Safe to swim? Water pollution at bathing sites on the River Wharfe

Kiera Chapman : 10 April 2022

Freshwater bathing, also called “wild swimming” is increasingly popular in the UK. And who can wonder, with sites as beautiful as the River Wharfe at Ilkley, one of the first UK rivers to be designated for bathing by Defra.

But despite the idyllic appearance of this river, all may not be well beneath the surface. When there is heavy rain, water companies are routinely discharging excess sewage into rivers. In 2019, the local sewage treatment works discharged diluted sewage into the river a total of 136 times. That makes a total of 77 days of polluted water on this site alone.

To make matters worse, it is possible that run-off from livestock in fields upstream is also contaminating river water.

The truth is that we simply don’t know how safe our waters are to swim. In 2020, just 17% of the bathing sites in the UK were rated excellent, and 1.9% were judged as having poor water quality. However, this data has been seriously affected by a decrease in water quality monitoring during the COVID pandemic.

The conventional methods used to keep tabs on water quality don’t necessarily give us all the information we need, either. They simply detect faecal indicator bacteria (i.e. germs in poo). But we have long known that this method does not detect all the microbial diversity in a water sample, meaning that it misses lots of microorganisms that are potentially dangerous. It tells us nothing about the origin of the pollution either: is it from animals in fields around the water, or is it human, from sewage discharges into our freshwater systems?

We need better monitoring methods that are faster, able to identify a much wider range of microorganisms, and that can tell us about the level of contamination. And we need to know the source (human vs. other animals) to better manage our aquatic ecosystems.

Fortunately, microbial molecular approaches enable us to do this. University of Sheffield researchers are using molecular techniques to sample water quality at several sites on the Wharfe, at Ilkley. The locations were selected to compare sewage discharges with run-off from agricultural land, and samples of water were collected monthly from June to September 2021.

Led by Isabel Douterelo Soler, scientists are now analysing these samples in the lab, using an approach called ‘Microbial Source Tracking’ or MST. It is based on the fact that different animals have different bacteria in their gastrointestinal tract. By looking for these specific types of bacteria, MST can tell whether pollution comes from human, ruminant (e.g. cow/sheep), or even bird faeces.

Scientists search for these host-specific bacteria using ‘primers’: short strands of nucleic acids that are designed to bind only to the DNA of the target bacteria. More copies of those genes are then created using Quantitative Polymerase Chain Reaction (qPCR) technology, which enables scientists to quantify the amount of those bacteria that are present in a sample.

However, MST only detects whether certain target bacteria are present. To explore what else might be lurking in the water, scientists are using DNA sequencing techniques. They look for a gene called 16S ribosomal RNA (rRNA). Some areas of this gene vary between related species, which means that they act as a kind of ‘bacterial signature’. Searching for 16S rRNA can therefore help researchers to identify a much wider range of harmful pathogens in water samples, including those that do not derive from faeces.

Once this comprehensive analysis of our samples from the Wharfe is complete, we should have a much better picture of the source of pollution (human/other animals) and the level of contamination in the water at Ilkley. This will provide a better indication of the health risks associated with bathing there, and help to create better mitigation strategies to keep swimmers (and other recreational water users) safe.

We have applied for funding to extend this work on both the Wharfe and the River Roding in London, using more sophisticated testing and modelling to find new ways of working with citizen scientists to monitor and mitigate water pollution.