(Inside Science) -- As far as most people are concerned, "down the drain" means gone forever -- vanished out of sight and mind. But most of the water we flush away ends up in our streams, rivers and lakes, along with a cocktail of hard-to-remove chemicals such as pharmaceuticals, according to Jacelyn Rice, an environmental engineer at the University of North Carolina at Charlotte. Now, a nationwide study by Rice and her colleagues suggests that those chemicals may become concentrated at levels dangerous to wildlife when stream levels are low.
Messing with hormones
More than 14,000 wastewater treatment plants take in sewage from American households, treat it to remove contaminants such as bacteria and heavy metals, and then get rid of the leftover water, either by using it for purposes like irrigation or by releasing it directly into the environment. This process can drastically alter the flow of surface water, with more than 900 streams in the U.S. composed of at least 50 percent treated wastewater, according to Rice's estimates. These high-wastewater streams are somewhat concentrated in the dry southwestern parts of the country, but many are located in more humid regions.
In some respects, adding wastewater can help the environment, replacing water that was taken out upstream and ensuring that wildlife has enough to drink and swim in. But treatment plants are required to test for only certain kinds of contaminants. According to Rice, there are no wastewater regulations specifying acceptable levels of most of the chemicals known as endocrine disruptors -- substances that can interfere with the body's hormone systems.
Around 800 chemicals are known or suspected to be endocrine disruptors, including certain drugs, industrial chemicals, pesticides, and ingredients in household products, according to the World Health Organization. If these chemicals reach waterways in high enough concentrations, they can harm ecosystems by altering the behavior and development of wildlife such as frogs and fish. Some researchers suspect that endocrine disruptors in the environment and consumer products may also be to blame for rising rates of certain human ailments, including breast and testicular cancer, infertility, diabetes, learning disabilities, and birth defects of the penis and testes.
Scientists and regulators have been growing increasingly worried about endocrine disruptors, and the U.S. Environmental Protection Agency now considers them to be "contaminants of emerging concern." Nevertheless, regulators have only recently begun to consider endocrine disruptors, and according to Rice, our water purifications systems weren't designed to deal with them. There are technologies that can effectively remove endocrine disruptors from wastewater, but these extra treatments cost money and energy, and few U.S. treatment plants use them, said Rice.
From 2011 to 2012, the EPA tested for a variety of endocrine disruptors in wastewater released by 50 of the largest treatment plants in the U.S. Rice and her colleagues focused on three of those chemicals, all types of estrogen -- the primary female sex hormone and a component of birth control medication. Using the EPA's measurements, they estimated the concentration of estrogens at additional plants that release water into 1,049 streams across the country.
At each of these streams, the U.S. Geological Survey had previously installed stream gauges to track water levels, which allowed the researchers to estimate what proportion of each stream was composed of wastewater. It also allowed them to estimate the estrogen levels that fish and other wildlife were exposed to. The researchers considered the concentrations potentially unsafe if they were greater than a tenth of what is known to cause harm to animals -- a threshold typical of contaminant regulations, said Rice.
Their findings suggested that at low flow levels, more than half of the streams exceeded the researchers' safety thresholds for one type of estrogen, and about a third of them exceeded thresholds for two types of estrogen. The streams would experience these low-flow conditions about 5 percent of the time; the rest of the time, they would have more water to dilute the contaminants. The study was published this month in Nature Geoscience.
"I was surprised," said Rice. "Looking at this at the national scale helps to really identify the scope of the potential problem."
The study may actually have underestimated the prevalence of contaminated streams, because the USGS is probably more likely to install stream gauges in larger streams and rivers, said Rice.
The findings emphasize the need to consider the most severe conditions wildlife will be exposed to, rather than assuming that pollutants are evenly distributed across time and space, said Bryan Brooks, an environmental scientist at Baylor University in Waco, Texas, who was not involved in the study.
"Organisms deal with extremes, not averages," he said.
While Rice and her colleagues only focused on three chemicals, their findings may point toward a larger problem, according to Dana Kolpin, a research hydrologist in Iowa City, Iowa, and head of the USGS' Emerging Contaminants project, who was not involved in the study. Endocrine disruptors are often considered individually, but past research has shown that their effects can add up when more than one type is present at once. Earlier this year, USGS and EPA researchers exposed cell cultures to water from 35 streams, and found that some of the cells' hormone receptors were activated in ways that couldn't be explained by any particular chemical detected in the water.
The new study should be treated with caution, since its estrogen concentrations were based on estimates, not direct measurements, noted Larry Barber, a research geochemist with the USGS in Boulder, Colorado. To confirm the findings and understand their implications, researchers will need to test at-risk streams for endocrine disrupting activity and chemicals, and look for actual impacts on wildlife.
Nevertheless, Barber hopes the findings will serve as a wake-up call about our interconnected water supply.
"As a hydrologist, I often ask people where their drinking water comes from, and they say their tap. Then I say, 'where does it go,' and they say, 'down the drain,'" said Barber.
Studies like this, he said, drive home the reality that one person's wastewater is another person's boating, fishing and drinking water -- and maybe we should all watch what we put into it.