Are Hawaiʻi beaches getting cleaner or more contaminated? The answer depends on where you look. Cesspool conversion is slowly helping the worst beaches, but climate change is making storm events worse. Here is what the data shows.
The single most predictable pattern in Hawaiʻi water quality is seasonal. Wet season (October through March) produces 3–4 times more advisories than dry season (April through September). This is driven entirely by rainfall — more rain means more runoff, more cesspool overflow, and more bacteria washing into the ocean.
This seasonal pattern holds across all four major islands, though the magnitude varies by location. Leeward (south/west) coasts see less seasonal variation than windward (north/east) coasts because they receive less rainfall year-round.
Oʻahu consistently has the highest advisory count of any island. The combination of urban density, the Ala Wai Canal system, aging wastewater infrastructure, and high cesspool concentration in residential areas creates chronic contamination sources. The North Shore receives heavy winter rain and has multiple stream-mouth beaches. South shore (Waikiki east end) and west side (Ko Olina) generally test clean.
South Maui (Wailea, Kīhei) has consistently excellent water quality — it is the driest coast in the state. West Maui has seen increased attention since the 2023 Lahaina wildfire, with concerns about increased runoff from exposed soil, contaminated debris, and damaged wastewater infrastructure affecting nearshore water quality.
The Kohala Coast on the Big Island’s west side is the cleanest resort area in Hawaiʻi. Hapuna Beach, Mauna Kea Beach, and Mauna Lani Bay have the lowest bacteria levels of any monitored beaches in the state. The dry climate, lack of streams, and modern wastewater systems keep contamination minimal. The exception is Kahaluʻu Beach Park in Kona (92% bacteria failure rate), where confirmed cesspool wastewater reaches the shoreline.
Kauaʻi has both some of the cleanest and most contaminated beaches in the state. Poipu on the dry south shore tests excellently. Hanalei Bay on the wet north shore has been listed as impaired by DOH since 2004, with four river discharges and 360+ cesspools in the watershed. Kalapaki Beach near Līhuʻe has chronic contamination at the stream mouth.
Hawaiʻi has approximately 88,000 cesspools — more than any other U.S. state. These unlined underground pits receive raw household sewage and let it seep into the ground without treatment. An estimated 53 million gallons of untreated sewage enters the ground every day statewide.
The state legislature passed Act 125 in 2017, setting a 2050 deadline for all cesspools to be converted to septic systems or connected to sewer lines. Progress has been slow. The cost per property ranges from $20,000 to $60,000 depending on location and soil conditions. State and federal grants cover a portion for qualifying homeowners, but the financial burden remains significant.
The impact of cesspool conversion is already visible at some locations. Areas where early conversions have occurred show measurably lower baseline bacteria levels. But at the current rate of conversion, meeting the 2050 deadline for all 88,000 properties is uncertain.
The March 2026 Kona Low was one of the largest storm events to hit Hawaiʻi in recent years. It triggered simultaneous Brown Water Advisories on multiple islands — a relatively rare occurrence. USGS stream gauges recorded discharge at 10–20x baseline across windward stations statewide. The event highlighted the vulnerability of even typically clean beaches during extreme weather, and the limitations of the DOH advisory system (brown water advisories are visual-only, and routine testing was paused during the event).
The August 2023 Lahaina wildfire destroyed thousands of structures and burned vegetation across West Maui. The environmental aftermath continues to affect water quality: exposed soil without vegetation increases erosion and runoff during rain; damaged wastewater infrastructure in the burn zone led to sewage spills; contaminated debris and heavy metals in soil may leach into coastal waters during storms. Monitoring by Hui O Ka Wai Ola and Surfrider has documented elevated turbidity and nutrient levels in West Maui coastal water after rainstorms.
On the positive side, the City & County of Honolulu has proposed 10 new stormwater monitoring programs in their FY27 budget. This would expand water quality data collection in urban areas of Oʻahu, providing better tracking of contamination sources in streams and storm drains before they reach the coast. More data means better early warning capabilities.
Climate models project that Hawaiʻi will experience more intense rainfall events, even if total annual precipitation does not change dramatically. The implication for water quality is significant: it is not the total rain that matters most, but the intensity. A 3-inch storm in 6 hours produces far more contamination than 3 inches spread over a week.
More intense storms = larger contamination spikes. Runoff volume is a function of rainfall rate, not just total amount. Intense rainfall overwhelms natural drainage, overflows cesspools faster, and produces higher-velocity stream flow that carries more sediment and bacteria to the coast.
Sea level rise threatens infrastructure. Rising seas increase the risk of coastal flooding at wastewater treatment plants, pump stations, and sewer lines. Saltwater intrusion into the groundwater system can also push cesspool wastewater toward the surface more rapidly.
Warmer ocean temperatures. Warmer water allows enterococcus and other bacteria to survive longer in the nearshore zone, potentially extending the contamination window after storm events.
Water quality in Hawaiʻi is likely to remain variable for decades. Cesspool conversion will slowly reduce chronic contamination at the worst beaches. But climate change is producing more extreme storm events that cause larger contamination spikes. The net result: baseline conditions may improve at specific locations, but the frequency and severity of episodic contamination events is likely to increase. Checking conditions before you swim will remain essential.
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⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent project — not affiliated with the Hawaii Department of Health or any government agency. Trend analysis is based on publicly available advisory data and may not capture all events. Always verify current conditions with the Hawaii DOH Clean Water Branch.
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