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Fresh water Groundwater quality
Nitrate in groundwater
Nitrogen 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 (therefore 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 1) 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. For the period 1995 to 2008, the national median nitrate level of monitored groundwater is 1.7 milligrams per litre. This is higher than the national median of 1.3 milligrams per litre reported for the period 1995 to 2006 due to the unavailability of data for the Gisborne region in the 1995 to 2008 period. Gisborne region is dominated by oxygen-poor groundwater with low concentrations of nitrate.
Between 1995 and 2008, median nitrate levels exceeded the health-related drinking water guideline of 11.3 milligrams per litre at almost 5 per cent of monitoring sites. However, the proportion of these sites used to supply drinking water for people is not known and is likely to be low
Median nitrate levels in groundwater, 1995–2008
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This map shows the median nitrate levels for the period 1995 to 2008 for 914 groundwater sites in New Zealand. The majority (80 per cent) of sites have median nitrate levels less than 5.65 milligrams per litre. Forty four (5 per cent) sites have median nitrate levels that exceed the health-related drinking water guideline of 11.3 milligrams per litre (from the Drinking Water Standards for New Zealand 2005). Regions with a significant proportion of sites that have median nitrate exceeding 5.65 milligrams per litre (i.e. half the drinking water guideline) are Canterbury, Manawatū, Southland, Taranaki, Waikato and Wairarapa.
Note: mg/L = milligrams per litre.
Source: Ministry for the Environment.
In New Zealand, nitrate concentrations are highest in shallow well-oxygenated groundwater in unconfined aquifers. The median nitrate concentration in this type of groundwater was 2.8 milligrams per litre in the period 1995 to 2006, which is more than twice the average of all monitored groundwater (1.3 milligrams per litre)1.
Monitored groundwater sites with nitrate concentrations that breach health standards are found in most regions, but are most common in the Manawatū and Waikato 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
Nationally, about one third of sites showed significant changes in nitrate levels between 1995 and 2008. Of these, more have increasing (deteriorating) trends in nitrate levels (20 per cent of all sites) than decreasing (improving) trends (11.9 per cent of all sites).
Regional trends in Nitrate, 1995-2008
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| Region | % of sites with significant decreasing trend | % of sites with no significant trend | % of sites with significant increasing trend |
|---|---|---|---|
| New Zealand total | 11.85 | 68.17 | 19.98 |
| Auckland | 28.6 | 57.1 | 14.3 |
| Bay of Plenty | 7.5 | 86.8 | 5.7 |
| Canterbury | 5.9 | 64.3 | 29.8 |
| Hawke's Bay | 8.9 | 73.3 | 17.8 |
| Manawatu | 19.0 | 71.4 | 9.5 |
| Marlborough | 8.7 | 69.6 | 21.7 |
| Northland | 8.1 | 83.8 | 8.1 |
| Otago | 11.2 | 76.5 | 12.2 |
| Southland | 12.2 | 68.3 | 19.5 |
| Taranaki | 12.9 | 80.0 | 7.1 |
| Tasman | 18.8 | 56.3 | 25.0 |
| Waikato | 20.0 | 47.3 | 32.7 |
| Wellington | 19.7 | 74.6 | 5.6 |
| West Coast | 0.0 | 50.0 | 50.0 |
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 well (or bore)? Make sure it is in good condition to avoid contamination of groundwater aquifers. Your local council should have more information on requirements for wells and bores . 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.
- Are you a farmer or horticulturalist? Adopt a nutrient management plan to minimise the amount of nitrogen that is wasted by leaching into the groundwater.
This information has come from the latest technical report National groundwater quality indicators update: state and trends 1995-2008 and the state of the environment report Environment New Zealand 2007.
Return to the groundwater quality page.
Last updated: January 2010
