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Nitrates in short term water supply?

What are the implications of higher nitrates than WHO Guideline figures in a borehole (55mg/l rather than 50mg/l) for short term supply?

What level should emergency figures be and for how long?

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Regarding Nitrates in a borehole WHO Guidelines for Drinking Water state that for emergencies (p108):

6.2.5 Chemical and radiological guidelines Many chemicals in drinking-water are of concern only after extended periods of exposure. Thus, to reduce the risk of outbreaks of waterborne and water-washed (e.g. trachoma, scabies, skin infections) disease, it is preferable to supply water in an emergency, even if it significantly exceeds the guideline values for some chemical parameters, rather than restrict access to water, provided the water can be treated to kill pathogens and can be supplied rapidly to the affected population. Where water sources are likely to be used for long periods, chemical and radiological contaminants of more long-term health concern should be given greater attention. In some situations, this may entail adding treatment processes or seeking alternative sources. Local actions that can be considered in the event of a short-term guideline exceedance or emergency are discussed in section 8.6.

Nitrates and nitrites do have a short term risk though for bottle fed babies:

Specifically the WHO Guidelines factsheet for nitrates and nitrites (p417) states that:

Guideline value for nitrate 50 mg/litre to protect against methaemoglobinaemia in bottle-fed infants (short-term exposure ) Guideline value for nitrites 3 mg/litre for methaemoglobinaemia in infants (short-term exposure)

(Provisional guideline value for nitrite 0.2 mg/litre (provisional) (long-term exposure) The guideline value for chronic effects of nitrite is considered provisional owing to uncertainty surrounding the susceptibility of humans compared with animals.)

Guideline value for combined nitrate plus nitrite The sum of the ratios of the concentrations of each to its guideline value should not exceed 1.

Guideline derived from:

Nitrate (bottle-fed infants): in epidemiological studies, methaemoglobinaemia was not reported in infants in areas where drinking-water consistently contained less than 50 mg of nitrate per litre

Nitrite (bottle-fed infants): application of body weight of 5 kg for an infant and drinking-water consumption of 0.75 litre to lowest level of toxic dose range, 0.4 mg/kg of body weight

The main issue is nitrites reacting with haemoglobin in red blood cells to block oxygen transfer, causing blue baby syndrome. Nitrates are reduced to nitrites by bacteria, which is why the nitrate guideline is there. High stomach pH (more likely in bottle-fed babies) and gastrointestinal infections increase this bacterial reduction of nitrates to nitrites. Using high nitrate water for bottle fed babies (the water being the major part of their fluid intake), especially with gastro-intestinal infections exacerbates this problem.

Recommendations:

These figures are primarily based on the effect of nitrites and nitrates on bottlefed babies. Mothers of bottlefed infants and expectant mothers should be the target of any intervention. If they can be properly identified and measures put in place for them then the issue is less of an emergency concern.

It is worth checking the nitrite values as well as nitrates and ensuring that ratios of nitrites and nitrate tested values are less than the minimum.

Where possible use other water sources, with large ... (more)

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The nitrates level should not cause too much of a problem, theoretical risks are somewhat academic if WHO guidelines are only breached by say 10%.. What would concern me would be if nitrate levels suddenly increased actually as a result of much higher borehole extraction rates. MONITOR levels daily as a matter of routine, particularly as spectrophotometers offer a simple technology option. Briefly, Nitrate and nitrite (NO3 and NO2 respectively) are, like ammonia, generally expressed as mg/l N. Nitrite is an intermediate oxidation state of nitrogen in the biochemical oxidation of ammonia to nitrate. Surface waters do not normally carry high levels of nitrite unless badly polluted (<0.1mg/lN).

High levels of NO2 and NH4 generally indicates gross sewage pollution in a river. Nitrates in groundwater can be reduced to nitrites. A prime source of nitrate contamination of surface waters is run-off containing fertiliser chemicals, particularly with the onset of winter rains.

