network connected. Any external monitoring and alarms are done via isolated digital signals the simple, old fashioned way - Relays for alarms, status, and remote safeties, and MAYBE a phone dialer. Any reprogramming is not via ethernet or even MODEM, but on site or by shipping a new PLC ROM module or HMI flash card or USB stick. Get it right and mail it, or plan a trip.
I've heard that DOD sites are tougher, but I'm totally ignorant about that.