DENVER, Sept. 10 (UPI) -- U.S. authorities say water pollution by "emerging contaminants" -- antidepressants, antibiotics, birth-control pills and cosmetics -- threatens public health.

Scientists who trace urban contaminants entering water supplies through human waste, bathing and flushing are concerned they may harm people, The Denver Post reported Friday.

In Colorado several years ago, Denver Water officials discovered trace amounts of antibiotics and pharmaceuticals in water sources used to supply 1.3 million metro-area residents with drinking water.

"The fact that some compounds were detected surprised us and shows that even the best watersheds are experiencing the impacts of consumer products," Denver Water spokeswoman Stacy Chesney said.

This summer, U.S. Environmental Protection Agency teams began testing at Denver Water's Dillon Reservoir and the South Platte and other rivers.

The EPA is making emerging water contaminants a top national research priority, agency spokesman Rich Mylott said.

Colorado, with EPA funding, is expanding a pharmaceutical take-back program launched in 2009.

Colorado is one of several states with take-back programs aimed at preventing improper disposal of harmful chemicals in sewers and trash.

"We recognize that pharmaceuticals and medications have greatly improved the health of Americans," but we need to deal with the consequences, EPA toxicologist Kristen Keteles told the Post. "We want to do what we can -- eliminate the improper disposal."