Hexavalent Chromium in U.S. Drinking Water

Chromium can be found in drinking water in both the trivalent and hexavalent forms as a result of natural (i.e. geologic) and man-made (i.e. anthropogenic) sources. The current U.S. EPA maximum contaminant level (MCL) for total chromium in drinking water is 100 ug/L. To understand the occurrence of hexavalent chromium [Cr(VI)] and other contaminants in U.S. drinking water that are currently not regulated under the Safe Drinking Water Act, the U.S. EPA is conducting a nationwide survey of public water systems (PWS) under the Third Unregulated Contaminant Monitoring Rule (UCMR3). Cr(VI) concentrations have been characterized in this survey using Method 218.7 with a minimum reporting level (MRL) of 0.03 ug/L.

Our analysis of the preliminary data for UCMR3 (USEPA 2017a, 2017b) shows that Cr(VI) detection in the U.S. drinking water supply is widespread, but occurs predominantly at low levels. Of the approximately 4900 PWS systems surveyed as of December 2015, 89% reported levels of Cr(VI) at or above the MRL. The average Cr(VI) concentration across all samples[1] (n=62,837) was 0.786 ug/L and reported concentrations ranged from the minimum reporting level of 0.03 ug/L to a maximum of 97.38 ug/L.

Figure 1 presents average Cr(VI) concentrations by state. State averages ranged from 0.06 ug/L (RI) to 6.31 ug/L (OK)

Figure 1. Map of average Cr(VI) concentrations in drinking water throughout the U.S. These values include non-detects as 1/2 the MRL of 0.03 μg/L. The total number of detects over the total number of samples collected in UCMR3 through December 2015 are also presented by state.

Background concentrations of Cr(VI) in U.S. drinking water are well below the Cr(VI) doses used in the NTP 2-yr animal carcinogenicity study (NTP 2008). Figure 2 compares the range of Cr(VI) detected in U.S. drinking water (USEPA 2015b) to the minimum Cr(VI) dose tested on male/female rats and mice that resulted in carcinogenicity (20,000 ug/L). The lowest observed carcinogenic Cr(VI) dose (based on historical control data) from the NTP study is approximately 20,000 times and 200 times greater than the average and maximum concentrations in U.S. drinking water, respectively.

Figure 2. Detected concentrations of Cr(VI) in U.S. drinking water samples collected through December 2015 by the UCMR3 survey (μg/L). The whiskers represent minimum and maximum concentrations detected; the blue box represents 25 to 75 percentiles of detected values; the dark black line represents the median concentration detected. The dotted black line marks the lowest carcinogenic dose of Cr(VI) in the 2-yr NTP carcinogenicity study (based on comparisons to historical control data).

Finally, presented below in Table 1 is a summary of the UCMR3 results for individual states.

Table 1: Concentration (ug/L) of Cr(VI) reported in UCMR3 (January 2017) in States and Territories*

State/Region	Samples	Detects	Avg Conc	Min	Max
Alaska	105	93	0.26	0.015	1.5
Alabama	1953	1462	0.16	0.015	3.2
Arkansas	508	373	0.18	0.015	3.51
American Samoa	78	72	0.77	0.015	3
Arizona	2017	1884	4.52	0.015	79
California	10008	9139	2.10	0.015	47
Colorado	1121	727	0.12	0.015	1.3
Connecticut	614	504	0.17	0.015	1.2
District of Columbia	32	32	0.08	0.042	0.12
Delaware	202	140	0.31	0.015	4.7
Florida	2761	1668	0.18	0.015	2.6
Georgia	1297	966	0.14	0.015	2.8
Guam	225	225	1.79	0.052	26
Hawaii	466	460	1.73	0.015	9.7
Iowa	471	316	0.57	0.015	2.75
Idaho	582	493	0.94	0.015	7.9
Illinois	2151	1511	0.28	0.015	4.5
Indiana	845	398	0.09	0.015	3.2
Kansas	389	353	0.75	0.015	8
Kentucky	1220	956	0.11	0.015	1.5
Louisiana	1011	410	0.09	0.015	5.1
Massachusetts	2271	1735	0.15	0.015	20
Maryland	433	294	0.24	0.015	3.8
Maine	147	104	0.09	0.015	2.6
Michigan	1371	1080	0.13	0.015	1.535
Minnesota	671	287	0.15	0.015	7.89
Missouri	878	542	0.56	0.015	6.2
Northern Mariana Islands	46	46	1.02	0.15	7.2
Mississippi	795	180	0.08	0.015	2.579
Montana	175	130	0.12	0.015	1.25
North Carolina	2110	1405	0.15	0.015	11
North Dakota	98	76	0.45	0.015	1.6
Nebraska	347	277	0.43	0.015	3.9
New Hampshire	227	163	0.09	0.015	0.46
New Jersey	2028	1428	0.19	0.015	3.8
New Mexico	602	462	0.95	0.015	10.63
Nevada	335	329	0.61	0.015	12
New York	2655	2205	0.26	0.015	7.3
Ohio	1246	960	0.18	0.015	3.2
Oklahoma	934	725	6.31	0.015	97.38
Oregon	769	614	0.25	0.015	2.2
Pennsylvania	2469	2075	0.18	0.015	11.587
Puerto Rico	667	542	0.82	0.015	26.251
Rhode Island	174	103	0.06	0.015	0.26
South Carolina	793	645	0.08	0.015	0.64
South Dakota	178	125	0.19	0.015	1
Tennessee	1294	1123	0.13	0.015	1.5
Texas	5808	3434	0.28	0.015	10.3
Utah	824	775	0.48	0.015	5.242
Virginia	927	779	0.13	0.015	1.2
Virgin Islands	18	11	0.27	0.015	1
Vermont	96	74	0.08	0.015	0.39
Washington	1830	1467	0.38	0.015	9.9
Wisconsin	1154	818	0.33	0.015	3.3
West Virginia	270	223	0.17	0.015	2.4
Wyoming	141	85	0.08	0.015	0.51
United States	62837	47503	0.79	0.015	97.38

*non-detects substituted with the MRL/2

References:

National Toxicology Program (NTP) (2008). Research Triangle Park, NC: National Toxicology Program; Toxicology and Carcinogenesis Studies of Sodium Dichromate Dihydrate (CAS No 7789-12-0) in F344/N Rats and B6C3F₁ Mice (Drinking Water Studies). TR 546. [PubMed]

USEPA (2017a). The Third Unregulated Contaminant Monitoring Rule (UCMR3): Data Summary. Last updated January 2017. https://www.epa.gov/sites/production/files/2017-02/documents/ucmr3-data-summary-january-2017.pdf

USEPA (2017b). The Third Unregulated Contaminant Monitoring Rule (UCMR3): Occurrence Data. Last updated January 2017. https://www.epa.gov/dwucmr/occurrence-data-unregulated-contaminant-monitoring-rule [1] The average was calculated using ½ the MRL of 0.03 ug/L for non-detects.