December 6, 2021

I, Science

The science magazine of Imperial College

Sarah Gaunt on how we know whether it's safe to go for a swim

dorset surfer shutterstock_9622858I remember the first time it happened with painful clarity.  The summer sun had burned the Cornish sand a brilliant gold that day, and glittering rainbows sprang from the spray of each wave.  Scores of neoprene-clad beach bums already floated out back, safe behind the breaking white water, and we dived head first into the foam, eager to join them.  Set after perfect, glassy set rolled in, and not even our abysmal surfing skills could take away from our enjoyment as the afternoon faded, and we left for home, exhausted.

Hours later I was wrenched awake, flushed and shivering; covered head to toe with a cold sheen of sweat.  My abdominal muscles clenched against the pain, and my guts knotted as if caught in a vice-like grip.  I’ll spare you the truly gory details of that night, but suffice to say, the next week dragged by in a misery of aching limbs, dizzying headaches and crippling stomach cramps.

Every year doctors’ surgeries are met with surfers and swimmers who have fallen foul of contaminated swimming water.  In the best instances, these are non-life threatening cases of mild stomach upsets, ear infections and sore throats.  More seriously however, is the risk of infection by more harmful toxins, such as E. coli bacteria or Hepatitis; both of which are rare, but very real threats from swimming in polluted water.

Kirsty Cavill, a veterinary nurse and mother of three from Devon, is a regular on the beaches of southwest England, visiting every week to surf and kayak with her boys.  “At least one of us has experienced stomach upsets, and the boys have contracted ear infections on at least two occasions following surfing trips,” she said.  “Water quality is very important to us, and we consciously plan visits to beaches we know are safe, and have no sewage output nearby.”

geograph-3334774-by-Richard-WestHere in the UK there are around 31,000 ‘combined sewer overflows’ dotted around the coast.  These are structures which, when sewers are put under strain from heavy rainfall, will discharge untreated sewage directly into the sea.  Last summer, nine beaches were closed in Kent alone due to sewage overflow.

As a way to combat the ill effects of sewer overflows, local authorities sample water from each of the UK’s 754 designated bathing areas, for the duration of the summer season.  Once a week, water is collected and tested in a laboratory to see how many harmful bacteria are floating in each teaspoon of swimming water.  If levels are too high (more than forty-eight bacteria per teaspoon, if you were wondering), beaches are closed for public safety.  Alarmingly however, results of these tests may take as long as two days to be returned, meaning hundreds of people may have already been exposed to contaminated water.

Across the Atlantic in America, scientists at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science think they may have found the solution to this exact problem.  By noting both the type and the source of harmful bacteria found on a particular beach, along with weather events and tidal patterns, Dr Brian Haus and his team have developed a predictive tool to accurately simulate how fast beach-dwelling bacteria grow.  “The predictive tool,” suggests Dr Haus, “provides a middle path between direct [water] sampling, and complex numerical modelling.  The purpose is to provide a predictive capability for the likely presence of potentially hazardous conditions for beach managers.”

At the moment, the tool is still in its testing phase, and being implemented across the sandy beaches of Miami-Dade, Florida, coupled with conventional sampling to ensure its accuracy.  Before he makes the tool fully available, Dr Haus is keen to test his model at a variety of beach types – “in particular, open ocean, high energy beaches”, just like those on the UK’s south coast.

Currently on British beaches however, despite the weekly testing scheme, the sewage in our seas remains a major problem, and a primary concern of environmental charity Surfers Against Sewage.  Andy Cummins, director of campaigns for the charity, was very excited at the prospect of a new predictive tool to eliminate the lengthy waits for results.  “That would be fantastic!” he said.  “The benefit would be rather than getting a one-day-in-seven sample, there would be continuous, ongoing information.  You would be able to identify where there are spills, and give people the right information to make an informed choice about how, when and where they use the sea.”

While the majority of UK beaches are kept to exceptional standards of cleanliness, it is worth pausing before diving head first into the crashing surf.  Do you know of any sewage outlets near your favourite swimming spot?  Are you desperate for a dip after a week of torrential rain?  As the summer days are lengthening, and the rippling waves look ever more inviting, just remember, there may be more life in the ocean than you had bargained for.

Sarah Gaunt is studying for an MSc in Science Communication

Images: Surfer riding a wave in Dorset by David Hughes (Shutterstock); Coastal East Lothian : Belhaven Bay Sewage Outflow Pipe (Copyright Richard West and licensed for reuse under this Creative Commons Licence)

Citation: Feng, Z. et al. (2015) A predictive model for microbial counts on beaches where intertidal sand is the primary source. Marine Pollution Bulletin 94:37-47. doi: 10.1016/j.marpolbul.2015.03.019.