Dr Jacqueline Rowarth: Check before you swim this summer

Photo / File

Comment: Is New Zealand's Clean Water proposal too stringent in its assessment of swimmability? Dr Jacqueline Rowarth looks into the matter of faecal measurements.

Warming temperatures mean swimming is on the horizon. Beach patrols started on Labour Day and hardy people have tested the waters. The LAWA website (LAWA.org.nz) is already active with all the latest information on "swimmability" – the microbiological testing that is part of the "can you swim here" safety net.

Swimmability is assessed by the quantity of the faecal bacterium Escherichia coli present in 100ml of water, measured in coliform units (cfu).

All proposed swimmable grades in Clean Water, the Government's new approach, include a requirement that the median E. Coli concentration be less than 130 per 100 ml.

Median is the midpoint of distribution of measurements, not the average. This is an issue with the proposed requirement for sampling "regardless of weather conditions". One high result could skew the median.

The requirement is despite the fact that swimming within two days of heavy rain has always been discouraged because of potential rushing water carrying debris and bugs.

The warning is common in developed countries, and the EU Directive on Swimming Water states that "samples taken during short-term pollution may be disregarded".

Short-term pollution is defined as microbial contamination from known sources and unlikely to affect bathing water beyond 72 hours from when the water was first affected.

NIWA has commented that the New Zealand requirement for sampling regardless of weather conditions "is likely to introduce a stringency to a site obtaining a swimming grade given that high E. Coli levels in rivers are often measured during higher flow events".

Different measurement system

The requirement would make New Zealand rivers appear less swimmable than those in the EU, simply because of a difference in measurement system.

NIWA highlighted this comparison in its comment that the EU "Sufficient" and "Good" grades are more permissive than the Clean Water proposal.

The EU reports annually and over 95 per cent of all sites are considered swimmable. No gastro-intestinal outbreaks in swimmers have been apparent.

The question should be asked why New Zealand would impose more stringent requirements than those in the EU when no ill-health has been recorded.

E. Coli, and other bacteria such as Enterococci and Campylobacter (which caused the gastrointestinal disease outbreak in Havelock North in 2016 and Darfield in 2012 when drinking water became contaminated), are common in faecal matter. E. Coli are used as an indicator for other bacterial species.

The Campylobacter infection risk model indicates that a median E. Coli of less than 130 means that the health risk is practically zero for at least half the time.

In waters that just meet the Clean Water swimming threshold (bottom of the yellow grade), NIWA calculates that the average infection risk faced by a random swimmer on a random day is no more than 3.1 per cent.

That is 3 in 100 people, using the median which includes samples taken in flood. Avoiding flooded waters, as advised, reduces risk.

Dr Jacqueline Rowarth. Photo / Supplied

Different faecal sources

A second issue is that of source of E. Coli. In reports on Southland river quality, ESR scientists have suggested that discriminating between different faecal sources (such as human, livestock and wildfowl) is an important aspect of effective water quality management.

Faecal contamination of human origin is generally considered to pose the greatest risk to human health and pathogens from ruminants and wildfowl are also known to present a risk. At the moment the analysis on swimmability is simply on E.coli presence.

The ESR report on the Oreti river indicated that under base flow (dry) conditions, over 20 per cent of samples contained faecal source tracking (FST) markers from humans, with a similar percentage from cows and approximately 30 per cent from sheep.

Around 90 per cent of samples showed FST from wildfowl. In rain the human FST was still above 20 per cent, cow increased to 50 per cent, sheep to just under 60 per cent and wildfowl to just over 60 per cent.

In the Aparima, the picture was broadly similar. Under base flow, human FST were found in approximately 8 per cent of samples, cow in 10 per cent, sheep in just under 50 per cent and avian in almost 90 per cent.

Following rainfall, human FST increased to over 20 per cent, cow to 50 per cent, sheep dropped slightly to 45 per cent and wildfowl to just under 80 per cent.

These results reflect leaking septic tanks, failing sewerage systems, freedom campers and rain washing faecal matter off river banks.

Fencing cows out of rivers has been standard on dairy farms and is increasing on hill country farms.

Sheep are not subject to the same strictures because they don't tend to wade, but their faecal matter can wash in from banks where they have been grazing. Sheep excrete less faecal matter than cows per day, but their faeces contain more E. Coli per gram.

An adult sheep has been calculated to produce 2.51 x 1010 E. Coli a day whereas a dairy cow produces 2.03 x 109 and a duck produces 3.18 x 1010.

Swans, geese and seagulls also contribute, and are attracted to anywhere where food is available. This includes popular swimming holes, where campers and dogs are also to be found.

ESR has suggested that if we identify sources of faecal contamination, we will be able to prioritise mitigation efforts. Human contamination needs addressing first, which would assist with beaches as well as rivers.

In the meantime, as the weather becomes warmer, and the waters are more tempting, maintain a watch on the LAWA "Can I swim here?" website and don't swim within 48 hours of heavy rain.

And remember that if there is a warning notice for bacterial contamination, the source might not be what you think.

- Dr Jacqueline Rowarth has a PhD in soil science which involved analysis of sheep faecal matter. She is an investor in a family-owned Waikato dairy farm where all waterways are fenced and a peat lake has been restored. Wildfowl numbers are controlled in accordance with regulations.

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