The Pacific Northwest may be the epicenter of U.S. coffee culture, and now a new study shows the region's elevated caffeine levels don't stop at the shoreline.

The discovery of caffeine pollution in the Pacific Ocean off Oregon is further evidence that contaminants in human waste are entering natural water systems, with unknown consequences for wildlife and humans alike, experts say.

(Read National Geographic magazine's "Caffeine: What's the Buzz?")

Scientists sampled both "potentially polluted" sites—near sewage-treatment plants, larger communities, and river mouths—and more remote waters, for example near a state park.

Surprisingly, caffeine levels off the potentially polluted areas were below the detectable limit, about 9 nanograms per liter. The wilder coastlines were comparatively highly caffeinated, at about 45 nanograms per liter.

"Our hypothesis from these results is that the bigger source of contamination here is probably on-site waste disposal systems like septic systems," said study co-author Elise Granek.

The difference may be due to more stringent monitoring in more developed areas.

"Wastewater-treatment plants, for the most part, have to do regular monitoring to ensure they are within certain limits," added Granek, a Portland State University marine ecologist. Granek noted, though, that caffeine is unregulated, and so is not specifically monitored.

By contrast, for on-site waste-disposal systems, "there is frequently not much monitoring going on."

The big sewage plants may also be at an advantage because Oregon cities are relatively small. The plants don't have to process the sheer volume of waste associated with a major city such as Boston, which one study has found to be pumping fairly high levels of caffeine into its harbor.

(Related: "Cocaine, Spices, Hormones Found in Drinking Water.")

"Contaminant Soup" Has Unknown Impacts

Hydrologist Dana Kolpin welcomed the new research, saying caffeine concentrations in water have been documented before but more often in freshwater than marine environments.

"Caffeine is pretty darn ubiquitous, and there is growing evidence that this and other understudied contaminants are out there,"  said Kolpin, of the USGS's Toxic Substances Hydrology Program in Iowa City, Iowa.

In our waste "there is a whole universe of potential contaminants including pharmaceuticals, hormones, personal-care products like detergents or fragrances, even artificial sweeteners."

Caffeine is something of a canary in a coal mine for elevated levels of human contaminants in water, said Kolpin, who wasn't part of the new study.

In other words, if caffeine's in the water, chances are there are other contaminants too.

"What does this mean?" he asked. "Aquatic organisms are getting hit with a soup of low-level contaminants.

"Are there environmental or human-health consequences from exposure to these compounds or different mixtures of compounds? Obviously that's the million-dollar question."

(Infographic: How Coffee Changed America.)

Caffeine and Cellular Stress in Animals

Caffeine has been documented in waters around the world, including Boston Harbor, Puget Sound, the Mediterranean, and the North Sea. It might persist for up to 30 days in marine waters, study co-author Granek noted.

But the stimulant's impact on natural ecosystems is unknown. Nonlethal effects may be invisible but could have repercussions up and down the food chain and from generation to generation.

Granek and colleagues have shown in lab experiments that caffeine at the levels found offshore does affect intertidal mussels, causing them to produce specialized proteins in response to environmental stress.

The levels found in the remote study areas, for example, "did cause these mussels to exhibit cellular stress," she said. "If we expose them to higher concentrations or longer terms, do we see changes in growth rates, or changes in reproductive output?" The team hopes to find out with future experiments.

Kolpin said some studies of other contaminants have shown more drastic effects, including one at a remote Ontario Lake, which concluded that estrogen from birth control pills can cause wild fish populations to collapse.

"With caffeine, we're not yet sure about its environmental effects," he said. "But it's a very nice tracer, even if it doesn't have a large effect, because in most parts of the world, you know that this is coming from a human waste source."

The Pacific Northwest caffeine research was published in the July 2012 edition of the Marine Pollution Bulletin.