Chromium is a moderately common transition metal; that is to say an elemental metal with atoms that can have different electrical configurations (valencies) enabling it to exhibit different chemical properties when forming bonds with other atoms. The most common form of chromium in the environment is trivalent chromium, or Chromium (III), however the rarer hexavalent chromium, or Chromium (VI), is sometimes found in water that has passed through some volcanic rocks, which is a cause for concern as this form of chromium is recognised to be carcinogenic to Humans (that is to say it can cause cancer in Humans). In 2014 the North Carolina Department of Environmental Quality detected raised levels of Chromium (VI) in well water from a number of sites close to coal-ash settling ponds in the Piedmont region of North Carolina, as well as in the ponds themselves, raising concerns that chromium (VI) might be entering local aquifers, presenting a threat to Human health.
In a paper published in the journal Environmental Science and Technology Letters on 26 October 2016, Avner Vengosh, Rachel Coyte, Jonathan Karr, Jennifer Harkness and Andrew Kondash of the Division of Earth and Ocean Sciences at Duke University, Laura Ruhl of the Department of Earth Sciences at the University of Arkansas at Little Rock, and Rose Merola and Gary Dywer, also of the Division of Earth and Ocean Sciences at Duke University, describe the results of a study of Chromium (VI) in well water from the Piedmont region of central North Carolina.
Vengosh et al. examined water samples from 376 domestic water wells in central North Carolina. These samples were tested for the presence of Chromium (VI), and also strontium and boron, other elements that were present in the settling ponds at high concentrations. The samples were found to contain varying levels of Chromium (VI) and strontium, but low levels of boron, and wells close to the settling ponds were particularly low in boron. Further examination of the strontium in the water samples revealed a different ratio of the two most abundant isotopes of the atom (⁸⁷strontium and ⁸⁶strontium). This suggests that water was not migrating from the settling pools into nearby aquifers (which is fortunate as the settling pools also contain high concentrations of sulfate, arsenic, selenium, molybdenum, and thallium).
Having ruled out the settling ponds as the source of Chromium (VI), Vengosh et al. then looked to geological formations in the area as a potential reservoir of the ions. Water from wells in Rowan County, close to the settling ponds, was found to be high in Chromium (VI), but water from wells in Forsyth County, the closest of which was 30 km from a pond, was also high in the ion, suggesting that the geological source of the chromium (VI) passed under these counties. The Piedmont Geological Province, which underlies central North Carolina, contains a number of potential sources, with the highest concentrations of Chromium (VI) found in the Floyd Church and Cid formations, but high levels also found in a variety of metamorphic and igneous rocks in the province.
Distribution of total Chromium concentrations (ranked by color, in micrograms per liter) in drinking water wells, coal ash ponds, and selective geological formations in the Piedmont region of North Carolina. The small inset map at the top left shows the distribution of the Piedmont geology in the southeastern United States. The bottom inset map shows the distribution of Chromium near coal ash ponds close to Salisbury, North Carolona. The felsic metavolcanic rock and granitic rock categories are primarily felsic formations. The Cid, Floyd Church, metamorphosed mafic rock, and metavolcanic formations are of mixed character with varying levels of mafic components. The biotite gneiss and schist and phyllite and schist categories are characterized as general metamorphic bodies. Vengosh et al. (2016).
While excluding the settling ponds as a source of chromium (VI) can be seen as a positive thing, the possibility of high levels of the ion being derived from rocks of the Piedmont Geological Province is potentially a far greater cause for concern, as these rocks extend far beyond North Carolina, reaching from Alabama to New Jersey.
The highest concentration of Chromium (VI) reported in this study was 33.8 μg/L. The U.S. Environmental Protection Agency does not issue guidelines on specific valencies of ions, though it does regulate for total chromium, which should not exceed 100 μg/L, while the World Health Organisation recommends total chromium should not exceed 50 μg/L. A maximum Chromium (VI) level of 10 μg/L has been legislated for in the State of California (the only state to address the issue to date) and has adopted a long term public health goal of reducing levels to 0.02 μg/L in all drinking water in the state.
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