By David P. Spath, Ph.D., Steven A. Book, Ph.D., and Gary H. Yamamoto

Summary: The waterborne contaminant discussed in this article was one of the hot emerging issues discussed at last June’s American Water Works Association convention in Denver. WC&P introduced the topic in the November 2000 “On Tap” column. Here is a more detailed overview.

With continued improvement in analytical technology and corresponding ability to measure chemical contaminants at increasingly lower concentrations, a myriad of new chemicals is being detected in drinking water supplies. One of these chemicals is N-nitrosodimethylamine (NDMA), a potential human carcinogen, which has become of increasing concern to public health officials and suppliers of drinking water in California.

NDMA is used primarily in research, but it has had prior use in the production of 1,1-dimethylhydrazine for liquid rocket fuel, and a variety of other industrial uses: a nematocide, a plasticizer for rubber, in polymers and copolymers, a component of batteries, a solvent, an antioxidant, and a lubricant additive. NDMA was reported to be present in a variety of foods, beverages, and drugs, and in tobacco smoke; it has been detected as an air pollutant as well as in treated industrial wastewater, treated sewage in proximity to a 1,1-dimethylhydrazine manufacturing facility, deionized water, high nitrate well water and chlorinated drinking water.1

Not closely followed
Because NDMA historically hasn’t been considered a common drinking water contaminant, it has no state or federal maximum contaminant level (MCL). There’s currently no standard or approved analytical method for NDMA at very low levels. In addition, technologies for the removal of NDMA from high volumes of water associated with drinking water aren’t readily available and are only now being developed.

Given these limitations, when NDMA was found in 1998 in California drinking water supplies, the state’s Department of Health Services (DHS) established an advisory “action level” to help protect consumers. Originally set at 0.002 micrograms per liter (µg/L), the action level was temporarily revised in late 1999 to 0.02 µg/L.

In addition, DHS investigated more sensitive analytical methods to help drinking water systems identify NDMA contamination.

Recent activities
In 1998, concern about the presence of NDMA contamination at a northern California aerospace facility prompted investigations of its presence in nearby drinking water sources.

Samples collected in February and March 1998 from a drinking water well in eastern Sacramento County confirmed the presence of NDMA at approximately 0.15 micrograms per liter (µg/L). Exposure to this concentration of NDMA would result in theoretical cancer risks to consumers of drinking water greater than the 10-6 cancer risk level typically considered de minimis (or below regulatory concern), and the same level DHS typically uses for action levels. Thus, this well was taken out of service by the utility.

NDMA was also detected in three drinking water wells in the San Gabriel Basin that were sampled in May 1998. Two wells with NDMA at concentrations of 0.07 µg/L were removed from service. The third well, already out of service because of trichloroethylene and perchlorate contamination, contained NDMA at 3 µg/L.

Several areas within San Gabriel Basin have been designated as federal Superfund sites as a result of the significant groundwater contamination. Projects are being finalized to clean up the contaminated groundwater and use the treated water as a source of drinking water. The use of ultraviolet (UV) light to remove NDMA has been tested at one of these sites and shown to be an effective treatment process.

In the spring of 1999, as interest in NDMA monitoring increased in the water treatment community, DHS was informed of NDMA findings in treated wastewater used for groundwater recharge. From the standpoint of protecting drinking water consumers and sources, DHS considered this finding to be important in the evaluation of proposed recycled water projects involving groundwater recharge.

Also in 1999, limited sampling indicated that NDMA appeared to be present at very low levels (<0.01 µg/L) in treated drinking water. Preliminary analyses suggested that NDMA’s presence in drinking water was related to disinfection processes, but very limited data were available, and often they appeared to be inconclusive.

The fallout
As a result of these findings, DHS initiated studies with drinking water utilities to investigate the occurrence of NDMA in raw, treated and distributed water, the role water quality and treatment processes may play in the production of NDMA, and the possible extent of NDMA production at various steps in the water treatment process. These studies looked at water treatment disinfection processes that employ chlorination and chloramination. The studies also evaluated the seasonal variation in NDMA to determine what role water temperature may play in the production process. Those studies have recently been completed and the results are being assessed.

A closer look
In May 2000, an agency in Orange County using treated wastewater for groundwater recharge detected NDMA in nearby monitoring wells as high as 0.15 µg/L. As a result, over 90 domestic water wells were tested for NDMA. Two drinking water wells in Orange County had NDMA at concentrations of approximately 0.03-to-0.04 µg/L, and were taken out of service. The sources of the NDMA appear to be discharges that make up the raw wastewater as well as production during chlorination of the treated wastewater prior to recharge.

DHS informed the wastewater treatment agency that its activities were impairing groundwater, and directed them to reduce the levels of NDMA accordingly.

Studies are presently under way examining the effectiveness of UV treatment to remove NDMA. Other groundwater recharge projects are also being investigated to determine the prevalence of NDMA. Levels as high as 0.09 µg/L have been detected in Los Angeles in treated wastewater that was blended before recharge.

NDMA has also been found in the effluent from an ion exchange treatment facility that’s being used for nitrate removal from a drinking water source. The concentrations were about 0.03 µg/L. The source of the NDMA was apparently related to the resins used in the ion exchange process.

