E.Coli Contamination in "Secure" Groundwater Sourced Drinking Water Supplies

The Drinking-water Standards for New Zealand 2005 (Revised 2008) define a secure groundwater source based on the absence of surface or climatic influences, the security of the bore head and an absence of E. coli from regularly collected water samples. However, in some instances, apparently “secure” bores are still at risk of E. coli contamination. This paper provides examples of two such incidences. 

Potential migration pathways for shallow contaminated groundwater into apparently secure boreholes include joins in the casing, possible migration down the borehole annulus and/ or an artificially increased hydraulic gradient through the natural strata. Based on the mixing model calculations used to define a secure groundwater source in the DWSNZ, some sources with high E. coli concentrations can still cause a threat of contamination to “secure” supplies. Detections of total coliforms are also an indicator of risk from surface contaminants that should not be ignored. 

These risks need to be managed through appropriate controls on land use activities and discharges that occur within the contributory capture zones around water supply bores and through careful review of regularly gathered water quality monitoring data.