Poo, E. coli and our lakes.
February 25 2019 by Dr Richard Muirhead
Recently there has been lots of chatter and coverage on the health of our environment and the quality of our water. The state of our lakes seems to be driven by the amount of E.coli entering the system so we asked an expert to explain more about what E.coli is and how it ends up in our lakes.
Poo, E. coli and our lakes. Most New Zealanders have an affinity with water and are concerned about the ‘swimability’ of our lakes and rivers. Articles on swimability always talk about E. coli but what is it and why is it so important? First, we need to talk about some ugly stuff – poo. Many human diseases are spread by what we call the ‘faecal-oral cycle’. I won’t go into the details but I’m sure we all have experienced vomiting or diarrhoea at some point. All faecal material has the potential to contain disease-causing organisms, including animal faeces (the fancy name for this is ‘zoonoses’). Unfortunately, New Zealand has some of the highest rates of zoonotic diseases in the developed world; higher than Australia, the UK or the USA.
To test if these disease-causing organisms are in our lakes and rivers, water samples are collected. It’s expensive and tricky to detect all the microorganisms that cause disease, so instead we look for E. coli. E. coli is a faecal indicator organism – it inhabits the gut of all warm-blooded animals, so its presence indicates that the water has been contaminated with poo. It’s a simple and inexpensive test and regular sampling builds up a long-term picture of ‘swimability’ of the site. A single cowpat or litre of sewerage can contain 1 billion E. coli, so a relatively small amount of faecal material near a test site can cause a spike. Furthermore, as E. coli is a living organism they can die off, so the spikes and come and go very quickly. Regular water quality sampling can’t tell you what the water quality is on a specific day, or where the contamination came from.
Faecal contamination can be generated by a number of sources: leaking sewerage pipes, direct inputs from bird populations, animals accessing the water to drink, or rainfall events causing runoff from farmland or urban areas. Rainfall on soil is usually absorbed, but in urban areas impervious roads, carparks and roofs can generate significant runoff even from relatively small rainfall events. This storm water typically ends up in our rivers or lakes.
One way to identify the exact source of faecal material is to use faecal source tracking (FST), where samples are sent to a specialist lab for biological and chemical testing. It’s great, but there’s a few challenges – you have to collect the sample when there’s a spike and you need to be aware that each spike may come from a different, or multiple, sources. Sanitary surveys using sampling and computer models is another good method, but they take time and are expensive. In summary, the identification and management of faecal sources impacting on swimability is a challenge. The simplest approach can be to observe, and record sources seen entering the water most days – but be prepared to work in the rain to see the storm runoff sources!
By Dr Richard Muirhead,
Science Team Leader at AgResearch and Scientist at Our Land & Water, Invermay, Dunedin.