Median Escherichia coli concentration

Source:
National Institute of Water and Atmospheric Research

Method:
"Escherichia coli concentration is measured by collecting water samples which are then sent to laboratories to be analysed. The concentration is expressed as the number of E.coli per 100 mL.
Estimates of median E.coli concentrations across New Zealand is based on monthly concentrations from 409 river sites monitored by the 16 regional councils and 77 sites along 35 major rivers measured monthly by the National Institute of Water and Atmospheric Research (NIWA) since 2001.
In the National Policy Statement for Freshwater Management (NPS–FM), the annual median concentration is used to specify the possible infection risk from activities that involve occasional immersion and some ingestion of water, such as wading or boating. The median represents the middle value when observations are ordered in size. When the annual median sampling statistic is 260 E.coli/100ml or lower, people who undertake an occasional immersion activity are exposed to a very low risk of infection (less than 0.1% risk) (Ministry for the Environment, 2014).
Infection risk for people undertaking activities involving full immersion in river water is often determined using the 95th percentile. This is the level that for 95 percent of sample observations, the E.coli concentration is at this level or below. When the 95th percentile sampling statistic is greater than 540 E.coli/100 ml, people who undertake a full immersion activity are exposed to a high risk of infection (greater than five percent risk). When the 95th percentile sampling statistic is 260 E.coli/100 ml or lower people are exposed to a low risk of infection (less than one percent risk) (Ministry for the Environment, 2014).
The accuracy of the data source is of medium quality.



Reference:

Ministry for the Environment (2014). National Policy Statement for Freshwater Management. Available from www.mfe.govt.nz.
"