Waters containing high levels of nitrates are potentially very harmful to young children and are increasingly being linked with enteric cancers; principally because of the formation of nitrosamines. In young children bacteria in the digestive tract can reduce nitrates to nitrites which are then absorbed into the blood stream across the gut wall and convert the oxygen-carrying haemoglobin into methhaemoglobin.

Infants have a high fluid intake relative to body weight and the proportionate reduction in the oxygen-carrying capacity of the blood has been known to make infants go blue, hence the conditions name "blue baby syndrome". In parts of the UK it was not uncommon for Water Plcs to issue bottled water for young children.

There are no straightforward methods for reducing nitrate levels during water treatment. Demineralisation and biological denitrification methods are both not suited for large scale operations.

Regards

Brian Clarke

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Reply from Jean McCluskey:

The point here is that WHO guideline is 50mg/l - how worried do we need to be about 55mg/l - where is the important threshold for nitrates in becoming a danger? For what period of time is 55mg/l okay for? If it increases, when would we really need to cut off the water - at how many mg/l do we really need to get worried?

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Regarding the nitrates issue see my previous email on nitrates. The emergency issue is short term risk to bottle fed babies, especially with gastro-intestinal infections. Babies taking in lots of high nitrate water (used for making up bottled milk) run greater risk of the bacteriological reduction of nitrate to nitrite (especially due to high bacterial loading during gastro-intestinal infections) and the nitrite blocking the oxygen transfer in haemoglobin. The risk is blue baby syndrome even over a short period. Targeting mothers with infants and pregnant mothers with a different source of water or promoting breast feeding and strong hygiene measures should negate this risk. Other emergency risks are minimal.

It is the combined figure of nitrites and nitrates that is the risk as nitrates reduce to nitrites which cause the problem. If the combination of ratios of tested nitrites to acceptable nitrites (3mg/l) and tested nitrates to acceptable nitrates (50mg/l) are above one then there is a risk to bottle fed babies. With water tested at above 50mg/l nitrates, and unknown nitrites it is better to try to provide other sources of water to pregnant women and mothers with young babies (who bottle feed).

Other possibilities are to blend the water with other sources of lower nitrate/nitrite concentrations. WHO says that a minimum is to ensure the water is well disinfected with a residual so that the bacteriological loading is minimal and therefore reduction of nitrates to nitrites is minimised. Better still is to provide other water sources to the bottle fed infants and mums.

Regards

Toby Gould

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Before I bore you to tears with the nerdy stuff it is important that I make the following clear; All of the available epidemiological evidence for humanitarian contexts indicates that the risk of mortality from diseases associated with reduced water quantities present a greater humanitarian hazard than diseases associated with water quality. As pointed out in previous posts, the health risks associated with nitrogen in the water are chronic, while risks associated with bacteriological contamination and reduced water quantities are acute. Given the context described in your post in the short term I would chlorinate the water to provide a minimum free residual chlorine level of 0.2mg/L at the collection point to protect against bacteriological contamination and seek to provide the minimum quantity of 20 L per person per day specified in the Sphere standards, irrespective of the nitrogen levels. Concurrently I would seek to carry out a risk assessment to determine possible hazards associated with long term use. In particular I would want to identify the source of the nitrogen contamination, preferably through a combination of sanitary surveys and testing. Determining the source of nitrogen is critical in the long term. While chlorination is an effective protection against bacteriological pathogens (e.g. Vibrio Cholerae and Shigella) , it is less so against protozoa (e.g. giardia, cryptosporidum) and viral pathogens If the presence of the nitrogen is from contamination with sewage then there is a risk of exposing users to viral (e.g. Hepatitis E, Polio) and protozoan pathogens. If this is the case then further treatment would be required.

From a risk assessment perspective, There is a higher likelihood of the Methamaeoglobinaemia (Blue baby syndrome) hazard in populations with vitamin C deficiency. To fully assess the risk you would need to consult local medical authorities to quantify risk factors associated with micro nutrient deficiencies, viral and protozoan pathogens.

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Asked:
2013-12-09 04:51:27 -0500
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Last updated:
Dec 09 '13