A grade is a summary of years of samples, not a same-day reading
A BeachGrade Beach Safety Grade answers one question: across all the bacteria samples a beach has on record, how often did the water exceed the safe-swimming threshold? A beach where samples almost never cross the line earns an A; one where they cross it a large share of the time earns an F. Of the 8,447 U.S. coastal and Great Lakes beach segments in our inventory, 3,000 had enough monitoring history to receive a grade — 1,136 A, 598 B, 428 C, 486 D, and 352 F.
This is deliberately different from the single "is there an advisory right now?" flag that beach programs post day to day. A same-day advisory tells you about today; a grade tells you about the pattern. A beach can be advisory-free this morning and still carry a D grade because it fails routinely after rain — and a beach can have one bad sample this week yet hold an A because its long record is clean. Both signals are useful, and BeachGrade shows the current advisory status alongside the grade so you can read them together.
Why enterococci and E. coli are the numbers on the report card
Testing beach water for every possible pathogen would be slow and impractical, so regulators measure "indicator" bacteria instead. Under its 2012 Recreational Water Quality Criteria, the EPA recommends enterococci as the indicator for marine and Great Lakes waters and E. coli for fresh water, because laboratory and epidemiological studies link their concentrations to the rate of gastrointestinal illness among swimmers. When indicator counts rise, the odds that disease-causing organisms are present rise with them.
The thresholds BeachGrade uses — enterococci at 104 CFU (colony-forming units) per 100 mL for marine water and E. coli at 235 CFU/100 mL for fresh water — are the EPA single-sample maximum values for designated bathing beaches. State and county beach programs widely adopt these values as the trigger for a beach notification. When a routine sample comes back above the applicable value, that sample counts as an exceedance in our grade and, in the real world, is the kind of result that prompts an advisory.
From a water sample to a posted advisory
The chain is short but has a built-in lag. A technician collects water at ankle-to-knee depth, the lab incubates it, and a count comes back — historically in about 18 to 24 hours for culture methods, though many programs now use rapid qPCR methods that return results the same day. If the count is over the threshold, the managing agency issues an advisory (a recommendation not to swim) or, in more serious cases, a closure (swimming prohibited).
Because of that testing lag, an advisory posted today often reflects yesterday's water. That is one reason a multi-year grade is valuable: it captures how a beach behaves over hundreds of samples and many conditions, rather than a single lab result that may already be stale. It is also why no grade can be read as a guarantee. BeachGrade classifies measured history; it does not clear the water for swimming on any given day.
How to read a BeachGrade page
On each beach page, the grade sits next to the exceedance rate — the percentage of samples that came back over the limit — plus the number of samples, the years they span, and how many monitoring stations feed the beach. A grade built on a few dozen samples over two seasons is less certain than one built on hundreds over a decade, so beaches with a thin record carry a "limited data" flag. Treat those grades as provisional.
The safest way to use the site is as a planning tool, not a green light. Use the grade to compare beaches and spot the ones that struggle, then check the official state or county advisory for the day you actually plan to swim. Conditions can change within hours, especially after heavy rain, and the current advisory — not the historical grade — is the signal that reflects today.