Issue Date: March 2, 2015
Persistence Pays Off In Studying Persistent Organic Pollutants
As an organic analytical chemist, Ronald A. Hites of Indiana University has been taking the measure of persistent organic pollutants in the environment for decades. In a career spanning 50 years, Hites has watched some of these chemicals survive only a few days in the atmosphere. But to his surprise, they never seem to go away completely—some have stuck around as long as he has.
For example, traces of the infamous malaria-fighting insecticide DDT remain 40 years after it was banned in the U.S.—nearly everyone has tiny amounts in their blood. Hites knows firsthand of at least one place it originated.
“Growing up in Detroit in the 1950s, the streets were lined by big elm trees that formed a beautiful canopy,” Hites recalls. “In the summer, men wearing yellow rain slickers would come down the street following a truck and spraying DDT up into the trees to kill mosquitoes and other bugs.”
Hites is one of a persistent group of environmental scientists who develop testing methods and make long-term measurements to track the fate of DDT and other chemicals such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and brominated flame retardants. These compounds were once widely used for industrial and agricultural applications and as key components of consumer products. But scientists discovered that the properties that make them useful also make them stable to natural degradation. As a result, the chemicals accumulate globally in water, soil, plants, and animals. They accumulate in people too—in blood, fat tissues, and milk—where, in some cases, they can cause neurotoxic and other negative health effects.
Many of these chemicals were banned or voluntarily discontinued starting in the 1970s. Some of their replacements were also found to be problematic and have been replaced. Each time a problematic compound is replaced, the researchers add more chemicals to the list they study.
“The questions we want to answer are simple ones,” Hites says. “Where do the chemicals come from? How fast are the concentrations decreasing? How are the concentrations varying from location to location? Are they ever going to go away?”
The answers to those questions help shape both environmental policies and business decisions, Hites says. The data he and his colleagues collect verify whether regulations are effective and provide insight for synthetic chemists and chemical companies to develop more environmentally benign alternatives.
A Dynamic Sea Of Chemicals
Hites and his team track the atmospheric concentrations of more than 200 chemicals in their goal to better understand how persistent organic pollutants cycle through the environment. Since 1990, they have been taking air and particulate matter samples every 12 days at urban sites in Chicago and Cleveland and at three rural sites in Michigan and New York and every 24 days at one site in Ontario.
The researchers work up their samples in the lab and then analyze them with gas chromatography and mass spectrometry. The effort is part of the Environmental Protection Agency’s Integrated Atmospheric Deposition Network (IADN), which was formed through mandates of the U.S.’s Clean Air Act and the U.S.-Canada Great Lakes Water Quality Agreement.
After a persistent organic pollutant is phased out, its concentration decreases at an exponential rate, Hites explains. His team uses its measurements to determine each compound’s “halving time,” which is the time it takes for the concentration to drop by half. In January, Hites and team members Amina Salamova and Marta Venier reported that the current halving times for DDT, PCBs, and PAHs are 10 to 15 years, depending on the exact compound (Environ. Sci. Technol. Lett. 2015, DOI: 10.1021/acs.estlett.5b00003).
Those figures are far greater than what you might expect based on how scientists think organic molecules degrade in the atmosphere, Hites notes. The hydroxyl radical is the primary agent in Earth’s atmosphere that chews up organic molecules. Hites’s team has determined the gas-phase reaction rate constants by which these radicals degrade persistent organic pollutants.
“Those halving times are on the order of a few days,” Hites says. “The natural processes by which you think these compounds degrade in the atmosphere are fast, compared with the overall rates we measure.”
The team concluded that the halving time is governed less by atmospheric degradation and more by slow leakage of the compounds into the atmosphere. The primary sources of those leaks, Hites says, are urban areas and landfills. But the compounds also seep back out of water and sediments, where they were first absorbed long ago.
As to whether we should still be concerned, Hites points out that the concentration of most of the compounds in the environment is relatively low, on the order of nanograms per gram of sample. There aren’t any limits or guidelines about safe atmospheric or water concentrations of any of these compounds, he says. One exception is PCBs; environmental agencies have advised the public to limit consumption of fish and shellfish that contain PCBs, dioxins, and mercury. And in that case, the concern is mostly for pregnant women and for children.
“The great thing about these long-term programs is that they give us the ability to look at the impacts of the toxics reduction efforts we have put into place,” says Todd G. Nettesheim, who manages IADN from EPA’s Great Lakes National Program Office, based in Chicago. “We are able to see trends with old chemicals and identify new chemicals as they start to appear, and that scientific data helps inform the national and international dialogue about chemicals. We are lucky to have scientists who are driven to do this work for many years.”
