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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 2015a, 2015b) shows that Cr(VI) detection in the U.S. drinking water supply is widespread, but occurs predominantly at low levels. Of the approximately 4400 PWS systems surveyed as of June 2015, 88% reported levels of Cr(VI) at or above the MRL. The average Cr(VI) concentration across all samples[1] (n=38,682) was 0.814 ug/L and reported concentrations ranged from the minimum reporting level of 0.03 ug/L to a maximum of 97.4 ug/L.

Figure 1 presents average Cr(VI) concentrations by state. State averages ranged from 0.057 ug/L (VT) to 7.5 ug/L (OK); the majority of states had average Cr(VI) concentrations between 0.10-1.00 ug/L.

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. Also presented by state, are the total number of detects over the total number of samples collected in UCMR3 through June 2015.

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 June 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 red 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 (June 2015) in States and Territories

State/RegionSamplesDetectsAvg ConcMinMax
US49079369340.580.01597.38
Alaska49460.160.0150.52
Alabama154111920.170.0153.2
Arkansas3262340.220.0153.31
American Samoa39360.810.0153
Arizona161115084.530.01562
California846277182.010.01544
Colorado9265810.120.0151.3
Connecticut4974130.150.0151
District of Columbia26260.080.0420.12
Delaware1681170.310.0154.7
Florida237314310.180.0152.6
Georgia9116460.130.0152.8
Guam84841.920.05226
Hawaii4584521.730.0159.7
Iowa3722530.560.0152.75
Idaho4343660.830.0157.9
Illinois173211870.260.0154.5
Indiana5582510.100.0153.2
Kansas2722450.800.0158
Kentucky9497350.110.0151.5
Louisiana7633180.010.0155.1
Massachusetts166312900.150.0154.4
Maryland2832070.310.0153.8
Maine119850.100.0152.6
Michigan10097680.120.0150.81
Minnesota5192110.120.0152.6
Missouri7494670.620.0156.2
Mississippi6341300.070.0152.579
Montana137980.0100.0150.35
North Carolina151910190.130.0159.1
North Dakota81590.450.0151.6
Nebraska2922430.470.0153.9
New Hampshire1961390.090.0150.46
New Jersey157510860.190.0153.8
New Mexico3973061.00.01510.63
Navajo Nation17140.600.0153.9
Nevada2322280.410.0153
New York230719350.270.0157.3
Ohio10217960.170.0153.2
Oklahoma6845417.510.01597.38
Oregon6625300.260.0152.2
Pennsylvania184515490.180.01511.587
Puerto Rico2311721.420.01526.251
Rhode Island118720.060.0150.254
South Carolina6345020.080.0150.64
South Dakota1561070.200.0151
Tennessee9548260.130.0151.5
Texas446524650.260.0159.71
Utah5585300.510.0154.212
Virginia7886800.130.0151.2
Virgin Islands1490.340.0151
Vermont76590.060.0150.26
Washington134810670.310.0159.7
Wisconsin9296640.330.0153.114
West Virginia2101830.210.0152.4
Wyoming106580.090.0150.51

 

 

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 (2015a).  The Third Unregulated Contaminant Monitoring Rule (UCMR3): Data Summary. Last updated January 2015.  http://water.epa.gov/lawsregs/rulesregs/sdwa/ucmr/upload/epa815s15001.pdf

USEPA (2015b).  The Third Unregulated Contaminant Monitoring Rule (UCMR3): Occurrence Data. Last updated January 2015.  http://water.epa.gov/lawsregs/rulesregs/sdwa/ucmr/data.cfm#ucmr2013

 [1] The average was calculated using ½ the MRL of 0.03 ug/L for non-detects.