Action levels
NDMA causes cancer in laboratory animals and, therefore, is considered to pose a potential risk of cancer in people. NDMA is identified as a carcinogen under California’s Health and Safety Code Section 25249.5, i.e., the Safe Drinking Water and Toxic Enforcement Act of 1986 (“Proposition 65”). In addition, the U.S. Environmental Protection Agency (USEPA) identifies NDMA as a “probable human carcinogen,”2 and the National Toxicology Program lists NDMA as “reasonably anticipated to be a human carcinogen.”1

Risk assessments from the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA) and USEPA identify lifetime de minimis (i.e., 10-6) risk levels of cancer from NDMA exposures as 0.002 parts per billion (ppb) and 0.0007 ppb, respectively. The 10-6 risk level corresponds to up to one excess case of cancer per million people drinking 2 liters of water per day for a 70-year lifetime.

OEHHA lists NDMA as a chemical known to cause cancer.3 OEHHA’s 10-5 risk level corresponds to an exposure of 0.04 µg per day,4 which equates to 0.02 µg/L in drinking water. For a 10-6 level, the NDMA concentration is 0.002 µg/L.

USEPA classifies NDMA as B2—probable human carcinogen—based on the induction of tumors at multiple sites in laboratory animals exposed by various routes. USEPA’s 10-6 risk level in drinking water corresponds to a concentration of NDMA of 0.0007 µg/L.2

In April 1998, DHS established an action level for NDMA of 0.002 ppb in drinking water. However, analytical capabilities didn’t enable detection at that concentration, so DHS’s approach was to consider any detectable quantity as exceeding the DHS action level for NDMA.

Coordinating efforts
In the fall of 1999, coincidental with more sensitive analytical methods becoming available allowing NDMA to be measured by some laboratories at the 0.001-to-0.002 µg/L level, DHS began working with utilities to investigate the production of NDMA in drinking water treatment processes. During this investigational phase, DHS established a temporary action level of 0.02 µg/L for NDMA, effective November 1999.

DHS has advised utilities about actions that should be taken when NDMA concentrations exceed the temporary action level in public water supplies.

In the fall of 1999, DHS began working with utilities to investigate the production of NDMA in drinking water treatment processes. During this investigational phase, DHS established a temporary action level of 0.02 µg/L for NDMA (effective November 1999). The temporary action level was set to ensure public health protection related to waterborne disease transmission and proper disinfection that didn’t compromise the investigations, and encourages drinking water systems to participate in investigations without concerns of immediate consumer notification if a positive finding at a low level occurs. A level of 0.02 µg/L NDMA corresponds to a theoretical 10-5 cancer risk, if it were to continue over a 70-year lifetime exposure.

DHS has given advice to utilities about actions that should be followed when NDMA concentrations exceed the temporary action level in public water supplies.

Implementing an action level
Consistent with DHS’s application of advisory drinking water action levels, the following apply for NDMA:

“If NDMA is in a drinking water well at concentrations greater than the action level, notification of local governing bodies is required by statute. Under Section 116455 of the California Health and Safety Code, whenever a well that’s used as a drinking water source for a public water system is discovered to contain a contaminant in excess of a MCL or an action level, the public water system is required to notify the governing body (e.g., the city council or county board of supervisors) within 30 days.”

Public information
If NDMA is in water served to the public at concentrations greater than the action level, DHS recommends that the utility inform its customers as soon as possible about the presence of NDMA and its potential for adverse health effects. DHS recommends that whenever such a public “right-to-know” notice occurs as a result of exceeding a drinking water action level, it should be provided to customers and to the water-consuming population in the affected area that wouldn’t directly receive such information, including renters, workers and students.

If NDMA isn’t present in raw water, but is present in treated water, the notice may include information about NDMA’s presence as a potential disinfection or treatment by-product. As with other consumer notification, the local district office of the DHS Drinking Water Program should be involved in the development of proposed notices.

If NDMA is present in a drinking water source at concentrations greater than 100 times the 10-6 de minimis  risk level (currently 10 times the temporary action level), DHS recommends that the source be removed from service. This is the concentration that corresponds to a 10-4 cancer risk, the upper bound of the risk range considered “acceptable” to regulatory agencies.

NDMA appears to be a by-product of certain disinfection practices used in water treatment. It’s uncertain whether the levels produced constitute a significant public health risk. The presence of NDMA in treated wastewater, particularly that which is intended for groundwater recharge, appears to be of greater concern. More studies need to be undertaken to determine the extent of the problem to ensure drinking water supplies recharged with treated wastewater are adequately protected.


  1. National Toxicology Program (NTP), “N-Nitrosodimethylamine CAS No. 62-75-9,” Eighth Report on Carcinogens, 1998 Summary, Public Health Service, U.S. Department of Health and Human Services, 1998.
  2. U.S. Environmental Protection Agency (USEPA), N-nitrosodimethylamine; CASRN 62-75-9 (04/01/97), Integrated Risk Information Service (IRIS) Substance File, Internet download (, 1997.
  3. Title 22, California Code of Regulations (22 CCR), Section 12000.
  4. 22 CCR Section 12705(b)(1).

About the authors
Dr. David P. Spath is chief of the division of Drinking Water & Environmental Management in the California Department of Health Services. He is the drinking water administrator for the State of California. Dr. Steven A. Book is a toxicologist with the Drinking Water Program in the California Department of Health Services. Gary H. Yamamoto is chief of the Drinking Water Technical Programs Branch in the California Department of Health Services.


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