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Faecal pollution (bacteria) in groundwater

There are about 700 groundwater monitoring sites that have sufficient data on bacteria levels to derive medians. Escherichia coli is the main bacteria used because it is a species that indicates the presence of faecal matter from warm-blooded animals, however, other bacterial indicators are also used.

Current situation

For the period 1995 to 2008, seventy seven per cent of the monitored sites have median bacterial levels that comply with the health-related drinking water guideline of less than 1 E. coli unit per 100 millilitres of water sampled. However, the proportion of these sites used to supply drinking water for people is not known and is likely to be low. The drinking water standard is breached most commonly in  Auckland, Otago and Taranaki.

For the period 1995 to 2008, only 2 per cent of the monitored sites have median bacterial levels that exceed the Australia and New Zealand Environment Committee Council (ANZECC) guidelines for stock drinking water quality of 100 coliform units per 100 millilitres of water sampled.

Like nitrate, bacteria concentrations are highest in shallow, unconfined groundwater. Bacteria generally do not survive the long travel times needed to reach deeper groundwater. However, bacteria can be widely dispersed within shallow groundwater systems because these aquifers typically have relatively fast-flowing water and porous sediments.

Elevated concentrations of bacteria in groundwater are commonly attributable to faecal matter leaching from stock dung on the land surface or from human waste disposal facilities such as septic tanks. However, high bacteria counts do not always represent general groundwater degradation. For example, poor groundwater well (or bore) design may allow faecal material from a localised source at the land surface (such as a farm animal defecating near the well head) to leak directly down the shaft.

Recent trend

Ninety eight per cent of the monitored groundwater sites showed no change in concentrations of bacteria between 1995 and 2008.

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.

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