Nitrosodimethylamine (NDMA) Information

Content updated April 10, 2007

Background

N-Nitrosodimethylamine (NDMA) is a member of a chemical class, the N-nitrosoamines, which are suspected carcinogens.  Their cancer potencies are reported to be higher than those of the trihalomethanes [I].  NDMA was first detected in groundwaters of Northern California (Rancho Cordova) in 1998 and Southern California in 1999 near rocket engine testing facilities at concentrations as high as 40,000 ng/L (nanograms per liter, or parts per trillion) on site and 20,000 ng/L off site [I] .  NDMA is a semi-volatile organic chemical and has the molecular formula O=N-N(CH3)2.  NDMA is soluble in water (3,978 mg/L) and is not likely to bioaccumulate, biodegrade, adsorb to particulate matter or volatilize. Sunlight may result in limited NDMA reduction.  Consistently, transport of NDMA is not retarded through soil columns [I] .

NDMA occurrence is not limited only to the regions near rocket fuel facilities.  NDMA detected at other sites appeared to be associated with chlorine/chloramine disinfection of water and wastewater, especially in locations where chlorinated wastewater effluent was used for aquifer recharge.  Chlorination of wastewater resulted in significant NDMA formation (hundreds ng/L), with a direct relationship between NDMA concentration and chlorine dose.  More recently NDMA was detected in treated drinking water at low levels (10 ng/L) [II] from sources that were not impacted by wastewater effluent or industrial sources, especially when monochloramine was used to maintain chlorine residual [I].

Sources

NDMA is found in the diet, in various meat and cured meat products (600 to 1,000 ng/kg in fried pork bacon), fish and fish products, beer (50 to 5,900 ng/kg), milk (90 to 100 ng/L) [III] , cheese, soybean oil, canned fruit, and apple brandy.  In the body, NDMA is formed when acidic conditions in the stomach catalyze the reaction between nitrite and dimethylamine (DMA).  Historical non-dietary sources of NDMA include unsymmetrical dimethylhydrazine (UDMH, a rocket fuel component which used NDMA during synthesis), cutting oils, tobacco smoke, herbicides, pesticides, rubber products, and drugs formulated with aminopyrine.  Recently discovered sources of NDMA precursors include carpet dyes, dithiocarbamates and methyl-dithiocarbamates from circuit board manufacturers.  NDMA occurrence in drinking water may result from industrial groundwater contamination (rocket fuel), from the chlorination/chloramination of cationic polymers, from the use of ion exchange resins, and as a chlorination/chloramination byproduct.  NDMA is also found in sewage influent from industrial sources (carbamate users, etc.) and is formed during the chlorination of secondary effluent at wastewater treatment plants.  Recent work shows that it can also be formed by a reaction between chloramine and natural organic matter that contains organic nitrogen.

Regulatory Considerations

The USEPA Integrated Risk Information System (IRIS) classification of NDMA is B2, meaning that it can be reasonably anticipated to be a human carcinogen based on animal studies, however inadequate human data exist.   Some animal studies have shown induction of tumors at multiple sites in both rodents and non-rodent mammals exposed by various routes.  The unit risk factor for one in one million risk is 0.00143 µg/L in drinking water, resulting in a hypothetical guidance concentration for a one in one million risk level of 0.7 ng/L (ppt).  The California Office of Environmental Health Hazard Assessment (OEHHA) has performed their own risk assessment, resulting in a public health goal of 3 ng/L (ppt) developed in December 2006.  There is currently no maximum contaminant level (MCL) for NDMA, though a notification level of 10 ng/L was set in 2002 by the California Department of Health Services, based in part on the discovery of NDMA as a byproduct of chlorination and chloramination of drinking water.  The detection limit for NDMA in drinking water matrices currently ranges from 0.5 to 2 ng/L.

Occurrence in Drinking Water

In 1989, a survey of 145 drinking water plants in Ontario, Canada indicated that NDMA concentrations in the treated water from most of the plants were less than 5 ng/L with some samples exceeding 9 ng/L [IV].   In 2001, the California Department of Health Services conducted a similar survey of drinking water systems [V].  Their results indicated that 3 of 20 chloraminated supplies contained NDMA levels greater than 10 ng/L; none of eight supplies using free chlorine disinfection had NDMA levels above 5 ng/L; and one of four water supplies using anion exchange treatment had NDMA levels above 10 ng/L.

A 2001-2002 survey of NDMA in 21 North American water systems [VI] indicated the median NDMA concentrations in treatment plant effluents was less than 1 ng/L, for either chlorinated or chloraminated systems.  The median distribution system concentrations were less than 2 ng/L for chloraminated water and less than 1 ng/L for chlorinated water.  The highest NDMA concentrations were found in groundwater treated with anion exchange resin and chlorination.  The next highest concentrations were found in groundwater treated with lime softening and chloramination.   Most samples were below the CDHS  action level of 10 ng/L.  There were more samples with NDMA values between 2.5 and 10 ng/L in chloraminated systems than for chlorinated systems, both in treatment plant effluents and distribution systems. 

Formation in Drinking Water

For NDMA to be formed, reactive chlorine, such as monochloramine or chlorine and ammonia must be present.  In addition, organic nitrogen containing organic matter must be present.  This can include poorly characterized substances such as common humic material (one form of natural organic matter), as well as substances produced during biological waste treatment [VII] .  Other  precursors such as dimethylamine or other organic amines, N-based cationic polyelectrolytes [VIII] , or ion exchange resins may also be important in certain situations. 

