From The Associated Press via The Colorado Springs Gazette:
The driver wasn’t severely injured, but about 9 cubic yards of waste sludge spilled into the creek.
The sludge is a byproduct of a U.S. Environmental Protection Agency treatment plant that is cleaning up water draining from the inactive Gold King mine. The EPA has said the sludge is not hazardous.
Authorities say it doesn’t appear the truck spilled any fuel.