Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/426767
wateronline.com ■ Water Online The Magazine O n July 1, 2014, California became the first state in the nation to regulate hexavalent chromium — Cr(VI) — in drinking water. The California Department of Public Health set a maximum contaminant level (MCL) of 0.010 mg/L (10 µg/L) for Cr(VI). This regulation is in addition to the existing California unique MCL of 0.050 mg/L for total chromium — Cr(tot). Total chromium is defined as the sum of the trivalent chromium — Cr(III) — and Cr(VI) species. Outside of California, Cr(VI) remains unregulated, but non-California drinking water utilities must meet the less stringent Cr(tot) MCL of 0.100 mg/L established by the U.S. EPA. The EPA has not announced if it will introduce a national Cr(VI) regulation, but it is likely that a Cr(VI) regulation will be promulgated in the future. The purpose of this article is to familiarize readers with the possible regulation and treatment of Cr(VI) in drinking water. Chemistry And Sources Of Chromium Chromium is the earth's 21st most abundant element. It can enter water through the erosion or dissolution of chromium-bearing rocks or minerals as well as through man-made contamination. In natural water, chromium exists in two oxidation states. The oxidized species, Cr(VI), is a negatively charged ion and highly soluble at the pH range of drinking water. The chemistry of the reduced species, Cr(III), is more complex. Depending on pH, it can be a negatively charged ion, an uncharged molecule, or a positively charged ion. At moderate pH (7-9), Cr(III) forms an insoluble hydroxide. Soluble Cr(III) can also associate with organic matter or metal oxides. Hence, while Cr(VI) is quite soluble in water, a large portion of Cr(III) in water may be in particulate form. In natural systems, the presence of manganese-bearing minerals can oxidize Cr(III) to Cr(VI), while the presence of natural organic matter tends to reduce Cr(VI) to Cr(III). Strong oxidants, like free chlorine, commonly used by utilities for disinfection, can affect chromium speciation by oxidizing Cr(III) to Cr(VI). The kinetics (speed) of the reaction is relatively slow under conditions seen in drinking water plants, but the reaction may become important in distribution systems where Cr(III) can have several days to oxidize to Cr(VI) under the influence of the residual disinfectant. Understanding the extent of this problem is the subject of current research. The national occurrence of chromium in drinking water is being studied. A 2004 investigation by the Water Research Foundation (WRF) of 400 utilities found an average raw water Cr(VI) concentration of 1.1 µg/L. In anticipation of a possible regulation of Cr(VI), the EPA is requiring utilities to monitor Cr(tot) and Cr(VI) in their treated water as part of the Third Unregulated Contaminants Monitoring Rule (UCMR3). The monitoring period started in January 2013 and will close in December 2015. Through July 2014, the EPA reports that of 2,640 monitored systems, 66 percent detected Cr(tot) (reporting level = 0.2 µg/L), and 87.6 percent detected Cr(VI) (reporting level = 0.03 µg/L). The reader should note that these reporting levels are far lower than the California Cr(VI) MCL. If the EPA establishes a national Cr(VI) MCL similar to California's, it is estimated that 2 to 4 percent of all drinking water systems will be impacted. Chromium's Health Effects The movie Erin Brockovich heightened the public's sensitivity to chromium, but the movie did not accurately reflect chromium's risks. Trivalent chromium is not harmful to humans at low concentrations. In fact, it is a micronutrient necessary for good health. The EPA classifies Cr(VI) as a carcinogen via exposure by inhalation, but the adverse health effects of low level Cr(VI) exposure via ingestion are less clear. Much of the debate about Cr(VI)'s human health risk centers on how effectively ingested Cr(VI) is detoxified by the gastrointestinal tract. At present, the EPA has not finalized its findings regarding the health risks of Cr(VI) via ingestion, although the State of California independently concluded that ingestion of Cr(VI) in water at levels greater than 10 µg/L represents an unacceptable public health risk. The Nationwide Impact Of California's Hexavalent Chromium Regulations California recently became the first U.S. state to regulate hexavalent chromium in drinking water. Will others follow suit? By Dr. Philip Brandhuber Regulatory Analysis 17