The data
Every grade is built from real public data only — no estimates, no synthetic values:
- Beach inventory & advisories come from the U.S. EPA BEACON program, which tracks the public beaches covered by the federal BEACH Act, their locations, and their advisory and closure history.
- Bacteria samples come from the Water Quality Portal (WQX/STORET), the shared federal repository where state and local agencies upload beach-monitoring results.
The indicator: fecal bacteria
Beach programs test for fecal-indicator bacteria — the signal that runoff or sewage may have reached the water and that the risk of gastrointestinal and other illness is elevated:
- Enterococcus at saltwater and Great Lakes beaches — Beach Action Value 104 CFU/100 mL.
- Escherichia coli (E. coli) at other freshwater beaches — Beach Action Value 235 CFU/100 mL.
The Beach Action Value (BAV) is the single-sample threshold at which water managers typically post a swimming advisory. A sample above it is an “exceedance.”
The Beach Safety Grade
For each beach we gather the monitoring stations within a few kilometers, pool their recent samples (multiple years), and compute the exceedance rate — the share of samples that came back above the Beach Action Value. That rate maps to a letter grade:
| Grade | Samples over the safe-swimming limit |
|---|---|
| A | Under 3% — almost never |
| B | 3–7% — rarely |
| C | 7–12% — occasionally |
| D | 12–25% — frequently |
| F | Over 25% — very often |
We only assign a letter grade when a beach has at least 20 samples spanning two or more years. Beaches with fewer samples are marked “not yet rated” rather than guessed at — we never treat missing data as a passing or failing grade. Grades based on a thin sample set are flagged as limited.
Grade vs. current advisory
The grade is a multi-year track record. A current advisory reflects today’s water. A well-graded beach can still be under a temporary advisory after a storm, because bacteria spike when rain flushes runoff into the water. We show both, and we always link to the official state or county advisory — check it the day you swim.
Limitations
- Monitoring is seasonal and uneven. Some beaches are sampled weekly; others rarely. Grades reflect what’s been reported.
- A monitoring station a short distance away is a good proxy for a beach but not a perfect one.
- Bacteria are an indicator, not a complete measure of safety — rip currents, algae, and chemical hazards are separate concerns.
- BeachGrade is informational and is not a substitute for official public-health guidance.
Data last updated 2026-07-13. Questions or a correction? Contact us.