From The Pueblo Chieftain (Chris Woodka):
“The report that was mailed to customers shows only the tests where we found something in the water,” said Don Colalancia, manager for water quality and treatment. “We do thousands of tests throughout the year.”[…]
Testing found traces of barium, fluoride, nitrates, nitrites and selenium in the water, at levels well below the federal standards. Turbidity, trihalomethanes, haloacetic acids, sodium and microscopic particles also were measured, and again to be well below acceptable levels…
The report also lists sources of water. Pueblo gets all of its water supply from Lake Pueblo, which is fed by the Arkansas River and its tributaries to the west.
More water treatment coverage here.