Bacteriological Monitoring 101: When is a Beach Closed to Swimming?
Routine Weekly Sampling
When a sample is taken at one of Cape Cod’s beaches, the water is brought back to the laboratory and tested for the presence of the indicator organism Enterococci (if the sample is salt/marine water) or E. coli (if the sample is fresh water). It takes 24 hours to determine if Enterococci or E. coli are present in the sample.
The maximum allowable number of Enterococci colony forming units per 100ml sample are 104. The maximum allowable number of E. coli colony forming units per 100ml sample are 235. If a single sample exceeds the limit for Enterococci or E. coli, we say that the beach “failed” or that an “exceedance” has occurred, and our samplers are sent to retrieve a re-sample. If the results of re-sample are below the allowable bacteriological standard, the beach remains open, no notifications are made, and routine testing resumes the following week.
Re-Testing and Conditions for Closure
If, after 24 hours, it is determined that the results of the re-sample are higher than the maximum allowable number of colony forming units described above, the health agent in the town where the beach is located is notified and the beach must be closed to swimming within 24 hours due to the occurrence of two consecutive exceedances.
In some cases, if a beach has experienced more than two consecutive exceedances (both the initial sample and re-sample exceeded the limits for bacteria in bathing water) within the last four bathing seasons, that beach is placed on a list of beaches that must be posted closed to swimming after only one exceedance. In other words, due to historical data showing that a beach is known to have consecutive exceedances, we don’t wait for the results of the re-sample; we notify the health agent that the beach must be closed within 24 hours (you can read a more detailed explanation for this below).
There is More than One Type of Beach Closure!
Under the Massachusetts bathing beach regulations, there are two ways a beach can be considered unsafe based on bacteria levels–two consecutive occurrences of a single count exceedance (described above) and geometric mean exceedance.
A single count exceedance is when one sample comes back with bacteria levels above the limit, likely from a sudden/transient spike in pollution from bird/animal waste or stormwater runoff. A single count exceedance triggers re-testing but not closure (with the exception of certain beaches that have a history of repeated exceedances; see explanation above). Two consecutive exceedances triggers a beach closure.
In contrast, the geometric mean is a running average of the most recent 5 samples in the bathing season. For marine beaches the geometric mean must not exceed 35 Enterococci CFU/100 ml, and for fresh water ponds the geometric mean must not exceed 126 E. coli CFU/100 ml. When these standards are exceeded, the beach is closed until re-testing shows the geometric mean to be within acceptable parameters.
Geometric Mean Explained
When beach water is tested for bacteria like enterococci or E. coli, we don’t just look at a single day’s result to decide if the beach is safe. Instead, we look at five days of test results to get a better picture of overall water quality. But instead of using a regular average (or arithmetic mean), the Massachusetts regulations use something called the geometric mean; a special kind of average that’s better at representing trends in numbers that are variable and change from day to day.
The geometric mean gives us a better sense of longer-term water quality trends that are less indicative of a transient issue and more indicative of an extended event.
Need more explanation? Feel free to reach out to us at [email protected]!
You can also visit our FAQs on our Bathing Beach Water Quality page.
