Nitrate in waterways - what's the story?
Nitrate contamination in waterways is cause for concern for everyone in Canterbury.
Updated 23 July 2021
Over the past 10 years, the Canterbury Water Management Strategy, Canterbury Land and Water Regional Plan, and its later, sub-regional updates have started to address the problems caused by land-use intensification.
Some of the impacts of these new rules are only starting to take effect, and there is a lot of work to be done by farmers, industry and Environment Canterbury in coming years.
In the meantime, nitrate concentrations will continue to cause problems in spring-fed streams and lakes for some years yet. While the drinking water from Christchurch’s deep aquifers remains clear of contaminants, and other council supplies are kept safe to drink, anyone using a private well for drinking water needs to be aware of the risk.
Read on to learn more about the actions being taken.
- Nitrate is a chemical made of nitrogen and oxygen. It occurs naturally in soil, where it is a major source of nitrogen for plants, helping them grow.
- When the soil contains more nitrate than plants can use, the excess can be washed through the soil by rainwater and carried into the underlying groundwater.
- If the groundwater feeds springs that flow into streams or lakes, the nitrate can cause excess weed growth, algal blooms and toxicity for fish and invertebrates.
- If that groundwater is used as a source of drinking water, the nitrate can pose a health risk to some people.
- For drinking water, the New Zealand Drinking Water Standards set a Maximum Acceptable Value (MAV) of 50 milligrams per litre (mg/l) for nitrate, which is equivalent to 11.3 mg/l nitrate-nitrogen.
- If nitrate-nitrogen concentrations reach half that level (5.65 mg/l) in a community supply, additional monitoring must be undertaken.
- High nitrate concentrations in drinking water can pose a health threat to bottle-fed babies. Taumata Arowai, the water services regulator established in 2021, sets the threshold to protect against this.
A study carried out in Denmark made a link to colorectal cancer at a much lower concentration, suggesting a threshold of 0.87 mg/l for nitrate-nitrogen. Another recent study in California has suggested links between nitrate contamination and premature births. These are important issues. Environment Canterbury supports calls for more research on any possible links between nitrate concentrations and human health. Taumata Arowai is responsible for the review and revision of the drinking water standards supported by public health policy advice from the Ministry of Health.
However, we should not and will not wait to take action. Environment Canterbury is addressing the cause by imposing strict regulations on farming practices and nitrogen loss to water. We do this by setting nitrogen leaching limits on individual farms, rather than limiting the number of cows.
- Yes, Christchurch drinking water is safe. Christchurch City Council monitors the quality of the city’s drinking water to make sure it is safe from pathogens, nitrate and other contaminants.
- Nitrate concentrations in the city’s water supply are well below the Maximum Acceptable Value set by Taumata Arowai.
- Christchurch’s drinking water comes from groundwater beneath the city, sourced from more than 100 wells.
- The groundwater comes from a mixture of water seeping out of the Waimakariri River and rainfall that percolates through soil in an area north-west of Christchurch.
Drinking water supplied by city and district councils throughout Canterbury is generally safe. Councils monitor the quality of the water to make sure it is safe from pathogens, nitrate and other contaminants.
Private wells are the responsibility of landowners and must be independently tested to ensure they are safe, particularly if they are being used as a source of drinking water.
- If you rely on a private well for your drinking water, you should make sure your well is secure and test your water regularly for E.coli and nitrate.
- This is the well owner’s responsibility, not that of the regional council or any other agency.
- In most cases, samples can be taken from a kitchen tap and sent to a laboratory for testing.
Farming is a major source of nitrate contamination in groundwater. Careful management of the entire farm system is required to ensure the farm does not maintain more nitrate in the soil than plants can use. Consequently, Environment Canterbury concentrates on limiting nitrogen losses rather than limiting cow numbers or the amount of fertiliser applied.
Wastewater disposal (septic tanks, other sewage treatment systems, industry wastewater) is another source of nitrate contamination.
In surface water, such as rivers and streams where rainfall often quickly becomes streamflow, the answer is usually no.
However, nitrate concentrations in Canterbury groundwater are generally increasing and this can affect some rivers and streams fed mostly by groundwater.
Because nitrate takes decades to travel through groundwater, nitrate levels may rise before the improvements from today’s strict farming rules become apparent.
Environment Canterbury’s role is to monitor and protect the quality of water in the region’s aquifers, rivers and lakes. We cannot knowingly let water quality decline.
We put controls on land-use activities that could threaten water quality. For example, we control land use to the west of Christchurch (where most of the Christchurch groundwater recharge rainfall lands) to protect the city’s drinking water supply. Much of this area is used for very low intensity stock grazing and recreational parks.
We have introduced some of the strictest farming rules in the country to help address water quality issues and, over time, deliver the improvements needed to protect our water.
This includes requiring farmers to operate within strict nitrogen leaching limits and adhere to industry agreed good management practices.
Higher-risk farming types are now required to measure and manage nitrogen leaching via a consent to farm. They are also required to have a Farm Environment Plan and their performance against this plan is graded by independent auditors as part of Environment Canterbury’s compliance programme.
Farmers are required to work hard to reduce their impact on the environment, including fencing off waterways and planting along stream margins.
Very few dairy conversions have been allowed in Canterbury in recent years. For farmers outside irrigation schemes, all the conversions and herd size changes have been made within allowable nitrogen loss rates, meaning the farmer has made environmental improvements to mitigate any additional nitrogen loss.
- Because of the time it takes for farming practices to change and for nitrate to travel through our groundwater, we do not expect to see clear improvements in groundwater quality for at least another 15 to 20 years.
- In some cases, we can expect the situation to get worse before it gets better.
In other cases, we are already seeing improvements. Recent analysis shows that nitrate concentrations are improving in some Canterbury streams and rivers. Recent trend analysis of Canterbury streams and rivers (surface water) has shown improvements in nitrate concentrations at more sites than are worsening in the past decade..
Environment Canterbury monitors groundwater quality in over 300 wells across the region each year. Read the findings from our latest survey in the 2020 Groundwater Quality Report (PDF File, 3.0MB).
The results from the latest survey in 2020 are in line with what we expected and similar to what we see most years.
Environment Canterbury publishes a Risk Maps of Nitrate in Canterbury Groundwater (PDF File, 8.04MB). Groundwater quality changes slowly, so a two-year review cycle captures regional-scale changes in nitrate risk.
Surface water is monitored at over 100 sites a month and the results are shown on the LAWA website.
- Read more about our drinking water
- To see water quality state and trend at individual sites on rivers in Canterbury visit the LAWA website.
- Read about What’s happening in your local water zone