Nitrate in groundwater

How we monitor

• Where and how we collect the data

  Monitoring sites

  Waikato Regional Council monitors nitrate concentrations in two networks:

  • Regional network: 110 mostly shallow wells (< 30 m deep) distributed throughout the region. This network represents generally vulnerable aquifers with relatively young groundwater in aerobic conditions. We exclude wells with significant iron concentrations from this network.
  • Community supplies: 80 school water supplies. These supplies are chosen as an indicator of risk to the community.

  Monitoring frequency

  • Community supplies are monitored every two years.
  • Regional network wells are monitored yearly. Thirty of these wells are also monitored on a quarterly basis, so we can examine the seasonal and land-use impacts.

  Monitoring history

  The regional network was established in 1996. It originally comprised of 108 sites and was monitored for two years on a six monthly basis. This network increased to 112 sites in 2002.  In 2010 we monitored 110 wells. A subset of 30 sites have been monitored quarterly.

  A total of 90 school supplies were sampled in 2000. 89 were sampled in 2002/3, 88 in 2004, and 89 in 2008. Other community supply information was collated from external district supply records in 2000. This information was not available in the period 2002-2004.

  Measurement technique

  Waikato Regional Council collects samples using the groundwater sampling protocol developed by the Institute of Geological and Nuclear Sciences. Samples are analysed in the laboratory for nitrate and other routine water analyses comprising pH; conductivity; total dissolved solids; alkalinity; carbon dioxide; calcium; magnesium; hardness; sodium; potassium; chloride, sulphate, boron, total iron, manganese copper and zinc. The detection limit for nitrate is 0.02 g/m³.
• How this indicator is compiled

  Waikato Regional Council has developed three categories of nitrate concentration in groundwater. The indicator reports the proportion of the region’s groundwater supplies in these three categories.

  This tells us the proportion of supplies with nitrate concentrations exceeding the drinking water guideline and those that need close monitoring (that is, those with concentrations of more than half the drinking water guideline). The elevated category is consistent with the Ministry of Health approach requiring close monitoring of community supplies with nitrate exceeding half the drinking water guideline.

  We also report low concentrations. The low category may still include some land uses that may affect sensitive environments. For example, in the Lake Taupo catchment even slightly raised nitrate concentrations will affect water quality.

  Guidelines and standards

  The Ministry of Health's (1995) maximum acceptable value for nitrate (NO₃) in drinking water is 50 g/m³. Expressed as nitrogen (N) this equals 11.3 g/m³ NO₃-N.

  Waikato Regional Council has developed three categories of nitrate concentration in groundwater that reflect this guideline:

  Category	Nitrate concentration (g/m3)	Comments
  Excessive	> 50	Exceeds the national drinking water guideline.
  Elevated	25 – 50	Safe for drinking but elevated from land use. Likely to produce adverse environmental effects.
  Low	< 25	May still affect sensitive environments.
• Limitations

  This indicator is limited by the extent to which the selected sites represent our very large, highly variable three dimensional groundwater resource. Care is taken to base this indicator on information from sites which are well documented (that is, where log, construction and location information are available) and distributed widely throughout the Region representing all major aquifer areas.

  The regional network comprises predominantly shallow wells (<30 m) with low iron concentrations. This may mean an overestimation of the region's likely ‘average’. By contrast, community water supplies with excessive nitrate concentrations are generally replaced or are not developed. Our monitoring of community supplies may under-estimate the occurrence of excessive nitrate concentrations.

  There is little ambient groundwater quality information to indicate temporal trends. We have not attempted to address spatial trends within the region, such as sub-regional comparison.

  Further indicator developments

  The regional monitoring network will be continuously reviewed to ensure appropriate sites are being monitored. Trends in nitrate contamination of groundwater require long data records, as the concentration changes very slowly. Future consideration of aquifer vulnerability and the age of groundwater will provide a better basis for our understanding of trends.