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Faecal pollution (bacteria) in groundwater

There are 520 groundwater monitoring sites that have sufficient data on bacteria levels to derive medians.

Current situation

Eighty per cent of these sites comply with New Zealand standards for drinking water quality of 1 bacteria coliform unit per 100 millilitres of water sampled. However, it is not known how many of these monitored groundwaters are used to supply human drinking water. The drinking water standard is breached most commonly in Northland, Southland and Canterbury.

Only 2 per cent of the monitored sites exceed the Australia and New Zealand Environment Committee Council (ANZECC) guidelines for stock drinking water quality (100 coliform units per 100 millilitres of water sampled).

Like nitrate, bacteria concentrations are highest in shallow, unconfined groundwater. Faecal 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 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 per cent of the monitored groundwaters showed no change in concentrations of bacteria between 1995 and 2006.

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.


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