Is Hawaiʻi’s ocean full of sewage?

Hawaiʻi is in the middle of a massive effort to eliminate cesspools. At first, that sounds like an infrastructure story. Then you learn that Hawaiʻi has about 83,000 cesspools — roughly 48,600 of them on Hawaiʻi Island — and that the island’s cesspools alone release an estimated 29 million gallons of untreated wastewater into the ground every day.

cesspools statewide (DOH, 2022)

83,000

cesspools statewide (DOH, 2022)

of them on Hawaiʻi Island

48,600

of them on Hawaiʻi Island

gallons of untreated wastewater a day, Hawaiʻi Island alone

29 million

gallons of untreated wastewater a day, Hawaiʻi Island alone

That changes the question. Are people swimming in sewage? Can you get an eye infection, a skin rash, or a stomach bug from going to the beach? And if the problem is serious enough that the State is requiring every cesspool to go, why don’t more people talk about it?

Sewage, a hole in the ground, and gravity

A cesspool is one of the simplest ways to dispose of household sewage. Wastewater from toilets, showers, sinks, and washing machines flows into a pit underground and then seeps into the surrounding soil and rock. There is very little treatment. Compared with a modern septic system — where solids settle in a tank and wastewater passes through a designed treatment area before entering the ground — a cesspool is essentially sewage, a hole in the ground, and gravity.

On Hawaiʻi Island, the geology makes this more serious. Much of the island is made of relatively young volcanic rock. That rock is porous, fractured, and full of underground pathways. Instead of wastewater slowly filtering through thick layers of soil over long periods, water can move through cracks and lava formations surprisingly quickly.

Researchers have measured just how quickly. At Puakō, University of Hawaiʻi at Hilo scientists put dye into home wastewater systems and watched for it at the shore. It came out of shoreline springs in as little as six hours — flush at high tide, and it can be in the ocean by low tide. Homes farther up the slope took up to ten days. In Keaukaha, near Hilo, dye from cesspools reached the shoreline within 20 hours to three days, moving one to two football fields a day.

That is the part of the cesspool story that deserves much more attention. The pathway is shockingly direct, and in some places the trip happens far faster than most people imagine.

So are the beaches contaminated?

Sometimes, yes. That does not mean Hawaiʻi’s beaches are generally unsafe, or that everyone swimming in the ocean is being exposed to dangerous concentrations of sewage. The ocean is enormous. Currents dilute contaminants. Sunlight kills microorganisms. Saltwater is hostile to many pathogens. Time and biology reduce contamination further.

But not every coastline has the same conditions. A deep beach exposed to strong open-ocean circulation is very different from a shallow, protected bay receiving groundwater directly from a nearby neighborhood full of cesspools. At Puakō, bacteria in the water in front of 81 percent of the homes sampled were above the State’s standards for swimming. That distinction — open ocean versus sheltered shoreline next to homes — matters.

According to the CDC, the most common illnesses from swimming in contaminated water are diarrhea, skin rashes, swimmer’s ear, flu-like illness, and eye or lung irritation. If you swallow water carrying human waste, stomach cramps, nausea, and vomiting are possible too. So yes, it is biologically possible to get a rash or an irritated eye after swimming in water with sewage in it.

That is very different from saying a swim at a Hawaiʻi beach means you are likely to get sick. For most people, at a clean beach in normal conditions, the most likely outcome is exactly what you’d expect. You swim, you get out, and nothing happens.

The risk rises when several things overlap: dense housing with lots of cesspools, shallow groundwater, freshwater springs entering the ocean, poor circulation, recent heavy rain, stream mouths, and brown water. Those conditions are very different from a big open beach on a sunny, dry day.

The bigger problem is the reef

When people hear the word cesspool, they naturally think of fecal bacteria. But pathogens are only part of the story. Wastewater also carries nutrients — especially nitrogen and phosphorus — and those move through groundwater too. When they reach the ocean, they feed algae. Algae compete with coral. Over time, that can wear down a reef and change a whole nearshore ecosystem.

At Puakō, the State’s Division of Aquatic Resources found coral cover down 35 percent and algae up 38 percent over 30 years. Bacteria there were elevated whether a home used a cesspool, a septic tank, or an aerobic unit — which is why researchers concluded that simply swapping each cesspool for an ordinary individual system might not fully solve the problem there, and recommended a community treatment plant. The geology itself is the challenge.

The same pattern shows up elsewhere. In Kona, UH Hilo researchers traced cesspool sewage from homes near Aliʻi Drive into Kahaluʻu Bay within about one tide cycle. In Keaukaha, every dye test turned up at the shoreline, and the study confirmed wastewater reaching nearshore waters and coral reefs. Concentrations vary enormously. Dilution matters. Location matters. Rain matters. But the connection between household wastewater and coastal water is real.

In some communities, the distance between a toilet and the ocean is much shorter than anyone should be comfortable with.

Why the State is spending serious money on this

Because this isn’t a small plumbing problem. It is infrastructure spread across tens of thousands of properties on several islands. State law requires every cesspool in Hawaiʻi to be upgraded, converted, or connected to sewer by 2050.

That means homeowners will need a modern septic system, an advanced treatment unit, a shared community system, or a sewer connection. Those projects commonly cost tens of thousands of dollars per property — Hawaiʻi County estimates $30,000 to $70,000 for an on-site conversion. Multiply that across tens of thousands of homes and the scale comes into focus. It’s why the Legislature created a new low-interest loan fund in 2026, which the Hawaiʻi Green Infrastructure Authority is still setting up.

None of this is because of a secret epidemic of swimmers getting sick. It’s because Hawaiʻi inherited a wastewater system built on technology that no longer makes sense in many of these places.

Should you still swim?

Absolutely. Just pay attention. Stay out when there’s a brown-water advisory — the Department of Health issues them after heavy rain, when runoff can carry overflowing cesspools and sewage into the ocean. Be cautious right after a storm. Avoid stream mouths when runoff is flowing, and think twice about swimming near drainage outlets. Try not to swallow the water. And if you have an open cut or wound, the CDC’s advice is to stay out.

The key isn’t fear. It’s knowing that the ocean isn’t one uniform body of water. Water quality can be excellent at one beach and questionable a few miles away, and it can change within hours.

The most striking thing about Hawaiʻi’s cesspool problem is that almost all of it happens out of sight. You can’t stand on the shoreline and watch wastewater moving beneath you. You can’t see nitrogen traveling through lava, or bacteria riding the groundwater. You can’t tell by looking at clear blue water that the spring bubbling up under it carries a neighborhood’s sewage.

But scientists can trace it. And increasingly, they are.

That doesn’t mean Hawaiʻi is surrounded by an ocean of sewage. It means that on an island, nearly everything eventually moves downhill.

And downhill usually means the sea.

Curious where your own property’s wastewater goes? Check My Property shows how far your cesspool is from the ocean, how deep the groundwater is, and how close the nearest drinking-water wells are — free, in about a minute.