Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Nitrate and Nitrite
Part I. Overview and Application
- The maximum acceptable concentration (MAC) for nitrate in drinking water is 45 mg/L. This is equivalent to 10 mg/L measured as nitrate-nitrogen.
- The MAC for nitrite in drinking water is 3 mg/L. This is equivalent to 1 mg/L measured as nitrite-nitrogen.
2.0 Executive summary
Nitrate and nitrite are widespread in the environment. They are naturally produced by the oxidation of nitrogen by microorganisms and, to a lesser extent, by lightning. The most common sources of these substances are human activities, including agricultural activities, wastewater treatment, and discharges from industrial processes and motor vehicles. Nitrate and nitrite can also be produced as a result of the nitrification process in source water or distribution systems. The concentration of free ammonia entering the distribution system can lead to nitrification and the potential increase of nitrate and nitrite in drinking water. This issue is fully discussed in the guideline technical document on ammonia in drinking water.
This guideline technical document reviews and assesses all identified health risks associated with nitrate and nitrite in drinking water. It assesses new studies and approaches and takes into consideration the availability of appropriate treatment technology in order to propose maximum acceptable concentrations that are protective of human health and achievable by both municipal and residential scale treatment technologies.
Based on this review, the drinking water guideline for nitrate is a maximum acceptable concentration of 45 mg/L (equivalent to 10 mg/L nitrate-nitrogen); the drinking water guideline for nitrite is a maximum acceptable concentration of 3 mg/L (equivalent to 1 mg/L nitrite-nitrogen).
2.1 Health effects
Methaemoglobinemia has long been considered to be the end-point of concern for humans from exposure to nitrate in drinking water. Scientific studies show cases of methaemoglobinemia occurring in bottle-fed infants, which are the vulnerable population for these effects. Recent evidence from animal and human studies suggests that effects on thyroid gland function are also an end-point of concern. Studies have seen an effect in school-age children, but no study has looked at this health effect in infants, who would also be expected to be the most vulnerable population for this health effect. In addition, current science suggests an association between cancer and exposure to nitrates in drinking water when conditions result in nitrosation within the human body.
Although no single study can be used to establish a guideline for nitrate in drinking water, available studies in humans show no adverse health effect (either methaemoglobinemia or thyroid effects) below 45 mg/L. At this level of exposure from drinking water, the estimated increased cancer risk would be in the range considered by Health Canada to be essentially negligible.
The maximum acceptable concentration for nitrate has been established at 45 mg/L (equivalent to 10 mg/L measured as nitrate-nitrogen), to be protective of the health of the most sensitive subpopulation, bottle-fed infants. As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area and recommend any change to the guideline that it deems necessary. Monitoring of science will focus particularly on thyroid effects, including neurodevelopmental effects, in the most sensitive subpopulation.
Current science maintains that the effect of concern for nitrite in drinking water is methaemoglobinemia. The maximum acceptable concentration of 3 mg/L for nitrite in drinking water (equivalent to 1 mg/L measured as nitrite-nitrogen) is established based on this effect in bottle-fed infants, the most sensitive subpopulation.
Canadians can be exposed to nitrates and nitrites through their presence in food, drinking water, air and soil. Both nitrate and nitrite have been found in meats - particularly wieners, sausages, luncheon meats and cold cuts. Although breast-fed infants have little exposure to nitrate, exposure for bottle-fed infants can be significant - from water used to reconstitute concentrated formula and/or from the use of soy-based formula. Nitrate is found more frequently in groundwater than in surface water, and is commonly detected in shallow wells. Nitrite is less frequently found in drinking water supplies, as it is converted to nitrate in the presence of oxygen. Approximately 80% of total exposure to nitrite is due to the reduction of ingested nitrate by oral bacteria. Nitrate and nitrite are not volatile, so exposure from drinking water is related to ingestion only.
2.3 Analysis and treatment
Nitrate and nitrite can be detected and analysed in drinking water supplies to levels well below the MACs. Treatment methods are generally focused on nitrate, as nitrite is relatively unstable and can be rapidly oxidized to nitrate.
