Nitrate in waterways - What's the story?

Recent news articles have discussed whether nitrate in our drinking water can cause health problems.  Naturally, this is cause for concern for all of us living in Canterbury.  In order to protect our water, farming practices are subject to the strictest farming rules in the country. 

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, anyone using a private well for drinking water needs to be aware of the risk. 

Regardless of where we get our drinking water, we all need to understand the problem and what is being done about it.

Read on to learn more about the actions being taken.

What is nitrate?

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 or nearby streams or lakes causing excess weed growth, algal blooms and toxicity for fish and invertebrates.

At what concentrations does nitrate become a health issue?
  • 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. The Ministry of Health has set a threshold to protect against this.
  • Nitrate in rivers, streams and lakes can cause water quality issues such as excess weed growth, algal blooms and toxicity to fish and invertebrates. 
  • A recent study carried out in Denmark showed a possible link between nitrate in drinking water and colorectal cancer.  A more recent review of all studies around the world concluded that there are too few studies to draw firm conclusions about the health risks of nitrate in drinking water in relation to colorectal cancer.  
  • The recent study in Denmark made a link to colorectal cancer at a much lower concentration, suggesting a threshold of 0.87 mg/l for nitrate-nitrogen. This is an important issue.   Environment Canterbury supports calls for more research, using data already available, on any possible links between nitrate concentrations and human health. It is the Ministry of Health’s role to undertake research and revise or set drinking water standards.   
  • 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.
Is Christchurch drinking water safe to drink?
  • 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 the Ministry of Health. 
Where does the city’s drinking water come from?
  • 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.
What about other council supplies?
  • 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.
Is private well water safe to drink?
  • 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 pathogens (such as 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.  
What causes nitrate contamination?
  • 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 and other sewage treatment systems) is another source of nitrate contamination. 
Are nitrate concentrations in Canterbury water increasing?
  • 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. 
Are nitrate levels increasing in any area that may have an impact on Christchurch drinking water, or drinking water anywhere else?
  • Recent scientific modelling tells us that any increase in nitrate in the Waimakariri area may have an impact on Christchurch drinking water many years into the future. 
  • If this is the case, the modelling tells us that any increase in nitrate would still be well below the safe standard for drinking water. 
  • Our monitoring showed a slight increase in nitrate concentrations in one deep water well in western Christchurch. However, the concentrations were very low – less than 0.5 milligrams per litre, where the Maximum Acceptable Value is 11.3 mg/l. We cannot tell whether the increase was caused by land use, or whether it was simply that pumping the well was drawing in shallower groundwater with slightly higher nitrate concentrations. The well was damaged in the 2011 earthquake and is no longer in use or accessible for monitoring.
  • We do not see increasing trends in any other wells in the city.
What is being done to stop the nitrate increase and improve water quality?
  • 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.  
When can we expect the nitrate situation in Canterbury to improve?
  • Because of the time it takes for nitrate from previous farming practices 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. 
What monitoring and reporting is being done?
  • Environment Canterbury undertakes a groundwater quality survey each year to better understand the region’s water quality and identify trends. 
  • The results from the latest survey in 2018 are in line with what we expected and similar to what we see most years. 
  • Environment Canterbury publishes a set of nitrate risk maps (PDF File, 4.27MB) every second year. The next set is due in early 2020 and is expected to be similar to the current maps. 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 .
Has chlorine been added to drinking water to protect it against nitrate?
  • No. Chlorine has been added to the city's drinking water by Christchurch City Council to ensure the water remains safe to drink while work is carried out on wellheads to make them secure from E. coli and other pathogens.  Chlorine does not treat nitrate. 

More information

Canterbury's drinking water

Read about water quality in our monitored rivers and streams

To see water quality state and trend at individual sites on rivers in Canterbury visit the LAWA website.

Read about aggregated water quality trends in Canterbury.

Read about Canterbury’s water and What’s happening in my water zone?

View the video -  Environment Canterbury's Chief Scientist's response to the claimed link between nitrate levels and health.