What's in the Water You Swim In?

Swimming water quality issues have been a recurring theme over the last couple of years, so now seems like a good time to go over some basics to help you be more aware of what you’re jumping into when starting a race. When it comes to triathlon open water swim quality, there are two basic areas to cover:

1) What’s in the water?
2) How do race organizations respond to what’s in the water?

So first, let’s go over the major players in terms of water quality for open water swimming:

E. coli
Bacteria. It will give you diarrhea. Or a UTI if you’re really unlucky.

Heavy metals
Usually thinking about things like mercury and lead. Obviously not “good” to swim in, but unless you’re swimming in a literal pool of mercury, a one-time exposure isn’t gonna do anything to you.

Chemical waste
Also obviously not “good” to swim in, but unless you’re swimming in a body of water contaminated by a known industrial polluter (a famous example is General Electric dumping PCBs into the Housatonic River in Pittsfield, MA), it’s not a scenario you have to think about all that much.

It’s obvious if you’re swimming in a body of water that recently had an oil spill. And only a psycho of a race director would hold a swim in water with an oil slick on top of it.

Large debris won’t give you any illnesses, but obviously isn’t fun to swim through. I’d expect any race to do a sweep for debris before the race.

Particulate matter
By “particulate matter”, I mean dirt and decomposed plant matter kicked up from the bottom of a river or lake. This is fairly common in triathlons and isn’t a health issue. It just makes visibility poor which can be annoying.

Fecal Coliform
Bacteria. It may not do much to you in and of itself, but it can be used as an indicator for the presence of other bacteria that will give you diarrhea.

Bacteria. It can give you UTIs, meningitis, and diarrhea. Usually not a huge concern in and of itself, but like fecal coliform, it is often measured to act as an indicator of the presence of other bacteria that will…. give you diarrhea.

Red Tide Algal bloom3
Huge bloom of ocean algae. Usually dinoflagellates. Can happen on any coast, but most common on the Gulf coast. Most algae won’t do anything to you, but some release toxins that you’d rather not swim through. They’re most likely to give you a small rash or diarrhea.

Bacteria. They’re the first organism on earth to produce oxygen, so they’re pretty great! But if you swim through a dense patch of them, they’ll give you...you guessed it, diarrhea.

For freshwater races, water quality will often decrease markedly after heavy rainfall, especially if the water is near agricultural land. But one positive on bacterial exposure is that the diarrhea won’t hit you until you’re done racing. So you got that going for you! (note: while these little bugs can cause more than GI distress and diarrhea, those are by far the most common effects)

Now, how do race organizations respond to all of those nasty things that can end up in the water?


Based on decision making processes used in prior events, it appears that Ironman defers to state and local authorities when determining water quality safety for events4. For reference: when setting water quality standards, states must set their limits to be at least as stringent as the EPA regulations. EPA guidance on the main bacterial offenders is shown below.

EPA’s recreational water quality criteria cover many more pollutants, but I’m guessing you don’t care that much and also don’t have 7 hours to read all of them.

World Triathlon

This is obviously more critical in 2024, given all the publicity surrounding the water quality of the Seine.

In 2010, World Triathlon adopted the EEC standards 2006/7/EC for bathing water quality, and ruled that swims should only be held in bodies of water deemed to be in the “excellent” category.

World Triathlon requires that venues measure and submit water quality testing data at least 4 times:
-when the venue site is announced (if this takes place at least 15 months before the first competition date)
- 12 months before the event
- Two months before the event 6
- 7 days before the competition.

More testing may be required if there are specific concerns for a body of water.

The exact WT standards are as follows:

Sea and Brackish waters (brackish waters are waters at the transition from fresh water to salt water)
- pH between 6 - 9
- Enterococci must be <100 cfu / 100 ml
- E. Coli must be <250 cfu / 100 ml
- Any visible amount of Red Tide Algal bloom is grounds for cancellation

Inland waters (lakes and rivers)
- pH between 6 - 9
- Enterococci must be <200 cfu / 100 ml
- E. Coli must be <500 cfu / 100 ml
- Blue-Green Algal blooms/scum (cyanobacteria) must be <100 cells/ml

However, there is some discretion involved. The WT Medical Committee may grant waivers if one of the above standards is violated but they deem the violation to not be a significant safety hazard.