Hawaii Beach Bacteria Guide

What Bacteria Lurks in Hawaiʻi's Ocean Water

The primary bacteria of concern at Hawaii beaches is enterococcus, a type of fecal indicator bacteria. Enterococcus lives in the intestines of warm-blooded animals — humans and animals alike. Its presence in ocean water signals that fecal contamination has entered the water, which can bring along pathogens like norovirus, E. coli, Salmonella, and other harmful organisms.

The Hawaii Department of Health (DOH) also historically tested for fecal coliform, another fecal indicator, but has shifted primarily to enterococcus as the EPA-recommended standard for marine recreational waters.

Enterococcus itself isn't always the direct cause of illness — it's a marker. When counts are high, it means contamination is present and the risk of encountering harmful pathogens is elevated. DOH uses an action level of 130 CFU per 100 mL for a single sample, or a geometric mean of 35 CFU per 100 mL over 30 days. Exceeding either threshold triggers a Beach Advisory.

The #1 Cause: Rain Runoff

When it rains in Hawaiʻi, water washes across the land and carries with it everything in its path: animal feces from birds, pigs, and cattle; organic debris from forests and farms; soil bacteria; and waste from cesspools and septic systems that sit close to the surface. This contaminated runoff flows into streams, storm drains, and ditches — all of which eventually empty into the ocean.

The effect on bacteria counts is dramatic and rapid. After even moderate rainfall, bacteria levels at many beaches can spike by 10x or more within hours. The closer a beach is to a stream mouth, storm drain outfall, or agricultural land, the worse this effect tends to be.

Other common causes of elevated bacteria at Hawaiʻi beaches include:

• Cesspool seepage — Hawaiʻi has more cesspools per capita than almost any other U.S. state. When cesspools are near the coast or sit in high-water-table areas, effluent can seep into groundwater and reach the ocean.
• Sewage spills and overflows — aging sewer infrastructure occasionally fails, sending raw sewage into streets, streams, and the ocean. These events trigger immediate closures.
• Stream discharge — perennial streams carry bacteria year-round, especially those flowing through populated or agricultural areas. The mouths of streams are almost always higher-risk zones.
• Wildlife feces — large concentrations of seabirds, monk seals, and other wildlife can locally elevate enterococcus counts at otherwise clean beaches.
• Poor water circulation — enclosed bays, lagoons, and areas with restricted tidal flushing trap bacteria longer than open-ocean beaches where wave action dilutes and disperses contamination quickly.

How Hawaiʻi Tests Its Beaches

The DOH Clean Water Branch runs the Beach Monitoring Program, which collects water samples at established monitoring stations across Oʻahu, Maui, the Big Island, and Kauaʻi. Field staff wade into the surf zone and collect water samples at ankle to knee depth, where swimmers are most often exposed.

Samples are transported to the DOH laboratory and analyzed using the EPA-approved Enterolert method, which uses a fluorogenic substrate to detect and quantify enterococcus bacteria. Results are typically available within 24 to 48 hours after collection.

Monitoring frequency varies: some high-use beaches like Waikiki are tested weekly. Others are tested monthly, or only in response to complaints or visible contamination. After significant rainfall events, DOH may conduct targeted testing of beaches known to be affected by runoff.

What the Numbers Mean

Beach water quality is measured in Colony Forming Units (CFU) per 100 mL of water. Here's how to interpret the key thresholds used by DOH:

These thresholds represent a statistical risk level, not a bright line between fine and dangerous. The action levels are set to protect the average healthy adult — children, elderly individuals, and immunocompromised people face higher risk at lower counts. When in doubt, don't go out.

How to Protect Yourself

• Check for advisories before you go. Visit the DOH Clean Water Branch website or Safe to Swim Hawaii for current advisories before heading to the beach.
• Always look at the water. Brown, murky, or discolored water means runoff is present. Do not enter regardless of whether an advisory is posted.
• Wait 72 hours after rain. This applies everywhere in Hawaiʻi, especially near stream mouths, storm drains, or areas with visible runoff staining.
• Avoid stream mouths and drain outfalls. These are the highest-risk zones at any beach. Stay well away from where streams or drains enter the ocean, even on clear-weather days.
• Don't swallow ocean water. This sounds obvious but is especially important for children and snorkelers.
• Rinse off after swimming. Shower promptly after leaving the water, especially if water quality is questionable.
• Cover open wounds. Bacteria can enter through cuts, abrasions, or piercings. If you have open wounds, skip the ocean.
• Be extra cautious with children and elderly family members. They are more susceptible to waterborne illness at lower contamination levels.

What to Do If You Get Sick

If you develop symptoms of waterborne illness after swimming in Hawaiʻi — nausea, vomiting, diarrhea, stomach cramps, ear pain, eye irritation, skin rash, or sore throat — take these steps:

• Contact a healthcare provider. Mention that you were swimming in the ocean and describe when and where. This information helps with diagnosis.
• Stay hydrated. Most waterborne GI illnesses resolve on their own with rest and fluids, but dehydration can become serious.
• Do not self-diagnose or ignore symptoms. Some infections can be serious if untreated. If you swam in streams, rivers, or inland waters, mention this specifically to your doctor.
• Report it. You can report suspected waterborne illness to the Hawaii Department of Health at (808) 586-4249. Reports help DOH target their testing efforts.