Christina Estes/KJZZ

Drinking fountain.

Almost all of Phoenix’s drinking water comes from the Salt, Verde and Colorado rivers — the rest comes from wells. But, a report released Tuesday shows higher levels of a carcinogen in drinking water sources across the nation — including Valley cities.

The Environmental Working Group’s report sifted through federal data on the contaminant chromium-6 in drinking water.

Chromium-6 is a carcinogen, and an element, that can occur naturally in certain rocks near groundwater wells.

Phoenix had an average in recent years of 7.9 parts per billion in the water samples — one part per billion is about one drop of water in an Olympic swimming pool.

City officials say the rate is closer to an average of 0.6 parts per billion, peaking at 1.7.

Phoenix Assistant Water Services Director Troy Hayes said the report neglects to include surface water that mixes with the higher contaminated groundwater.

“We feel that number is very misleading because it doesn’t really take into consideration where the chromium-6 comes from and the volume of water that makes it up,” he said.

The Environmental Protection Agency threshold for total chromium is 100 parts per billion, but California’s is set at 10 parts per billion.

West Valley cities that utilize groundwater wells also had high average amounts.

The EPA is reviewing data to see whether to lower the chromium threshold.