San Gabriel River Regional Monitoring Program
  1. Swim Safety
Swim Safety

What is E. coli and why do we measure it?

Escherichia coli is a Gram-negative bacterium that is common to the lower intestine of warm blooded animals. While most E. coli strains are harmless, some can cause serious food poisoning, but that is not why we measure E. coli in recreational waters. Instead we use the presence E. coli to determine if fecal contamination has occurred. As a result, E. coli is many times referred to as a 'fecal indicator bacteria' or FIB. If E. coli are measured in significant amounts in a waterbody, there is a chance that pathogens (bacteria or virus that cause illness) are also present since they also live in the intestine of warm blooded animals.

As the concentrations of E. coli increase the chances are greater that you might contract a water borne illness, usually gastroenteritis.

How we determine if it’s safe to swim

In 2007 the SGRRMP bacteria monitoring program was established with weekly water sampling for fecal indicator (FIB) bacteria during the summer months (May to September) at high use recreational swimming areas. Until that time, little was known about the risk of becoming sick for people who swam in the streams and rivers of the San Gabriel River watershed despite the fact that thousands of people swim in these waters during the summer months.

Early in the program we learned that the majority of water samples collected on or near weekends and holidays had the highest concentrations of FIBs. This is associated with increased numbers of people that are swimming.

Below are the regulatory thresholds for E. coli. When concentrations exceed these standards, there is a human health risk for swimming.

Fecal Indicator Bacteria Single Sample 30 Day Geometric Mean
E. coli 235 MPN/100 mL 126 MPN/100 mL


The 30-day geometric mean standard is based on a set of 5 samples collected within a 30-day period. When the geometric mean is elevated it indicates that there have been persistently elevated bacteria concentrations.