What they actually test for
Beach programs do not test for every germ that could make you sick. That would be impossibly slow. Instead they measure a fecal indicator bacterium, one that is easy to count and reliably signals that fecal contamination, and the pathogens that ride along with it, is present. Which indicator depends on the water:
- Fresh water uses E. coli. In lakes, ponds, and rivers, E. coli is the best indicator that studies link to swimming-related illness.
- Marine water uses enterococci. In salt water, E. coli dies off too quickly to be reliable, so coastal beaches count enterococci instead. Enterococci also work in fresh water and some Great Lakes programs use them there too.
The thresholds
A beach is flagged when a sample exceeds a single-sample threshold, sometimes called a beach action value, set to keep the estimated illness rate acceptably low. The widely used values are:
| Water | Indicator | Single-sample threshold |
|---|---|---|
| Fresh | E. coli | 235 MPN/100 mL |
| Marine | Enterococci | ~104 CFU/100 mL |
MPN and CFU per 100 mL are two ways of counting bacteria in a fixed volume of water. The fresh-water figure of 235is the same E. coli limit SwimZone shows on a water page next to the tested value, so a "over limit" read means a recent sample came in above it. States can set stricter values or add a running geometric-mean limit over several samples, so exact numbers vary a little by program.
Why closures lag the conditions
Here is the catch. The standard lab method has to grow the bacteria to count them, and that culture takes about 18 to 24 hours. So the timeline of a typical advisory looks like this:
- 1A technician collects a water sample in the morning.
- 2The lab incubates it overnight, roughly a full day.
- 3The result comes back the next day, and only then is an advisory posted.
By the time the sign goes up, the water it describes is a day old. Conditions can swing hard in that gap: a beach can be posted as unsafe after the contamination has already flushed out, or, more dangerously, look clear while a fresh runoff spike has not yet been sampled. Sampling is also intermittent, often weekly, and blooms and plumes move along a shoreline with wind and current, so a single sample point does not describe the whole beach.
This lag is exactly why the daily read layers recent rainfall on top of the last official sample: a heavy rain since the sample is a reason to use caution even before the lab catches up. That overlay is described in swimming after rain and in the methodology. The underlying advisories come from EPA's BEACON beach-advisory database and state beach programs.