Cr is the 21th most abundant element in the earth crust, and almost all Cr is from chromite (FeCr2O4) ore. The natural weathering, such as rock corrosion, and industrial activities including leather tanning, stainless steel welding, plating solutions, and Cr pigment production, are the common ways Cr are transmitted to our air, water, soil, and food.
Cr has seven forms or species in nature, but only trivalent (III) and hexavalent (VI) exist in water. Both forms can interconvert under different oxidative and pH conditions. Cr(III), prevalent in different plants, is nontoxic and found to be an essential nutrient involved in blood sugar metabolism. Cr(III) is often used in nutrient supplement and has recently been found to be oxidized to Cr(VI). Cr(VI) if inhaled, on the other hand, is a highly toxic carcinogen. However, there is still some debate on whether Cr(VI) is a carcinogen when ingested.
What is the debate about?
From the very start, the cause and effect of chromium exposure to humans has been controversial. In the now famous 1996 case, Erin Brockovich successfully won the case for Hinkley residents against PG&E for Cr(VI) contamination in local groundwater by citing a technical article concluding that an occupational exposure to chromium resulted in human cancer. The California EPA delayed regulating total Cr at this time because the article’s results were challenged, followed by the author retracting his results, despite under suspicious conditions. After the National Toxicology Program (NTP) 2008 report on Cr (VI), the California EPA proposed to regulate Cr(VI) based on studies with laboratory rodents which concluded consuming Cr(VI) in food and drinking water is carcinogenic.
The NTP findings were controversial, and in response the American Chemistry Council (ACC) hired ToxStrategies, an environmental consulting firm, in 2011to conduct a series of studies. ToxStrategies’ findings published from 2013 to 2016 discount the NTP conclusions, stating that humans had a different “mode of action” to Cr(VI) exposure and respond differently than rodents. Specifically, the human ingestion of Cr(VI) did not cause mutations as it did with rodents. Further the report concluded that humans could effectively convert Cr(VI) at the 100 ppb drinking water limit to the non-toxic Cr(III). The report in 2010 also discounted historical human occupational exposure because the exposure was at much higher concentrations than the drinking water limit and by a different exposure mode (inhalation rather than ingestion). The ToxStrategies reports concluded that the current 100 ppb total Cr regulation was acceptable and the proposed reduction to ...
The EPA drafted the health effect estimate for Cr in 2010 but decided to delay the decision for revision of current Cr regulations. The EPA then placed Cr(VI) and total Cr on the Unregulated Contaminant Monitoring Rule (UCMR) 3 list for monitoring from 2013 to 2015. This delay and ACC’s involvement have led to criticism from multiple organizations. Before the national report on chromium (VI) was released on September 20, 2016, the EPA was and is currently still working on the health assessment of chromium (VI). They will release the information for public comment in 2017.
The U. S. EPA set both Maximum Contamination Level Goal (MCLG) and Maximum Contamination Level (MCL) at 50 ppb for total Cr in 1977. In 1991, the U.S. EPA increased the MCL to 100 ppb based on the research conclusion at that time that Cr was not carcinogenic. The U.S. regulation has not been revised since 1991.
California has been more restrictive, retaining the original 50 ppb MCL for total Cr. In addition, California’s Office of Environmental Hazard and Heath Assessment (OEHHA), set the Public Health Goal (PHG) of Cr(VI) to 0.02 ppb in 2011. In 2013, the California Department of Public Health (CDPH) specifically set the Cr(VI) MCL at 10 ppb, effective on July 1, 2014.
What are Cr(VI) levels in the U.S.?
The 2011 data from 6500 water samples across the U.S. for Cr(VI) shows that only 10% of the samples contain greater than 6.89 ppb Cr (VI) and 75% of the samples have Cr(VI) concentrations above 0.04 ppb. In the 2011 to 2012 study, which examined 10,000 U.S. water samples, 75% were above the UCMR 3 Minimum Reporting Limit (MRL) of 0.03 ppb Cr(VI), and 60% of the samples contained more than 0.1 ppb of Cr (VI). If the current California 10 ppb regulatory standard was applied to the 10,000 U.S. water samples, less than 10% of the national samples would exceed the limit.
Currently, dissolved Cr (VI) is measured as chromate by EPA Methods 218.6 and 218.7 using ion-exchange chromatography with spectrophotometric detection. EPA Method 218.7 is more sensitive and required for Cr(VI) monitoring in UCMR 3 with a MRL of 0.3 ppb. The method also provides sufficient sensitivity to measure Cr(VI) at 0.02 ppb, the California PHG for Cr(VI).
UCMR3 requires both total Cr and Cr(VI) to be analyzed by EPA Method 200.8 (and other standard methods) and Method 218.7, respectively. A report from the Water Research Foundation and Dr. Eaton’s presentation in 2014 has demonstrated that Method 218.7 is much more sensitive (by at least one order of magnitude) than Method 200.8, especially when the Cr concentration is low. This suggests there may be bias in detecting sub-ppb levels of total Cr even when the samples are digested by acid. It is also worth noting that the MRL of 0.2 ppb for total Cr is much lower than that for previous analysis, including the analyses done during the EPA’s second 6-year review. Not surprisingly, the low MRL increased the locations with detectable Cr(VI) in the nation.