How this toxic form of chromium is linked to NC's water supply
November 29, 2016
Ever since a storm water pipe burst beneath a coal ash pond in Eden in 2014, coal ash has been the most widely recognized and reported source of water pollution in North Carolina.
New research from Duke University’s Nicholas School of the Environment, however, is showing that one of the most prominent water contaminants in the state doesn’t come from coal ash at all.
Avner Vengosh, a professor of geochemistry and water quality at Nicholas sampled water from 376 wells in the Piedmont region of central North Carolina and found that a vast majority of those wells contain a metal ion called hexavalent chromium or Chromium VI.
Further, hexavalent chromium showed up in ground water regardless of whether the wells were near or far (more than 30 kilometers) from a coal ash pond. Vengosh and his colleagues published their work in the journal Environmental Science and Technology Letters.
Chromium VI—named because chromium metal gives up six electrons to make this ion—is a carcinogen and powerful oxidant, meaning that it can perform chemistry in human cells that causes cancer and rips electrons away from other atoms and molecules. You might remember it as the primary pollutant in the movie "Erin Brockovich."
So how did Vengosh and his team figure out that the hexavalent chromium was not coming from coal ash? Apart from the fact that it showed up in wells both near to and far from coal ash ponds, they used chemical tracers to determine the ratios of certain elements in the ground water.
Different types of rocks and soils have different chemical makeups. Different ratios of aluminum, silicon and iron oxides separate granite from red clay. Add some decaying organic matter for thick black soil. Ground water flowing through these different rocks and soils will, therefore, pick up different elements along the way.
Coal ash is the leftover product from burning coal, but chemically it possesses the same aluminum, silicon and iron oxides that most soils do, just with a higher concentration of harmful metals. As such, water that soaks through coal ash (also known as leachate) has a specific chemical signature, just like ground waters from other soils.
According to Vengosh, the overall chemical composition of the groundwater the researchers tested did not match the usual composition of coal ash leachate. Further, the ratio of boron to chromium, as well as the ratio of isotopes of the metal strontium in the groundwater were dissimilar to coal ash leachate. It is more likely, according to Vengosh, that the hexavalent chromium came from common soils in the Piedmont region.
“This doesn’t mean [chromium] poses less of a threat,” Vengosh said in a press release. “If anything, because the contamination stems from water-rock interactions that are common across the Piedmont region, people in a much larger geographic area may be at risk. This is not limited only to wells near coal ash ponds.”
Chromium, like many other metals can make multiple different ions. Another common one, called trivalent chromium or chromium III, is actually an essential nutrient that helps in pancreas function. Unfortunately, Vengosh and his team found that most of the chromium in the Piedmont wells was actually the toxic hexavalent chromium.
Currently, the prescribed limit for chromium in drinking water is 100 parts per billion, but Vengosh says, that standard assumes that most of the chromium in the water is the safe chromium III. Further, the Environmental Working Group recently published a report based on federal data, showing more than 200 million Americans have at least some hexavalent chromium in their drinking water.
The good news is that there are proven methods to remove Chromium VI from drinking water. Reverse osmosis filters, which force water through a tiny filter, leaving dissolved ions and metals behind. They are normally installed under the sink and come with an extra faucet on the sink.
Reverse osmosis filters are not exactly cheap. They range from about $70 to more than $1,000. Many homes with hard well water have these systems in place already. Not all homes need these filters, but Vengosh’s research shows that some North Carolinians using well water might benefit from them.
Many home water tests do not test for chromium, but the Water Quality Association, an industry group for water testing and treatment, has information on certified water testers that can detect hexavalent chromium. For more information on chromium testing, click here.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.