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Fresh water Groundwater quality
Nitrate in groundwater
Nitrogen is found in groundwater in the form of nitrate and is monitored for health and environmental reasons. Excessive levels of nitrate in drinking water have been linked with blood disease in infants (commonly known as ‘blue baby syndrome’).
From an environmental perspective, elevated levels of nitrate often indicate the potential presence of other pollutants from human activities, such as faecal pathogens and pesticides (that is, nitrate can be a good indicator of general groundwater degradation). In addition, groundwater that is rich in nitrate has the potential to elevate nutrient levels in the surface water it drains into.
Current situation
More than one-third (39 per cent) of groundwater monitoring sites in New Zealand have levels of nitrate that are elevated above natural background levels, probably as the result of human activities, such as the leaching of fertiliser and stock effluent. The median nitrate level in New Zealand groundwater that is monitored is 1.3 milligrams per litre. Nitrate levels exceed the 2005 New Zealand drinking water standard of 11.3 milligrams per litre at almost 5 per cent of monitoring sites. However, the proportion of bores at these sites used to supply drinking water for people is not known.
Median nitrate levels in groundwater, 1995–2006
Note: mg/L = milligrams per litre.
Source: Ministry for the Environment.
Text description
The map above shows all New Zealand monitoring wells colour-coded according to the nitrate category they fall into (three categories covering the range 0 to >11.3 mg/L). Regions with a significant proportion of wells that have median nitrate exceeding 5.65 mg/L (i.e. half the New Zealand Drinking Water Standard) are Waikato, Manawatū, Wairarapa, Taranaki, Canterbury and Southland.
In New Zealand, nitrate concentrations are highest in shallow well-oxygenated groundwater in unconfined aquifers. The median nitrate concentration in this type of groundwater is 2.8 milligrams per litre, which is more than twice the average of all monitored groundwater (1.3 milligrams per litre).
Monitored groundwater sites with nitrate concentrations that breach health standards are found in most regions, but are most common in the Waikato and Manawatū regions. In the Waikato, elevated nitrate concentrations have been attributed to intensive land uses such as dairying and market gardening in areas where free-draining soils overlie a shallow water table.
As well as affecting drinking water quality, excessive levels of nitrate in groundwater can lead to nutrient enrichment of downstream surface water. This has important implications for regional freshwater management, particularly because there can be a lengthy time lag between groundwater being polluted and its emergence at a downstream water body.
Recent trends
There has been no clear nationwide trend in nitrate concentrations over recent years. Approximately equal numbers of monitored groundwater sites have shown increasing concentrations of nitrate (13 per cent) and decreasing concentrations of nitrate (11 per cent) over 1995–2006.
At a regional scale, increasing trends of nitrate are more widespread in some areas than others. Increasing nitrate concentrations have been reported in rural parts of Canterbury, probably due to the increasing intensity of human activities in the region such as dairy farming and wastewater disposal. Increasing concentrations of nitrate have also been recorded at some Waikato sites for which records are available from the 1950s.
What can you do?
- Do you have a groundwater bore? Make sure your bore or well is in good condition to avoid contamination of groundwater aquifers. Your local council should have more information on requirements for bores and wells. For example, Environment Southland has produced How well is your well? - a guide to protecting the quality and reliability of your groundwater supply, and Northland Regional Council has produced Save our groundwater.
- Farmer or horticulturalist? Adopt a nutrient management plan to minimise the amount of nitrogen that is wasted by leaching into the groundwater table.
This information has come from the latest state of the environment report, Environment New Zealand 2007.
Return to the groundwater quality page.
Last updated: December 2007
