Nitrate in drinking water - facts and figures
Find out about nitrate in drinking water, how the maximum acceptable value of nitrate in New Zealand's Drinking Water Standards is set, and how suppliers know their water is safe.
The Water Services Authority – Taumata Arowai wants to ensure that safe drinking water is available for everyone in Aotearoa New Zealand, every day.
New Zealand’s Drinking Water Standardsoutbound set out the maximum amount of nitrate and other substances that are acceptable in drinking water. In the public health and water sectors, these maximum amounts are known as the “maximum acceptable value” or “MAV’.
Nitrate is a naturally occurring chemical and essential for life. It is often found at low levels in surface water and groundwater. Human activities such as farming and wastewater discharges can increase the levels of nitrates.
The maximum amount of nitrate acceptable in drinking water under New Zealand’s Drinking Water Standards is 50 milligrams per litre. See below for further information.
There are a number of ways that people can consume nitrate. Drinking water is just one of these. For example, nitrate can be found in processed meats and other foods.
How MAVs are set
New Zealand’s MAVs are generally based on World Health Organization (WHO) drinking water guideline values, which reflect the weight of expert opinion informed by the body of scientific evidence available internationally. New Zealand’s Standards are also consistent with those used by comparable international jurisdictions.
New Zealand’s MAV for nitrate is set at a level to protect against an acute health issue called methaemoglobinaemia – also known as blue baby syndrome. This is a serious condition that affects the ability of the blood to carry oxygen. The most vulnerable group is bottle-fed infants.
How does a drinking water supplier know if their water meets the MAV?
Registered drinking water suppliers are required to regularly test their water for a range of substances including nitrate using an accredited laboratory.
Nitrate exceedances are rare in public or council drinking water supplies. Council supplies using groundwater tend to draw from deeper aquifers. Since the Authority was established in 2021, only two public supplies (Waimate and Gore) have reported exceedances of the MAV for nitrate. Most New Zealanders (about 84%) drink water provided by council suppliers.
People on unregistered supplies where the source is a shallow bore may be at increased risk of elevated nitrate.
The Authority recommends that all drinking water suppliers and those on domestic supplies test their drinking water regularly.
Both the drinking water supplier and the accredited laboratory used for the testing must notify the Authority if there are any drinking water sample results that exceed the MAV. Suppliers must then take immediate action to protect public health.
The Authority informs the National Public Health Service when testing results exceed the MAV for nitrate. The Authority can take action if we consider a supplier is not adequately responding to a situation where the MAV has been exceeded.
If a member of the public has any questions or concerns regarding their drinking water, they should contact their drinking water supplier.
Monitoring research into health impacts from nitrates
The Ministry of Health provides advice to the Authority on matters concerning New Zealand’s drinking water standards and related public health policy. We keep a close watch on research into the area of nitrates, given the potential health impacts.
Based on the available health evidence, the Ministry of Health considers the current MAV for nitrate to be appropriate.
There is reported evidence of a potential correlation with low birth weights and pre-term births. However, a review by the Liggins Institute in 2021outbound found that there was no consistent evidence of a relationship between nitrate in drinking water and adverse reproductive outcomes. The Institute, New Zealand’s authority on pregnancy and baby health, recommended that this area of research should continue to be monitored as more evidence becomes available.
There have also been reports of an association between nitrate levels in drinking-water supplies and bowel cancer risk in adults. However, the WHO states that the weight of evidence indicates that there is unlikely to be a causal association between gastric cancer and nitrate in drinking water.
The Authority and the Ministry of Health are aware of the Danish report, 'Evaluation of the parametric value for nitrate in drinking water’. The Danish Government has not lowered the limit, rather it has initiated work to consider what work is needed to change the limit value.
The Authority and the Ministry of Health will continue to monitor how those findings could apply in a New Zealand context and how other governments are updating their guidelines and regulations in response to new evidence about the risks of nitrate including this study. We will also be guided by any changes recommended by the World Health Organization.
New evidence and regulatory change overseas will inform any future advice to the Government on whether regulation changes are needed in New Zealand.
More information
Information from other agencies
• Nitrates in drinking water | Office of the Prime Minister’s Chief Science Advisoroutbound
• Addressing risks associated with nitrates in drinking water | Ministry for the Environmentoutbound
Are there two MAVs for nitrate?
There is just one maximum amount of nitrate that is acceptable in drinking water.
However, the MAV for nitrate is unique as it includes two different ways to express how much nitrate is in water. This is because there are two different ways to measure this.
In chemistry, a nitrate molecule is NO3. This means it’s made up of one nitrogen atom and three oxygen atoms.
The amount of nitrate in water can be expressed in two ways:
| If the following is being measured… | …you’ll see this is written as… | …and the maximum amount acceptable under NZ’s Drinking Water Standards is… |
|
both the nitrogen and oxygen atoms that make up a nitrate molecule
|
nitrate or NO3 |
50 milligrams per litre of water |
|
just the nitrogen atoms that make up a nitrate molecule
|
nitrate-nitrogen or NO3-N
|
11.3 milligrams per litre of water |
However, no matter what approach is used, the maximum amount of nitrate acceptable in drinking water is the same.
How often does a drinking water supplier need to test water for nitrate?
How often a drinking water supplier must test for nitrate depends on the size of the community that a supply serves.
For small drinking water supplies (serving 26-100 people), source water must be tested for nitrate every three years. If a result exceeds 50% of the MAV, drinking water must be tested every three months and source water annually.
For medium supplies (serving 101-500 people), source water must be tested for nitrate annually. If a result exceeds 50% of the MAV, drinking water must be tested every month.
For large supplies (serving more than 500 people), source water must be tested for nitrate every month. Additional testing of drinking water is required where the nitrate exceeds 50% in source water.
In summary, when sampling in small and medium supplies indicates a result that exceeds 50% of the MAV, sampling must take place every three months until three consecutive test results are less than 50% of the MAV. Large supplies must always test for nitrate monthly.