Setting water quality standards is a tricky business. Suitability for use is currently judged against guidelines that suggest water suddenly becomes ‘unsafe’ when a particular variable changes beyond the guideline value. For instance, according to MfE microbiological water quality guidelines (published in 2003), water is considered unsuitable for swimming once levels of E. coli – a microbe that indicates the presence of faecal bacteria – exceed 550 E. coli per 100 millilitres. But is it suitable at 500 per 100 ml? Or 100 per 100 ml?
In fact, swimming safety does not decline abruptly above or below this ‘magic number’, and is typically a combination of many factors, such as visibility, water temperature, pH, nitrogen, and phosphorus. says Dr Davies-Colley.
“What’s needed is a water quality index that transforms complex water quality data into information that is widely understood.”
In 2001, NIWA scientists published an index for contact recreation (swimming, kayaking, water skiing, etc.) in New Zealand waters. This index recognises that suitability for use doesn’t precipitously change above or below a particular water quality guideline. By way of a pilot study for Water & Atmosphere, NIWA scientists applied this contact recreation index to ‘characteristic’ values (medians) of river water quality data from 77 NRWQN sites over four years (2005–08).