Cesspools, sewage, agricultural runoff, and urban stormwater explained
Hawaiʻi’s beaches are world-famous, but the water quality at many of them is affected by preventable pollution sources. Understanding where beach contamination comes from helps you make better decisions about when and where to swim.
Hawaiʻi has approximately 88,000 cesspools — the most per capita of any U.S. state. A cesspool is essentially an unlined pit that receives raw, untreated sewage from homes and businesses. Unlike septic systems (which provide some treatment), cesspools simply allow sewage to seep into surrounding soil and groundwater.
These cesspools collectively release an estimated 53 million gallons of untreated sewage per day into the ground. In Hawaiʻi’s porous volcanic rock, this sewage can travel quickly through underground channels to reach nearshore waters, carrying bacteria, viruses, pharmaceuticals, and nutrients.
Areas most affected include older residential neighborhoods across all islands. The Big Island has the highest concentration, with many rural communities still relying entirely on cesspools. Hawaiʻi passed Act 125 in 2017 mandating conversion of all cesspools by 2050, but progress has been slow due to the enormous cost.
What this means for swimmers: Beaches near older residential areas with cesspool infrastructure may have chronically elevated bacteria levels, even on dry days, due to continuous groundwater contamination. Our page on Hawaiʻi cesspool contamination goes deeper into this issue.
Beyond cesspools, Hawaiʻi’s municipal sewage collection and treatment systems experience regular failures. Aging sewer lines crack and leak, pump stations malfunction, and treatment plants can overflow during heavy rain events.
Honolulu’s sewage system has been particularly problematic. The city has been under a federal consent decree since 2010 to upgrade its aging infrastructure after repeated large-scale sewage spills into waterways and the ocean. The Ala Wai Canal, which runs along Waikīkī, is a chronic hotspot where sewage overflows during heavy rain.
Sewage spill events can release tens of thousands to millions of gallons of raw sewage. DOH typically issues advisories after reported spills, but detection and reporting are not instantaneous. Our sewage spill guide covers this in more detail.
Hawaiʻi’s agricultural operations contribute multiple types of contamination to nearshore waters:
Cattle ranching (particularly on the Big Island’s Parker Ranch and Maui’s upcountry) produces animal waste containing high levels of bacteria. Rain washes this waste into streams that flow to the coast.
Chemical and organic fertilizers add excess nitrogen and phosphorus to waterways. These nutrients fuel algae blooms in nearshore waters, which can deplete oxygen and harm marine ecosystems.
Exposed agricultural land without cover crops erodes during rain, sending Hawaiʻi’s distinctive red dirt into streams and the ocean. Sediment smothers coral reefs and reduces water clarity.
In urban areas like Honolulu, Kailua, and Kahului, impervious surfaces (roads, parking lots, buildings) prevent rain from soaking into the ground. Instead, rainwater flows across these surfaces picking up contaminants:
This contaminated water flows through storm drains directly into the ocean, often with no treatment whatsoever. Storm drain outfalls are visible at many beaches, particularly in urban areas. The concentration of pollutants is highest during the first flush of rain after a dry spell.
Hawaiʻi’s streams are the primary pathway for land-based pollution to reach the ocean. During rain events, streams collect contaminants from their entire watershed — everything from mountain forests (wild animal waste, decomposing vegetation) to developed lowlands (sewage, chemicals, trash).
Beaches located near stream mouths consistently show higher bacteria levels than those without nearby freshwater input. This is why the 72-hour rain rule is so important, and why beaches on dry, streamless coastlines (like the Kohala Coast or South Maui) tend to have the cleanest water.
Feral animals — pigs, goats, and chickens — also contribute significant amounts of fecal bacteria to Hawaiʻi’s streams. This is a particularly large factor on Kauaʻi (feral chickens), the Big Island (feral pigs and cattle), and in Oʻahu’s mountain watersheds.
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⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent passion project — not affiliated with the Hawaii Department of Health or any government agency. Information presented here is for educational purposes based on publicly available data.
Always verify current water quality conditions with the Hawaii DOH Clean Water Branch before entering the water.
When in doubt, don't go out. 🤙