Health risks of nitrates in drinking water

Water pouring from a tap into a glass. Nitrate (NO₃) is a compound that is formed when nitrogen combines with oxygen. Sometimes high amounts of nitrate get into drinking water. Typical sources of nitrate include:

fertilisers,
animal wastes, particularly in areas of intensified farming,
unreticulated sewage disposal systems, and
industrial and food processing waste.

Nitrate is highly soluble in water, making it readily transported through the soil to groundwater.

Negative health effects of nitrates in drinking water

High levels of nitrate can pose a risk to babies less than six months who are formula fed and the unborn foetus of pregnant women. Adults with specific rare metabolic disorders may also be at risk.

Nitrate is converted into nitrite by bacteria in the gut. This nitrite combines with foetal haemoglobin in the foetus or infant less than 6 months old, preventing oxygen from binding and being distributed around the body. Symptoms include blueness around the mouth, hands and feet – hence the name ‘blue baby’ syndrome. In severe cases it can affect breathing and be life-threatening.

Fully breastfed infants are not affected as nitrites do not enter the breast-milk. Very few cases of ‘blue baby’ syndrome have been reported in New Zealand, though nitrates in groundwater have been rising in the last twenty years.

Download more information on the health risks of nitrate and blue baby syndrome. You can order copies of this resource from your local Community Health Information Centre.

Nitrate levels in your drinking water

Council water supplies in Canterbury currently have safe nitrate levels.

Many rural drinking water bores in Canterbury are at risk of elevated nitrate levels. Some private bores exceeding the recommended safe level of nitrates. Environment Canterbury (ECan) has produced maps identifying where nitrate levels in drinking water may be a concern – last updated in August 2022.

View the nitrate risk maps for drinking water for the Canterbury region.

The maps identify green, yellow and red areas:

Green areas are where nitrate concentrations in groundwater are always below the Maximum Acceptable Level (MAV) and the water is therefore safe to drink.
Red areas are where nitrate concentrations in groundwater are above the MAV most or all of the time. Alternative water sources should be used for drinking in these areas.
Yellow areas are where it is not known if a sample collected from a well will have nitrate concentrations exceeding the MAV. Testing is recommended in these areas.

Get your bore water tested for nitrates

The drinking water consumed by pregnant women, or formula fed babies under 6 months coming from a private bore in a medium to high risk area should be tested for nitrates.

Testing is the only way to detect nitrate as it is tasteless, odourless and colourless. There are several laboratories that are able to test for nitrate.

Find an accredited laboratory to test your water for nitrates.

Drinking Water Standards for New Zealand set a Maximum Acceptable Level (MAV) of 50mg/L for nitrate, which is equivalent to 11.3mg/l nitrate-nitrogen. Some laboratories report nitrate levels whereas other report nitrate-nitrogen. Ensure that you are aware which they are reporting if you are getting your water tested.

What do if your drinking water has high nitrate levels

If tests show that nitrate levels are above or close to the MAV then pregnant women and formula-fed infants less than 6 months should use an alternative water source for drinking or making up formula.

If tests reveal that nitrate levels are above half the MAV then the water is safe to drink. However the water should be tested monthly to ensure that it does not increase over the MAV.

Nitrate levels do vary over the year. Often results are highest in spring (following rain and snow melt). High nitrate levels have been found in late summer in areas where there is extensive irrigation.