Nitrate contamination of groundwater

Waikato Regional Council monitors nitrate in groundwater to detect changes in contamination levels that may affect human health and the environment. We have two monitoring networks - a regional and a community supply network of wells. 

Excessive nitrate and iron are the most common groundwater contaminants in the region.

Iron occurs naturally, but high nitrate levels indicate contamination from human activities. Nitrate levels are generally highest in shallow groundwater aquifers.

Nitrate contamination of groundwater is a concern for human health and the environment. High nitrate concentrations have been linked to a blood disorder in bottle-fed babies, known as ‘blue baby’ syndrome (or methaemoglobinaemia).

The effect of contaminated groundwater on our rivers and streams is also important. In the Hamilton Basin, up to 85 percent of the base flow of small streams comes from groundwater. Much of this water is already enriched with nutrients including nitrate. Find out more in our indicator of the sources of nitrogen in our rivers and streams.

Excessive nitrate concentrations in groundwater are related to land use activities, including:

• pastoral farming
• market gardening
• application of nitrogenous fertilisers
• industrial and sewage waste disposal (for example, septic tanks).

Studies have shown that high use of nitrogen fertilisers on pasture can make the groundwater unsuitable for human consumption.

Levels of nitrate in groundwater vary a lot across the region. Areas where nitrate levels most commonly exceed drinking water guidelines are:

• Hamilton-Mangaouna – an area of intense farming.
• Pukekohe - an area of mainly market gardening.

We have few records to indicate long-term nitrate trends. But long-term information from some schools supplies (since 1950s) indicates a steady increase in nitrate at these sites.

Find out more in our indicator ‘Nitrate in Groundwater.

This map shows the results of nitrate monitoring at sites throughout the region. These sites are mainly shallow aquifers.

More than nine percent of sites have nitrate concentrations above the accepted drinking water guideline of 50 g/m³ (11.3 g m^-3 as nitrate-N)¹. Close to a third have concentrations over half this value.

Higher nitrate concentrations are found at market gardening sites.

It is important to note that for healthy ecosystems nitrate concentrations need to be much lower than the drinking water guideline, possibly as low as 13 g/m³ (3 g m^-3 as nitrate –N).

Footnotes

1. Drinking water standard for nitrate is 50 g/m³ (11.3 g m^-3 as nitrate-N).
   From: Ministry of Health. 2000: Drinking Water Standards for New Zealand. Ministry of Health, Wellington.

Remember that it is much easier to prevent groundwater contamination than to fix it.

Look after your well

• Prevent contaminants from entering your well by sealing the top - for example with a concrete pad around the casing.
• Fence around your well or bore to keep animals away.
• Don’t store, mix or use chemicals (such as fertilisers) too close to a well or bore.

Be careful with nitrogen fertiliser

• Avoid excessive nitrogen fertiliser use. Use nitrogen fertiliser on pasture to supplement nitrogen produced by clover - not to replace it.
• Use nutrient budgets to work out a sale level of nitrogen fertiliser use.
• Avoid heavy use of nitrogen fertiliser during winter when there is higher rainfall and slower pasture growth, and a greater likelihood of leaching to groundwater.
• Apply nitrogen fertiliser in split dressings, rather than all at once, so pasture can use it for growth (reduces the amount lost to leaching and is more cost effective).

Growing commercial vegetables

Reduce the amount of nitrogen leaching when growing vegetables commercially:

• Break the application of nitrogen fertiliser for commercial vegetable growing into several smaller applications over the growing season (tailored to the crop development stage) - reducing leaching while maximising yield.
• During non-cropping times, cover crops such as oats and mustard can be cultivated and ploughed into the soil. This adds organic matter and traps unused nitrogen in the soil, making it available for the next crop.

Remember that most Waikato soils are over-fertilised.

Find out more about soil management in Franklin district where areas of intensive agriculture such as commercial vegetable growing are damaging soils.

Learn more in our indicators that measure groundwater quality in the Waikato region, including nitrate and pesticide levels.

Follow the guidelines on pesticide use in Growsafe, the New Zealand Standard for Agrichemical Users' Code of Practice.