Brorby G, Ring C, Loko F, Harris MA. Characterization of hexavalent chromium and total chromium in drinking water monitoring data. Poster MP 117. Society of Environmental Toxicology and Chemistry (SETAC) North America 39th Annual Meeting, Sacramento, CA, November 4–8, 2018.
In 2014, the State of California promulgated a maximum contaminant level (MCL) for hexavalent chromium [Cr(VI)] of 10 µg/L, as compared to the MCL for total chromium of 50 µ/L. Although the California MCL for hexavalent chromium is no longer in effect as a result of a court decision, its adoption created immediate interest because hexavalent chromium had been reported in public drinking supplies in California at concentrations exceeding 10 µg/L, but rarely above 50 µg/L. Since that time, a substantial amount of data (over 60,000 samples) on Cr(VI) and total chromium concentrations in public water systems (PWSs) nation-wide has been collected as part of the Third Unregulated Contaminant Monitoring Rule (UCMR3). From these data, the Cr(VI) concentration was less than 10 µg/L in approximately 97.6% of groundwater samples and in >99.6% to 100% of other samples (surface water, mixed surface water/groundwater; groundwater directly influenced by surface water). In California, the percentages were approximately 93.9% and >99.2% to 100%, respectively. The total chromium concentration was less than 50 µg/L in >99.8 to 100% of all sample types whether evaluated nationwide or California alone. Compared to the Federal MCL for total chromium of 100 µg/L, 100% of the Cr(VI) concentrations and all but one of the total chromium concentrations were less than 100 µg/L. To evaluate the geographical distribution of higher Cr(VI) and total chromium concentrations, UCMR3 sample locations were estimated for each PWS as the centroid of all ZIP codes that the PWS reported serving (exact locations are not publicly available for security reasons). Using these estimated sample locations, PWSs with higher concentrations of Cr(VI) and total chromium tended to cluster regionally in California, as well as in Arizona and Oklahoma. This analysis could serve as a basis for further investigation of geospatial occurrence or sources of Cr(VI) and total chromium in drinking water.