Hites and the IADN team are only one part of the Great Lakes monitoring system. Another is the Great Lakes Sediment Surveillance Program led by environmental chemist An Li of the University of Illinois, Chicago. Li and her Illinois colleague Karl J. Rockne take crews out on EPA’s research shipLake Guardian from which they use a sampling rig to go deep in the lakes to collect sediment cores for testing.
Li started out working on persistent organic pollutants as a student in the late 1980s. By the time she finished her postdoc in the mid-1990s, she thought environmental monitoring of halogenated compounds was on its way out. “The levels of DDT and PCBs were declining everywhere,” Li says. “I was happy for the environment, but not optimistic about my career.”
But a few years later, Li began hearing about polybrominated diphenyl ether (PBDE) flame retardants accumulating in the environment. PBDEs are a family of more than 200 organobromine compounds structurally similar to PCBs. The chemicals are used in plastics, textiles, and other materials to prevent fires during an electrical short circuit or automobile accident and to block the spread of house fires.
The most common types, which are mixtures of isomers, go by the nicknames pentaBDE, octaBDE, and decaBDE for the number of bromine atoms they contain. Penta- and octaBDE were phased out in the U.S. in 2004, and decaBDE was discontinued in 2013. They have been replaced by a new generation of flame retardants, such as bis(tribromophenoxy)ethane (BTBPE), which on average have a lower density of bromine atoms.
“I came to realize there will always be replacement chemicals and a need for making long-term environmental measurements,” Li says. She persevered in her work, and in 2001 Li received the first EPA-funded project for studying PBDEs in the Great Lakes.
Her team has now collected more than 1,100 sediment samples from across the Great Lakes. The researchers have found that concentrations of legacy pollutants such as DDT and PCBs are low and decreasing. That is also true in more recent years for penta- and octaBDE at most sites. Newer compounds such as BTBPE are present in lower concentrations than PBDEs, although BTBPE is doubling in concentration about every six years. Li estimates that roughly 6 metric tons of BTBPE now sits in the sediments of the Great Lakes, compared with about 430 metric tons of PCBs.
Sediment sampling has an advantage over air, water, and fish sampling in that the cores provide an instant chronology of pollution, Li says. “There is a history in each core, layer by layer.”
That history has revealed that in some cases older compounds such as PCBs are now exiting sediments rather than building up in them. “In the 1970s, when PCB production was high, the compounds were dissolving in water and accumulating in sediments,” Li explains. “In the 1990s, two decades after PCBs were no longer being manufactured, that process reversed in some places.” The lower concentration in the air has shifted the equilibrium so that PCBs are exiting sediment to the water, where they redeposit, accumulate in fish, or evaporate back into the air where some degrade and some redistribute.
Li’s team is also exploring sediment degradation processes. Microorganisms have evolved to feed on and break down natural and synthetic organohalogens that end up in the sediments, she notes. Researchers have known since the 1970s that PCBs undergo partial dechlorination at heavily contaminated sites, such as New York’s Hudson River. Because C–Br bonds are weaker than C–Cl bonds, brominated compounds such as decaBDE should be more readily degraded, Li notes. But the data show that process is quite slow in sediment. “Nature is practicing self-purification,” Li says. “However, we have to be patient.”
Li hopes that genomic analysis will reveal the microbial communities that are breaking down the halogenated compounds. The idea is that scientists might be able to engineer microbes to use in remediation efforts to clean up persistent pollutants. “It’s very challenging,” Li says. “We have a long way to go to translate the lab results into reality for something that might work.”
The Great Lakes region is just one place where researchers are studying trends in chemical pollutants. Environmental organic chemist Robert C. Hale of the Virginia Institute of Marine Science (VIMS), a state research, education, and advisory organization, has spent the past 30 years wading around in streams and poking around in the sludge accumulating in wastewater treatment plants. But he’s not just looking for the usual suspects.
“I like going out and hunting around for new chemicals,” Hale says. “When I find one that looks like it might be problematic based on its structure, I want to find out where it might have come from, what it is used for, how long it has been out there, and how widespread it might be.”
An environmental scandal in Virginia in the 1970s involving the now-banned insecticide Kepone helped start Hale’s career. The cyclic organochlorine compound was being manufactured at an Allied Chemical facility near Richmond. The company improperly handled the material and dumped waste into the James River.
“The problem came to light,” Hale says, “when workers at the plant started twitching and having convulsions.” Repeated exposure to Kepone and related organochlorine compounds causes degradation of the synaptic junctions of nerve cells. In 1975, Virginia closed the Kepone-contaminated James River to fishing for 100 miles, from Richmond to the Chesapeake Bay. The ban remained in effect for 13 years.
“Kepone hadn’t been on anyone’s radar screen, but it suddenly became a regional problem,” Hale notes. Hale became part of the VIMS team that studied river sediment and fish to track the progress of Kepone clearing from the ecosystem. That led him to begin investigating sewage sludge as a repository of synthetic chemicals, studying how he could use contaminants in sewage sludge to track commercial and agricultural chemical production and use.
