Micro-organisms in groundwater

Why we monitor micro-organisms in groundwater •
What's happening? •
Methods - how we monitor •
Results - data and trends •
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Why we monitor micro-organisms in groundwater

The occurrence of micro-organisms in groundwater is a top priority concern from a health perspective, as their presence can lead to rapid and major outbreaks of illness, sometimes with fatal consequences.

Escherichia coli (E.coli) are the preferred indicator group for the occurrence of micro-organisms in groundwater because they are easy to detect and count, and have better survival in fresh water than enterococci. They are used as criteria in Ministry of Health drinking water guidelines. Specific measurement of viral occurrence, such as MS2, is by comparison expensive and difficult. Total coliforms were not included as they indicate more general bacterial occurrence, often unrelated to human health risk.

What's happening?

In the Waikato region, groundwater is widely used for potable supply - particularly in rural areas. About 25 per cent of the Waikato population still relies on septic tanks, which are a common source of groundwater contamination by faecal matter. Other potential sources of microbial contamination include offal holes and animal wastes, particularly where there is access via poorly constructed wells.

Localised microbial contamination of groundwater has been reported in some 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 per cent of groundwater samples collected from 17 wells contained faecal bacteria. In 1997, monitoring showed about 10 per cent of samples from 19 wells at Whangapoua and just over 50 per cent of groundwater samples from 15 wells at Te Puru contained faecal bacteria.

There has been little information available on the extent of microbial contamination in individual rural groundwater supplies. However, a study of groundwater from 40 wells sampled in Matangi (near Hamilton) found five (12.5 per cent) were contaminated with faecal coliforms.

This indicator shows the proportion of groundwater wells in the Waikato region in which E-coli are detected. E-coli, which is a common gut bacterium living in warm blooded animals, is an indicator of water contamination by excrement and the potential presence of pathogenic viruses, bacteria and protozoa.

Waikato Regional Council monitors 82 wells for the occurrence of micro-organisms (E-coli). These include 14 wide diameter wells (900 mm or more). Wells were selected primarily on the basis of the vulnerability of groundwater to contamination from the land surface. Sufficient wide diameter concrete wells were included to enable a comparison between the occurrence of micro-organisms in these and narrower generally steel cased bores. There has been no consideration of proximity to microbial sources or conduits. The indicator reports the percentage of wide diameter wells and narrower diameter wells monitored at which E-coli were found to be present or absent.

Methods - how we monitor

Monitoring sites
Micro-organisms (E. coli) are monitored at 82 sites throughout the Waikato region. These comprise 21 school supply wells and 61 regional monitoring wells.
Fourteen of these are wide diameter wells (approximately 900 mm), only one of which is a school supply well. Wide diameter wells are generally more likely to be contaminated as they are shallow and more difficult to seal.
These wells were selected from a larger network, prioritised on the basis of the following criteria:
- vulnerability to contamination from the land surface assessed using the DRASTIC index (Aller et al., 1987)
- availability of in-situ pumps to minimise the potential to introduce contamination during sampling
- availability of well log and construction information (with the exception of some wide diameter wells)
- spatial distribution and land-use representation (for further statistical comparison).
Microbial contamination is more likely in this group than a completely random sample.
Monitoring frequency
The 82 monitoring wells selected will be sampled every 4 years. The indicator will be next updated in 2016.
Monitoring history
The data for this indicator was first collected in late 2004.
Earlier information reported localised microbial contamination of groundwater in some 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 per cent of 17 wells sampled contained faecal bacteria. In 1997, monitoring showed about 10 per cent of 19 wells sampled at Whangapoua and just over 50 per cent of 15 wells sampled at Te Puru contained faecal bacteria.
There has been little information on the extent of microbial contamination in individual rural groundwater supplies. However, a study of 40 wells in Matangi (near Hamilton) found five (12.5 per cent) were contaminated with faecal coliforms.
Measurement technique
Sites have been selected to utilise in-situ pumping equipment, wherever possible. Samples are collected in sterile bottles which are kept cool and analysis is undertaken within 24 hours.
The raw data is stored in Waikato Regional Council's Hydrol database.
Waikato Regional Council reports the presence of E. coli as a simple percentage in respect to the total wells monitored. A comparison is also provided between occurrence in wide diameter wells (approximately 900 mm) and the selected network of narrower diameter wells.
The Ministry of Health (2005) specifies that the maximum acceptable values for E. coli is <1 organism per 100 ml.
The indicator is limited by the extent to which the selected sites represent the very large, highly variable three dimensional groundwater resource.
Care has been taken to base the indicator, where possible, on quality information from sites which are well documented (for example, sites which have log, construction and location information) and distributed widely throughout the region - representing all major aquifer areas.
It should be noted that the monitoring networks have been selected based primarily on their vulnerability as expressed by the DRASTIC index of the hydrogeological setting, rather than proximity to likely sources.
The monitoring network will be reviewed four yearly to ensure appropriate sites are being monitored.