EPA Considers Establishing an Enforceable MCL for Hexavalent Chromium

By: Greg Kinsall, CPG

Chromium, a metallic element, occurs naturally and is found in rocks, soil, plants, and animals. Chromium is also widely used in steel making, metal plating, leather tanning, paints, dyes and wood preservatives. The most common forms of chromium in the environment are trivalent (chromium-3), hexavalent (chromium-6), and the metal form (chromium-0).

In January 1991 (56 FR 3526), EPA established a National Primary Drinking Water Regulation (NPDWR) maximum contaminant level (MCL) for total chromium in drinking water of 100 ppb. The MCL does not differentiate between chromium-6 and chromium-3; however, the MCL was established on the basis of potential adverse health effects from chromium-6, the more toxic species of the metal.  To understand the difference in the toxicity of the two species, consider the EPA’s Regional Screening Levels (RSLs) for tap water of 110 ppb for chromium-6 (which closely matches the total chromium MCL), and 55,000 ppb for the less toxic chromium-3. 

Hexavalent Chromium (CR+6) symbolThe health risk of chromium-6 has been previously assessed as part of the EPA’s Integrated Risk Information System (IRIS); however, the existing assessment is for inhalation of chromium-6 and did not include an evaluation of the carcinogenicity of chromium-6 via oral ingestion. Therefore, EPA began a rigorous and comprehensive review of chromium-6 health effects following the release of toxicity studies by the National Toxicology Program in 2008.  In September 2010 (75 FR 60454), EPA announced the availability of a draft report entitled Toxicological Review of Hexavalent Chromium: In Support of Summary Information on the Integrated Risk Information System (IRIS) for public comment and external peer review.  EPA has stated that when this human health assessment is finalized in 2011, they will carefully review the conclusions and consider all relevant information to determine if a new standard needs to be set specific to chromium-6.  In a nutshell, the draft EPA risk assessment says that orally ingested chromium-6 causes cancer by a mutagenic biological pathway (key word...mutagenic).  A finding of mutagenicity leads to a more stringent assessment under EPA’s cancer guideline, almost assuring that a very conservative chromium-6 drinking water MCL will be established by EPA.

Some states, most notably California and New Jersey, have already been pursuing stand-alone standards for chromium-6.  In August 2009, California prepared a draft public health goal for chromium-6 in water of 0.06 ppb, a staggering 1,700 times lower than EPA’s existing MCL for total chromium.  According to InsideEPA.com (January 13, 2011), California is now proposing an even tighter goal of 0.02 ppb (5,000 times lower than the current total chromium standard). 

EPA is being pressured about the findings of the draft assessment by both industry (claiming overestimate of risk) and environmental groups (claiming underestimate of risk).  Industry, through the American Chemistry Council (ACC), has mounted a challenge to the EPA’s contention that chromium-6 is mutagenic; if the challenge is successful, EPA would have the flexibility to establish a less stringent MCL. Environmental groups are embracing the EPA’s initial findings that chromium-6 is mutagenic, but also charge that EPA is underestimating the risk of chromium-6 due to flaws in their evaluation, and that the assessment should show an even greater risk than that presented in the draft report.

In closely related news, on December 20, 2010 the Washington-based Environmental Working Group (EWG), a non-profit environmental organization that specializes in environmental research and advocacy in the areas of toxic chemicals and corporate accountability, issued a report entitled Chromium-6 in U.S. Tap Water.  The controversial report claims that drinking water supplies in many U.S. cities have chromium-6 concentrations above California’s proposed drinking water standard (0.06 ppb).  According to InsideEPA.com (December 21, 2010), based on the EWG report, Senators Barbara Boxer (D-CA) and Dianne Feinstein (D-CA) said that they would introduce legislation that would set a deadline for EPA to set an “enforceable drinking water safeguard” for chromium-6.

So what does all this mean?  Certainly, we're very likely to see EPA set an enforceable MCL for chromium-6 in the near future.  The question becomes... how low will the drinking water standard be?  And what if the environmental groups and California are successful in their push for a chromium-6 standard that is 1,700 to 5,000 times lower than current standards?  In a January 11, 2011 memorandum to state drinking water agencies and public water supply owners, EPA provided suggestions for drinking water suppliers on how to “enhance chromium monitoring through additional sampling and analysis specifically for chromium-6.”  Cox-Colvin contacted several analytical laboratories about achieving the 0.06 ppb reporting level for chromium-6 using the EPA’s suggested analytical methods.  This level is achievable; however, the cost of conducting the chromium-6 analysis will be on the order of $225 per sample, about 10 times the cost of today’s typical chromium-6 analysis.  What will be the cost implications for treatment at our nations municipal drinking water systems, soil and groundwater remediation at sites that have contributed chromium-6 to groundwater, or the impact that this might have on waste water treatment systems for manufacturing facilities?  Is treatment to these levels feasible, or affordable?  As reported in by InsideEPA.com (January 13, 2011), the ACC states “EPA has not considered the cost to investigate and remediate soil at trace levels, which would be necessary [to protect groundwater] based on the draft IRIS assessment.  Conservatively, these costs could be in the range of billions of dollars.”   The ACC also points out that the California Department of Toxic Substances Control has estimated that chromium-6 cleanup in the San Fernando Valley alone will cost $100 to $200 million dollars.   

There's a lot of wrangling left and many questions that need to be asked and answered.  Cox-Colvin will continue to follow the progress of these arguments and will report on the outcome as more information becomes available.  But for now, expect to see additional interest by EPA and state agencies in chromium related investigations, monitoring, and cleanups.

Greg Kinsall is a Senior Scientist at Cox-Colvin & Associates, Inc. Mr. Kinsall's expertise centers around the cost-effective planning, design, implementation, and management of groundwater monitoring programs under the Ohio EPA Solid Waste Management Program and RCRA projects under both federal and state programs. He has nearly 24 years of experience in environmental consulting practice, working for numerous industrial, municipal, and State and Federal governmental clients throughout the country.


© 2014 Cox-Colvin & Associates. All Rights Reserved.