Urban wells most affected by volatile organic compounds
About 6 percent of urban wells and 1.5 percent of rural wells in the US contain levels of volatile organic compounds (VOCs) in excess of drinking water criteria, according to a US Geological Survey (USGS) study.
The study estimates that about 7 percent of US groundwater contains at least one VOC and that groundwater in the eastern half of the US is more likely to contain VOCs than in the western half of the country.
The study also found that people living in more populated areas, such as the northeast and the west coast, were most likely to use groundwater containing VOCs. In urban areas, about half of the sampled wells had at least one VOC and about 30 percent had two or more VOCs. In rural areas, about 15 percent of the wells had one VOC and 6 percent had two or more VOCs. “The larger the population density, the greater the detection frequency of VOCs in the aquifers,” notes report’s lead author Paul Squillace.
Groundwater in these areas is the source of drinking water for about 42 million people. 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 says.
When VOCs were detected in the samples, they most commonly occur with other VOCs, the study found. “One common pattern that the USGS is seeing in its assessment of the quality of US water is that chemical compounds occur in mixtures, a finding which is reinforced by this study of groundwater aquifers for drinking water supply,” said John Zogorski, chief of the VOC component of the USGS National Water Quality Assessment Program.
But, adds Squillace, the health implications of these mixtures are not known because current health criteria are based on exposure to a single [VOC] contaminant. Meanwhile, Dr Jim Pankow, Department Head of the Oregon Graduate Institute’s Department of Environmental Science and Engineering has told edie that his department is working on developing a procedure to understand the health implications of USGS data. This procedure will take ‘co-occurrence’ into account, Dr Pankow says.
Under the Safe Drinking Water Act, the US EPA has established drinking water standards and monitoring requirements for community water systems for 27 VOCs. The EPA standards are four times more likely to be exceeded in urban areas than in rural areas, according to the report.
Chlorination of public water, the most common treatment method, generally does not reduce VOC contamination, says Squillace. However, he notes, treatment facilities routinely monitor for VOCs and must use other treatment methods, such as aeration, if levels get above allowable limits. Such is not the case with private wells, he adds.
“The problem here is that you can’t tell people that they have to treat their water,” Dr Pankow told edie. “The possibility does exist, however, to ask well-drillers to provide data on the water they access before the well could be certified for use.”
The USGS collected samples from almost 3,000 US wells between 1985-1995. The sampled wells were primarily located along the east and west coasts, and in the central USA. The lowest concentration detected in the wells sampled was 0.2 micrograms per litre (2 parts per billion).
In urban and rural wells in the USGS study, the four most frequently detected compounds, in order, were chloroform, methyl tert-butyl ether, tetrachloroethene (PCE), and trichloroethene (TCE).
The USGS put a positive spin on the study’s findings. “The good news is that VOC concentrations that were below drinking water criteria were much more common, than those that exceeded those standards,” said Zogorski. “It is in those areas where wells were found to have levels that exceeded drinking water standards that we need to focus increased attention to better understand the vulnerability of these aquifers to contamination by VOCs. Because many people obtain their drinking water from aquifers, monitoring and protection of these aquifers are needed.”