Chlorination

Agricultural runoff can introduce much heterogeneous and ill characterized but reactive organic nitrogen containing material surface water streams that can produce NDMA in water treatment plant using chlorination processes such as sodium hypochlorite [IX] [X].  Other precursor substances that may be associated with this include DMA and nitrite.

Chloramination

The concentration of NDMA formed as a byproduct of chloramination appears to depend on the chloramine dose. In a 2001 study [XI] , it was found that increasing the chloramine dose from 1 mg/L to 5 mg/L more than doubled the resultant NDMA concentration.  The findings suggested that chloramine doses in the ranges generally used for drinking water treatment might result in NDMA formation in the range of the potential regulatory window.

Treatment – Removal

NDMA is highly soluble and cannot be removed by granular activated carbon (GAC) adsorption or air stripping.  Ozone does not directly react with NDMA or its precursors in water or air.  Although hydroxyl radicals produced from ozone and hydrogen peroxide could be used to remove NDMA, the ozonation efficiency is limited by the presence of hydroxyl radical scavengers.  Iron oxides can be used to reduce NDMA to DMA and ammonia; however, due to the slow kinetics of the reaction, this treatment method is not cost effective.  Bioremediation has the potential for treatment of NDMA contaminated water, but field results of biodegradation of NDMA are limited.  Despite the existence of NDMA-degrading bacteria in soil, for example at the Rocky Mountain Arsenal, no significant loss of NDMA was observed during the passage through the aquifer.   It is likely that a complex interaction exists between dissolved organic nutrients necessary for the growth of bacteria capable of degrading NDMA [I].  In addition to the lack of clear evidence for bioremediation of NDMA in groundwater, there is no information regarding the potential for biological removal of NDMA within drinking water treatment systems such as biofiltration units [I]. 

Direct photolysis with UV light in the 200 - 260 nm range is highly effective at NDMA removal, but this process may simply produce the NDMA precursors, DMA and nitrite.  UV can be applied through the use of low and medium pressure UV lamps and pulsed UV systems.  UV is currently considered the best available technology, as it is the most effective process for NDMA destruction.   The NDMA absorbs the UV energy, which breaks the N-N bond.  The UV dosage required for 90% decrease in NDMA concentration is approximately 1,000 mJ/cm2, which is approximately 25 times higher than that required for equivalent Cryptosporidium inactivation.  Therefore, UV treatment for NDMA will be feasible but more expensive than UV treatment for disinfection [I].  Advanced oxidation with UV and hydrogen peroxide can prevent formation and re-formation of NDMA by destroying precursors, and can also destroy NDMA once it is formed, though the use of hydrogen peroxide may interfere with maintaining disinfectant residuals in drinking water systems.  Reverse osmosis removes approximately 50% of NDMA, as shown by thin-film composite membranes in wastewater recycling plants [I]. 

Removal of NDMA can also be achieved indirectly by removing NDMA precursors, such as DMA and trimethylamine.  Like NDMA, however, these precursors are not susceptible to treatment by air stripping and adsorption filters such as GAC.  Although UV is effective at treating NDMA, it is relatively ineffective on the NDMA precursors.  Biological treatment methods and advanced treatment systems, such as microfiltration and reverse osmosis, are more effective in removing the NDMA precursors, DMA and trimethylamine.  Hydrogen peroxide also may inhibit NDMA reformation by oxidizing precursors. 

SFPUC Monitoring

SFPUC currently monitors for NDMA quarterly at its treatment plants and in the distribution system.  This is a voluntary non-regulatory monitoring program.  The Hetch Hetchy and Sunol Valley water blend, which constitutes the vast majority of the water served to SFPUC customers, was sampled 47 times for NDMA between 2004 and 2006 after chloramine conversion at several different sampling locations throughout the system:  100% of these samples had non-detected NDMA below the detection limit of 2 ng/L (parts per trillion).  This is similar to the results obtained when the distribution system was free chlorinated between 1999 and 2000:  100% of Hetch Hetchy and Sunol Valley water blend had non-detected NDMA.

Another SFPUC source water from the San Andreas Reservoir is treated at the Harry Tracy Water Treatment Plant (HTWTP).  The Harry Tracy WTP water was sampled 27 times for NDMA between 2004 and 2006 after chloramine conversion at several different sampling locations throughout the system:  26% of these samples had detected NDMA concentrations slightly above the detection limit of 2 ng/L (parts per trillion) but less than 5 ng/L (California Department of Health Services Notification Level is 10 ng/L).  This is similar to the results obtained when the distribution system was free chlorinated between 1999 and 2000:  25% of the samples had detected NDMA.

Summary

NDMA has not been detected in the Hetch Hetchy raw and treated water regardless of the disinfectant used in the distribution system (either chlorine or chloramine).  This is due to the excellent quality of this pristine source water low in organic nitrogen and free from agricultural or municipal run-off.

NDMA has been detected occasionally in the treated Harry Tracy WTP water at very low levels near the detection limit of 2 ng/L but less than the California Department of Health Services Notification Level of 10 ng/L.  NDMA has been detected in approximately 25% of the samples, regardless of the disinfectant used in the distribution system (either chlorine or chloramine), although the levels detected in chloraminated water appear slightly higher than when the system was chlorinated.  The source of NDMA is likely the cationic polymer applied at the HTWTP for turbidity control.  SFPUC is in the process of upgrading treatment at HTWTP to optimize filtration. Future treatment at HTWTP will allow SFPUC to better control turbidity in filter effluent, which now sometimes requires increasing cationic polymer dose, and to minimize NDMA formation, which would require lowering the polymer dose.