The strategy has helped Hale pinpoint other surprising sources of persistent organic pollutants. For example, he was part of a team working on the Hyco River in Virginia in 2001 that found the flame retardant pentaBDE present at nearly 50 ppm in fish, the highest level ever recorded at the time. “That was a shocker for people,” Hale says. The researchers used data from EPA’s Toxics Release Inventory and sampled sewage sludge at wastewater treatment plants to trace the flame retardant to a plastics manufacturing facility upstream in Roxboro, N.C. The company has since gone out of business.
The Value Of Humanure
Hale says environmental scientists have for years bounced around the idea that the data they glean from sewage sludge, which people in the industry sometimes call “humanure,” could be put to better use to forecast ecological and human health risks.
To that end, last year Rolf U. Halden of Arizona State University and coworkers formally proposed that wastewater treatment plants could be used as “chemical observatories” and created the National Sewage Sludge Repository (Sci. Rep. 2014, DOI: 10.1038/srep03731). The researchers currently are collecting municipal sewage sludge from 164 wastewater treatment plants to catalog data on contaminants.
Knowing that PCBs, flame retardants, pesticides, and antibiotics are lurking in sewage sludge creates a concern over the growing use of the material as fertilizer and compost in agriculture, landscaping, and home gardening, Hale says. “All sludge is not the same—it’s not generic,” he points out. “Sludge can range from being relatively benign to god-awful.”
Sewage sludge is rich in organic matter and traps a lot of nitrogen and phosphorus, making it a good fertilizer, Hale notes. “But the material also contains chemicals that we don’t want getting into our food supply.”
The situation reminds Hale of an environmental problem in his native Michigan that helped spark his desire to be an environmental chemist. In 1973, when Hale was still in high school, a Michigan animal-feed distributor accidentally added polybrominated biphenyl flame retardants to feed. The tainted feed gradually poisoned many animals across Michigan and contaminated milk, eggs, and meat in the state. He remembers some farmers being forced to shoot their cattle because they couldn’t sell them.
“Now we are back to looking at flame retardants again. If we contaminate soil with sludge containing PBDEs, we might have a renewed problem.”
Sludge might best be packed away in landfills engineered to keep pollutants out of the environment. But landfills are expensive. Wastewater treatment plants can offset operating costs by giving away or selling the sludge, which is an inexpensive option for farmers.
EPA has established regulations on the use of sludge, but the emphasis is on heavy metals and pathogenic microorganisms. The agency assigned a no-risk value to organic compounds, Hale notes, making the assumption that the worst persistent organic chemicals, such as PCBs, aren’t being made anymore and that newer ones are more biodegradable.
Someone asked Hale recently what it would take to do a complete analysis of sewage sludge. Given how small the list of known problematic chemicals in the environment is versus how many chemicals are in commerce, he says the cost of the task would rival that of looking for a cure for cancer.
But the question has gotten Hale to thinking. He envisions that a more thorough analysis of new and archived sludge samples could be synced with human health data stemming from blood and DNA testing to help map out how people react to a lifetime of chemical exposures.
“With metabolomics and proteomics, researchers can now explore relationships between mother and daughter for the onset of breast cancer and overlay information on the chemicals and their metabolic profiles found in blood,” Hale says. If there is a link and they can understand the mechanism, the researchers could have a shot at figuring out a new way to treat cancer and other diseases or possibly accurately predict and prevent diseases, he suggests.
Hale thinks similar connections could be made from parallel studies of sewage sludge. But it won’t be easy. “Finding a cure for cancer is now my analogy for how much money it would take to look for and study the impacts of all the chemicals in the environment. It would be immense. But we could start tying together measurements we did 25 years ago that didn’t seem like they would lead anywhere. Maybe now there’s an opportunity.”
Environmental measurements on persistent pollutants have gone a long way in helping clean up and protect the environment. But eliminating the bad actors doesn’t eliminate the need for the chemicals, so the industry responds with replacements, renewing the need for the measurements.
“It’s been like playing a game of Whac-A-Mole,” Indiana’s Hites says.
But he thinks society has become wiser in the process. “We are learning that the best approach is not to attack the science that develops the compounds, or the companies that make them, or anyone who brings a potential problem to the public’s attention.”
Instead, he says, the focus is on finding better substitutes. “My experience is that the chemical industry responds to public awareness and bad publicity—no one wants to hear about toxic chemicals in babies and bald eagles.” Hites says he’s glad to see that the chemical industry has become more receptive to making improvements rather than staying on the defensive.
Hites could have passed off his project to someone else long ago and retired to focus on his woodworking hobby. But he keeps making measurements because of his professional interest and natural curiosity. “We need to keep making these measurements to statistically make sense of the rates of change,” he says. “Plus I want to see if these chemicals are ever going to go away.”
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