Microbial contamination of groundwater can happen when wells, septic tanks and offal holes are poorly constructed. Small communities using septic tanks are especially vulnerable to microbial contamination of their drinking water. Community groundwater supplies are routinely monitored for drinking water standard compliance and are generally free of contamination.

The most common health risk associated with drinking water is microbial contamination from disease-causing micro-organisms, such as bacteria, viruses and protozoa.

Gut bacteria from warm-blooded animals (E-coli) pose a threat to human health. Drinking water guidelines state that there should be no bacteria (E-coli) present in drinking water¹. E-coli are measured in drinking water as indicators of microbial contamination.

Groundwater is better protected from the risk of microbial contamination than surface water because of the soil which acts as a barrier above it. However, direct contamination can occur from:

• poorly constructed groundwater wells
• septic tanks
• offal holes.

Localised microbial contamination of groundwater is a problem in many coastal areas where small communities rely on septic tanks for sewage disposal. On the Coromandel Peninsula, monitoring at Cooks Beach in 1990 showed about 40 percent of 17 wells sampled contained faecal bacteria. In 1997, monitoring showed about 10 percent of 19 wells sampled at Whangapoua and just over 50 percent of 15 wells sampled at Te Puru contained faecal bacteria.

At present there is little information on the extent of microbial contamination in individual rural groundwater supplies. However, a study of 40 wells in Matangi (near Hamilton) found 5 (12.5 percent) were contaminated with faecal coliforms.

Routinely monitored community groundwater supplies are generally free of faecal contamination.

What you can do

• Fix any leaking septic tanks or pipes.
• Seal the top of your well (for example with a concrete pad around the casing).
• Fence around your well or bore to keep animals away.
• Don’t drill offal holes closer than one metre above the water table.
• Don’t place offal in offal holes with water in the bottom of them.

Check out the national drinking water standards on the Ministry of Health website (online publications).

Learn more in our indicators that measure groundwater quality in the Waikato region, including nitrate and pesticide levels.

Footnotes

1. Ministry of Health. 2000: Drinking Water Standards for New Zealand 2000. Ministry of Health, Wellington.