California Drinking Water

How the Combination of Multiple Contaminants Raises Cancer Risks

April 30, 2019

California Drinking Water: Cancer Risk From Multiple Contaminants in California Drinking Water: Methods and Data

For the cumulative cancer risk assessment, we included all contaminants whose average concentration exceeded the one-in-a-million risk level in more than 20 community water systems in California. The cancer risk benchmarks were obtained from the websites of the California Office of Environmental Health Hazard Assessment, or OEHHA, and the U.S. Environmental Protection Agency Integrated Risk Information System.

Contaminants in California Tap Water That Pose More Than 1-In-1-Million Cancer Risk

Contaminant

Drinking water concentration corresponding to 1-in-1-million lifetime cancer risk*

Public water systems with contaminant levels exceeding the one-in-one-million

cancer risk

Population exposed to contaminant

at levels exceeding the 1-in-1-million cancer risk level

 

Estimated number of lifetime cancer cases for California

 

Agency defining 1-in-1-million lifetime cancer risk level

Arsenic

0.004 ppb

959

15,003,196

7,251

OEHHA, 2004

Hexavalent chromium

0.02 ppb

1250

32,947,698

2,448

OEHHA, 2011

Bromodichloro-methane

0.06 ppb

845

24,331,056

2,385

OEHHA, 2018 (proposed)

Dibromochloro-methane

0.1 ppb

821

23,802,319

1,620

OEHHA, 2018 (proposed)

Chloroform

0.4 ppb

713

22,386,686

564

OEHHA, 2018 (proposed)

Bromoform

0.5 ppb

678

20,501,727

187

OEHHA, 2018 (proposed)

Trichloroacetic acid

0.5 ppb

486

18,066,035

178

U.S. EPA IRIS, 2011

Dichloroacetic acid

0.7 ppb

550

19,275,893

164

U.S. EPA IRIS, 2003

Bromate

0.1 ppb

22

4,279,718

146

OEHHA, 2009

Sum of uranium-234, uranium-235 and uranium-238

0.43 pCi/L

763

19,201,427

162

OEHHA, 2001

Radium-228

0.019 pCi/L

225

5,967,996

154

OEHHA, 2006

Radium-226

0.05 pCi/L

99

6,611,524

29

OEHHA, 2006

1,2-Dibromo-3-chloropropane

0.0017 ppb

103

2,663,104

 

66

OEHHA, 1999

1,2,3-Trichloropropane

0.0007 ppb

62

2,548,031

40

OEHHA, 2009

Tetrachloroethylene

0.06 ppb

88

3,590,968

37

OEHHA, 2001

1,4-Dioxane

0.35 ppb

49

2,450,876

11

U.S. EPA IRIS, 2013

Trichloroethylene

0.5 ppb

25

1,266,183

7

U.S. EPA IRIS, 2011

* Drinking water concentration corresponding to 1-in-1-million lifetime cancer risk obtained from the websites of the California OEHHA and the U.S. EPA Integrated Risk Information System. Concentrations expressed in parts per billion (ppb) or in picocuries per liter of water (pCi/L), which is a measure of radioactive activity.

Source: EWG, from California OEHHA and U.S. EPA IRIS data for 2011 to 2015. Contaminants listed are those with levels that exceeded the one-in-a-million cancer risk in at least 20 communities.

Calculations

Chemical concentration corresponding to one-in-a-million risk level (also presented as 1x10-6 risk) is viewed by state and federal health agencies as posing de minimis risk.

The expected lifetime cancer risks for individual contaminants can be calculated by dividing the exposure levels by the one-in-a-million cancer risk level:

Lifetime cancer risk for a contaminant = [Exposure] / [Contaminant concentration corresponding to 10-6 lifetime cancer risk]

And the expected number of lifetime cancer cases can be incorporated by including the populations served by the water system:

Estimated number of lifetime cancer cases for a water system, a contaminant or for the entire state = S  [lifetime cancer risk for a contaminant or group of contaminants] x [population served by the water system or systems]

In this formula, exposure is represented by the average contaminant concentration, and overall cancer risks are treated as additive.

For the cumulative assessment, we calculated the representative contaminant concentrations for each individual water utility represented by the arithmetic means for all available test results for a contaminant within the time frame analyzed.

Data sources

For federally regulated water contaminants that are monitored annually, we used the 2015 data. For contaminants monitored once in several years, we used a longer time frame: 2010 to 2015 for radiological contaminants and 2013 to 2015 for arsenic. And for unregulated contaminants monitored in the EPA’s UCMR 3 program we used the entire data range of 2013 to 2015. Test results reported as non-detects were assigned a value of zero and included in the overall data array for the calculation of averages.

Data for populations served by utilities were obtained from California Safe Drinking Water Information System. Adding the total population served that is listed for the systems in our dataset, we found that population data overestimated by 12 percent the number of California residents served by public water systems, according to OEHHA’s information on how many Californians use private wells or public water systems. A 12 percent population correction factor was applied to estimates of exposed population.