Conventional water treatment processes used at municipal water treatment plants (coagulation, sedimentation, filtration and chlorination) are not effective for nitrate removal. Effective technologies for the removal of nitrate from municipal water supplies include ion exchange, biological denitrification, reverse osmosis and electrodialysis. Available treatment methods can consistently achieve levels as low as 22 mg/L (equivalent to 5 mg/L as nitrate-nitrogen).
At the residential scale, there are drinking water treatment devices available that are certified for the removal of nitrate, based on achieving a concentration of nitrate at or below the MAC. Residential reverse osmosis devices are capable of achieving lower treated water concentrations. Ion exchange technology can also be used, but finished water quality must be monitored regularly to ensure the process is working effectively and that no break-through of nitrate is occurring.
3.0 Application of the guidelines
Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.
The MAC for nitrate has been established at 45 mg/L (equivalent to 10 mg/L measured as nitrate-nitrogen), to be protective of the health of the most sensitive subpopulation, bottle-fed infants. However, it is recommended that levels be kept as low as reasonably practicable to account for (1) the lack of studies on thyroid effects in the most sensitive subpopulation; (2) the significance of the potential health effect in infants (neuro-developmental) related to thyroid effects; and (3) the fact that ingested nitrate under conditions that result in endogenous nitrosation is probably carcinogenic to humans.
In municipal systems with a water source containing naturally occurring ammonia or that add ammonia for chloramination, free ammonia entering the distribution system can be one of the causative factors of nitrification and the potential increase of nitrate and nitrite in the distribution system. Utilities that are chloraminating, as well as utilities with ammonia in the source water, should ensure good operational practices, such as limiting excess free ammonia entering the distribution system to concentrations below 0.1 mg/L, and preferably below 0.05 mg/L (measured as nitrogen), will help prevent nitrification. Utilities using ammonia as part of their disinfection strategy should ensure that the appropriate chlorine to ammonia ratio is maintained.
Homeowners with a well should test concentrations of nitrate and of nitrite in their water supply. Wells that are located in agricultural areas are susceptible to nitrate and nitrite contamination, particularly shallow wells. Water containing levels of nitrate and/or nitrite above the MACs should not be used to prepare formula or other foods for infants.
Homeowners that have nitrate or nitrite test results consistently above the MACs should consider installing a drinking water treatment device, using an alternative drinking water source, or relocating or drilling a deeper well that has been tested or verified and deemed to be a safe supply. It is important to note that boiling water will not reduce or remove nitrate or nitrite.
The approach to reducing exposure to nitrate from drinking water generally includes management of activities within the watershed/aquifer, treatment to decrease nitrate levels in the water supply and management of nitrification in the distribution system.
Routine monitoring of nitrate and nitrite in surface water and groundwater is recommended in agricultural areas and other areas where discharges of nitrogen compounds may occur. It is recommended that utilities that treat their water to remove nitrate conduct continuous monitoring and recording of nitrate concentrations in both source and treated water. Alternatively, it is recommended that utilities measure and record the treated water nitrate concentrations daily, during a period when treatment operations are likely to result in the highest concentrations (such as just prior to regeneration of an ion exchange unit). In cases where nitrite has been detected in the source water or where utilities are using biological denitrification treatment processes, it is recommended that routine monitoring of nitrite in the source and treated water be conducted in addition to nitrate monitoring.
Utilities that are chloraminating, as well as utilities with ammonia in the source water, should also monitor for nitrite and nitrate since ammonia entering the distribution system can be one of the causative factors of nitrification. Monitoring of nitrite and nitrate should be done in addition to other parameters such as free ammonia, total chlorine residual and heterotrophic plate count, at key locations in the distribution system as part of a nitrification monitoring program. It is recommended that locations such as entry points, reservoir outlets, and areas with long water detention times (e.g., dead ends) be monitored for nitrite and nitrate weekly. Utilities that undertake comprehensive preventive measures and have baseline data indicating that nitrification does not occur in the system may conduct less frequent nitrate and nitrite monitoring.
Owners of private supplies are also encouraged to have their water tested for nitrate and nitrite periodically. Shallow wells that are located in agricultural areas are particularly susceptible to nitrate and nitrite contamination and it is recommended that homeowners with these types of wells test their water for nitrate and nitrite at least once a year either in the spring or fall, when nitrate concentrations are typically the highest.
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