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:
Microbial contamination is more likely in this group than a completely random sample.
The 82 monitoring wells selected will be sampled every 4 years. The indicator will be next updated in 2016.
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.
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 Environment Waikato's Hydrol database.
Environment Waikato 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.