The gasoline additive MTBE (methyl tertbutyl ether) and solvent compounds were among the most frequently detected VOCs in urban and rural areas, according to the report. The other most frequently detected VOCs were tetrachloroethene, trichloroethene, and trichloromethane.
Volatile organic compounds are found in a variety of products including gasoline, paints, plastics and solvents. In a report released last September, an Environmental Protection Agency panel on oxygenates in gasoline attributed MTBE groundwater contamination to releases from underground gasoline storage systems, as well as small and large gasoline spills to ground and surface water including releases from recreational water craft. The panel stated that, "MTBE, due to its persistence and mobility in water, is more likely to contaminate ground and surface water than the other components of gasoline."
"VOCs can be important environmental contaminants because many are mobile, persistent and toxic," the USGS report stated. "The U.S. EPA has established maximum contaminant levels in drinking water for 27 VOCs because of human health concerns."
The USGS as part of its National Water Quality Assessment Program has sampled 2,948 wells for 60 different volatile organic compounds. Researchers collected and analyzed well water samples between 1985 and 1995. The researchers looked where contamination would not be expected, and avoided areas of known contamination.
The USGS study is not a complete assessment of the low-level occurrence of VOCs in drinking water aquifers, because it sampled selected wells. The USGS said its study "should be viewed as a progress report" assessing the occurrence, distribution, and status of VOCs in groundwater.
In urban areas where surface water and precipitation are used to recharge aquifers, and in areas impacted by spills, the groundwater contamination problem could be greater.
The sampled wells primarily were located along the eastern and western seaboards as well as some in the central United States. Although the study estimates that groundwater vulnerable to low-level contamination by volatile organic compounds may be used by as many as 42 million people, it did not determine the extent of human exposure. The researchers pulled samples from untreated aquifers, and not from the tap as delivered, and reported concentration levels as low as 0.2 ug/L.
The reports lead author, Paul Squillace, a research hydrologist with the USGS office in Rapid City, S.D., said "The samples were collected to represent the groundwater quality in the aquifer and not the water quality at the tap. This resource assessment is the first national scale assessment of [untreated water from] drinking water aquifers."
"Untreated groundwater in urban areas was four times more likely to exceed a drinking-water criterion than untreated groundwater in rural areas," according to the report. It also found that people living in more populated areas, the northeast and the West Coast, were most likely to use groundwater containing VOCs.
"The larger the population density, the greater the detection frequency of VOCs in the aquifers," Squillace said.
Chlorination, the most common drinking water treatment method, generally does not reduce VOC contamination. However, treatment facilities routinely monitor for VOCs and must use other treatment methods, such as aeration, if VOC levels rise above allowable limits. Such is not the case with private wells, he said.
In 1997, EPA issued the report, Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Methyl Tertiary-Butyl Ether. It stated: "There is little likelihood that an MTBE concentration of 20 to 40 ug/L in drinking water would cause adverse effects in humans."
The concern over the low concentrations found by the USGS study, levels mostly well below the current standards, arises because many wells had samples with multiple VOCs. Twenty-nine percent of the wells in urban areas and six percent in rural areas had two or more VOCs.
The VOCs co-occur because of their widespread use, and not only because they are used together. In fact, trichloromethane, which can form during chlorination or lawn irrigation, can be introduced into the groundwater through leaking water mains.
"Because current health criteria are based on exposure to a single [VOC] contaminant, the health implications of these mixtures are not know," Squillace said.
Drinking water criteria do not consider the co-occurrence of volatile organic compounds. As studies are conducted to determine whether a synergistic effect occurs when more than one VOC is present, new risk-based criteria may have to take into consideration the numbers and kinds of VOCs present even at very low levels.
How much actual human exposure there might be to VOCs in aquifers is "uncertain," says Squillace. But, since so many people obtain their water from aquifers, "monitoring and proactive protection of these aquifers would seem prudent," he said. While most of the detected VOC concentrations were below current criteria, the study cautions that health risk based criteria may be revised downward as new information becomes available.
An EPA Office of Water fact sheet estimates, based on 1998 Safe Drinking Water Information System reports, that 43,607 systems relied on groundwater and served 83.8 million people. Because the USGS study sampled selected wells, it remains to be seen how many more Americans actually may be affected.
A complete copy of the USGS report, Volatile Organic Compounds in Untreated Ambient Groundwater of the United States, 1985-1995 is available at the American Chemical Society website, http://pubx.acs.org.
In July, EPAs Blue Ribbon Panel on Oxygenates in Gasoline urged Congress, EPA and individual states to take action to improve Federal Reformulated Gas to protect national water supplies while continuing to try and maintain air quality benefits from the oxygenate fuel additives.
Panel chairman Dan Greenbaum said "We urge all parties to take quick action to preserve these benefits while minimizing current and future contamination of the nations drinking water supplies."
The panel called for reform to protect water supplies that included removing the Clean Air Act requirement for two percent oxygen in reformulated gas (RFG). According to the panel, federal law should be amended to allow states to regulate or eliminate gasoline additives that threatened water supplies. Its many recommendations also included accelerating the currently planned MTBE testing and reporting for public drinking water supplies to before 2001.
The Blue Ribbon Panel was appointed by EPA in 1998 and consists of experts on air and water quality, industry and environmental community representatives. The panels entire report is available at the EPA web site: http://www.epa.gov/oms/consumer/fuels/oxypanel/blueribb.htm. The site also contains links to other sites discussing VOCs and oxygenates.
A recently released U.S. Geological Survey (USGS) study found volatile organic compound (VOC) concentrations exceeded federal drinking water criteria in about six percent of urban wells and 1.5 percent of rural wells. The study estimated as many as 42 million Americans rely on the contaminated groundwater.
