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Limitations of the groundwater quality indicator

While the indicators for fresh water provide an overview of the quality and level of allocation in New Zealand, they do not provide a complete picture of the health of the freshwater resource. The indicators do not include other pollutants that can affect water quality, such as herbicide and pesticide residues, dissolved metals, pharmaceuticals and hydrocarbons.

In New Zealand, groundwater monitoring tends to focus on aquifers that are considered by water managers to be either an important source of supply or particularly vulnerable to pollution. Therefore, the results of monitoring surveys presented in this section should be considered to represent areas where contamination is likely, rather than representing the overall groundwater resource in New Zealand.

Although results from national and regional state of the environment groundwater monitoring are compared to the health-related drinking water guidelines from the Drinking Water Standards for New Zealand the results cannot be considered representative of drinking water quality in New Zealand. Many of the monitoring sites are not used for potable water supply but used for other purposes such as irrigation and stock water. Also, for those monitoring sites that are used for small scale supply of potable water, it is possible that the water will be treated to improve water quality after abstraction and before human consumption. For a detailed assessment of drinking water quality in New Zealand refer to the Ministry of Health’s Annual Review of Drinking Water Quality reports.

The indicator does not take into account the age and origin of groundwater, as the availability of this information is limited. The age and origin of groundwater is important information to help determine the drivers of groundwater quality, particularly in relation to the impacts of land use.

The relationships between groundwater quality and land use are difficult to detect because:

  • land cover is often used as a proxy for land use and land use intensity is usually not considered.
  • groundwater often does not enter the aquifer near the groundwater monitoring site. Therefore land use where the groundwater enters the aquifer needs to be considered as well as the land use at the monitoring site. However, where the groundwater enters the aquifer is not always known.
  • groundwater impacted by land use may not have had time to travel from where it entered the aquifer to the monitoring site
  • some substances can be converted into other forms or degrade before reaching the monitoring site. For example, nitrate can be converted to other forms of nitrogen, such as ammoniacal nitrogen or nitrite.

Faecal pollution

The Escherichia coli results presented should be interpreted with caution as they include other proxy bacterial indicators such as total coliforms and faecal coliforms These other indicators are often used by councils, particularly in the Bay of Plenty, Hawke’s Bay and Wellington regions. The presence of E. coli indicates faecal contamination from warm-blooded animals, whereas total coliforms include bacteria from faecal and environmental sources eg naturally occurring soil bacteria. Therefore, total coliform counts are always greater than E. coli counts. Also some regions only started monitoring bacterial indicators in recent years, such as Auckland, Marlborough and Taranaki regions and therefore the results presented should be interpreted with caution as they are for a short time period.  

The detection limit for E. coli is the same as the drinking water guideline of 1 E. coli unit per 100 millilitres of water sampled. Therefore, compared to other indicators of groundwater quality such as nitrate, an E. coli level that exceeds the guidelines has a greater chance of being caused by contamination during sampling or laboratory analysis.

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