When good compounds go bad
The same gasoline additive used to improve air quality is also contaminating the nation’s groundwater—used as a source of drinking water for 35 million to 50 million people.
A U.S. Geological Survey (USGS) study of almost 3,000 untreated wells in the United States showed that methyl tert-butyl ether (MTBE), an oxygen additive used in gasoline for cities with high carbon monoxide and tropospheric ozone levels, is one of four top contaminants to groundwater.
The report, published last month in Environmental Science & Technology, is the first to look at the amount of volatile organic compounds, chemicals found in manufactured products, in the nation’s untreated groundwater. The data came from water collected between 1985 and 1995.
The three other main culprits contaminating groundwater were trichloromethane, tetrachloroethene and trichloroethene. Chloroform, the common name for trichloromethane, is a byproduct of water treatment and distribution that is then recycled back into the groundwater by lawn irrigation, leaking pipes and sewers. Tetrachloroethene (PCE), an industrial solvent, degrades into trichloroethene (TCE), which is also used.
The Clean Air Act Amendments passed in 1990 require cities with poor air quality to use oxygen additives in gasoline to help it burn more efficiently. “MTBE is widely used because it’s cheaper and more economically feasible,” says Rachel Sakata, environmental protection specialist at the Environmental Protection Agency.
Volatile organic compounds (VOC) are produced in abundance in the United States. They often evaporate quickly and sometimes have half-lives of only a few hours. But in some environments they can persist for long periods of time, evading detection for years.
“It is in those areas where wells were found to have levels that exceeded drinking water standards that we need to focus increased attention,” says John Zogorski, a hydrologist who headed the study. Some aquifers, especially those near populated areas, may be more vulnerable to contamination by VOCs, he says.
Zogorski and colleagues evaluated urban and rural wells across the conterminous United States for 60 VOCs, 27 of which have a health advisory set by the Environmental Protection Agency for drinking water. One or more of the VOCs studied were found in 47 percent of the 406 urban wells tested and in only 14 percent of the 2,542 rural wells tested.
Although drinking water treatment facilities must watch for VOCs, private wells do not. Aquifers came out clean for most of the lower 48 states, but it is four times more likely for groundwater contamination to exceed drinking water criteria in metropolitan areas than in rural areas. Tighter controls may be needed to ensure that the untreated water in cities is safe. And the first compound to question may be methyl tert-butyl ether (MTBE).
That MTBE came out as one of the four top contaminants in Zogorski’s national study may open questions about the chemical’s benefits. “Some states are concerned because they don’t want to see any degradation to their drinking water,” Zogorski says. On the other hand, he says, using it to reduce the carbon dioxide and ozone in the air also helps remove benzene, a known human carcinogen.
Although high concentrations of MTBE will degrade
water quality to undrinkable levels, the EPA does not consider it a health
hazard but does offer regulatory guidance to limit taste and odor problems.
One example Zogorski and colleagues include in their report to the Water
Resource Conference is a case of MTBE poisoning in Santa Monica, Calif.:
“In 1997, the City ceased pumping groundwater from two well fields due
to persistent and increasing concentrations of MTBE. This loss accounted
for 50 percent of Santa Monica’s total drinking water supply.” Concentrations
of MTBE in the water ranged from 8.2 to 610 micrograms per liter; the EPA
considers 20 to 40 micrograms enough to affect taste and odor, according
to the report. Other cases in Illinois, Texas and California, Zogorski
says, called for the treatment of water or removal of wells after public
water supply systems reported groundwater concentrations of MTBE greater
than 20 micrograms per liter.
While contaminated groundwater can carry it to some showers,
methyl tert-butyl ether (MTBE) is not as dangerous as showering
in the Bates Motel, as Anne Heche discovered in the 1998 remake of "Psycho." Universal Studios.
|“It smells like turpentine,” Sakata says. Although
most people can smell or taste MTBE at levels between 20 and 40 micrograms
per liter, the EPA benchmark is still a subjective number. “More sensitive
people may detect MTBE at five micrograms and others may not detect it
until 60,” she says.
But even if the water smells like fresh paint there’s no real understanding of the health effects. “All the health studies so far have been on rodents,” Sakata says. The EPA is conducting voluntary human exposure studies to investigate oral and dermal routes of MTBE through showering, bathing and drinking. “It’s on our radar screen,” she says. “We’ve been detecting it more often but at low levels. We will have to wait for the national occurrence information to find out how prevalent it is in drinking water and get results back on our health studies.”