April 2013, Issue 70: Editors’ Notes

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New federal regulatory standards are looming for hexavalent chromium. What will this mean for Arizona?

The EPA’s evolving efforts to set a standard for hexavalent chromium (chromium-6) may have profound implications for Arizona groundwater suppliers. Currently, public water systems are only required to meet the standard for total chromium (100 parts per billion [ppb], set in 1991), which includes both chromium-6 and its more benign counterpart, trivalent chromium (chromium-3). The California EPA adopted a drinking water goal of 0.02 ppb in July 2011 after determining that chromium-6 is likely a carcinogen.

The EPA began a rigorous review of the health effects of chromium-6 after the Department of Health & Human Services published toxicity studies in 2008. In September 2010, the agency released a draft of the human health assessment for public comment and external peer review. Once this draft is finalized, the EPA will use it, along with other relevant information, to determine if a new chromium-6 standard is warranted; this process is anticipated to take another year. In the interim, the EPA has required most public water systems to monitor for chromium-6 to gather more data on its distribution and concentrations.

So, why, after 5 years of investigation, are additional studies needed? One reason is that chromium has a Jekyll-and-Hyde personality in water: It may occur in either its trivalent (benign) or hexavalent (toxic) state depending principally on pH and oxidation conditions. Since EPA standards are risk-based, it’s important to understand which form of chromium is present in water supplies to evaluate how it will ultimately affect receptors.

Unfortunately, the conditions in Arizona’s deep aquifers — which tend to be alkaline and highly oxidized — favor the formation of chromium-6 over chromium-3. A 1975 USGS study found that chromium-6 concentrations in 436 samples in Paradise Valley ranged from 0 to 220 ppb. A more recent study by the Environmental Working Group looked at chromium-6 levels in tap water from 35 cities and found that it ranged from 0 to 12.9 ppb; chromium-6 levels were 0.19 and 0.05 ppb, respectively, in Phoenix and Scottsdale. In addition, chromium can partially or completely oxidize to chromium-6 when it occurs in water supplies that are subjected to chlorination or certain other disinfection processes.

If the EPA follows California’s lead in setting a low value for the new chromium-6 standard, it’s likely that many Arizona water providers will be required to treat their sources or develop new ones. The costs related to compliance and treatment may be similar to those for arsenic, which is now regulated under a standard established in 2006. In the short term, at least, this new standard has the power to impact both the availability and cost of municipal water supplies in Arizona — perhaps as much as any other market factor we have seen in a long time.