Cesspools increase contamination risks for areas hit hard by Kona lows

Sunday, April 19, 2026

By Chloe Jones cjones@staradvertiser.com

Today • Last updated 5:23 p.m.

Brown water was visible at Waimea Bay on Oahu’s North Shore in the aftermath of last month’s Kona-low storms. Following excessive rainfall, the state Department of Health issued a brown water advisory for the entire island of Oahu and advised the public to stay out of nearshore waters due to potentially harmful pathogens.

The state Department of Health gives the same warning to beachgoers each time the islands are hit with a heavy storm: Don’t swim in brown water.

The back-to-back “Kona lows” that inundated Hawaii in March and a third low-pressure system earlier this month were no different.

As of Saturday, the Health Department had issued 36 separate brown-water advisories statewide since March 11 — when the first of the trio of storms hit the island chain — and all of Oahu, Kauai and parts of Maui remained under advisories.

High bacteria counts also were recorded at five locations on Oahu, with four remaining active at Puaena Point, Haleiwa Beach Park, Mokuleia at Kiapoko Point, and Kawaihapai 1. Also late Friday, the Waialua Water Association issued a boil-water notice for customers served by the Waialua Sugar Pump 2 water system Opens in a new tab after tests detected E. coli bacteria, an indication the water may be contaminated with human or animal waste.

Heavy rain was listed as the cause of two ongoing sewage spills, one at the Waimea Wastewater Treatment Plant on Kauai and the other at the Wahiawa Wastewater Treatment Plant on Oahu, while an unpermitted wastewater discharge from the Kaneohe Marine Corps Base Hawaii wastewater treatment plant, which did not offer a cause for the spill, also remained active Saturday.

Most of the areas that received advisories during the storm — and those that remain under one — were among the hardest-hit regions during the recent string of intense storms that soaked the state, but they also share another factor: They are all in or near high-priority cesspool zones.

The Department of Health has identified 43,000 of 88,000 cesspools as posing a risk to water resources Opens in a new tab, with a total of 53 million gallons of raw sewage released each day. Several experts told the Honolulu Star-Advertiser that heavy rain flushes even more raw sewage out of cesspools, increasing bacteria levels and environmental risks.

The University of Hawaii Sea Grant program created the Hawaii Cesspool Prioritization tool Opens in a new tab in 2021 to classify Hawaii’s cesspools by their impacts on water quality and risk to human and environmental health, according to Christopher Shuler, faculty member of the university’s Water Resources Research Center. He said cesspools release untreated wastewater, which releases both nutrients and bacteria into the environment.

“Those are the two main things we care about, that we don’t want,” Shuler said. “Nutrients are bad for the reef. Bacteria is bad for people.”

He said the map can inform DOH and lawmakers to target cesspool conversions to have more impact. Factors that increase a cesspool’s risk include its proximity to the coastline or other waterways, the depth of the groundwater table and height from sea level, which are also factors considered to increase flooding risks.

“There’s a very natural kind of co-occurrence of where flooding is going to impact and where you would see the impacts from cesspools based on all those factors,” Shuler said.

On Oahu, the entire North Shore is considered to have high-priority cesspools, with heavy concentrations in Waialua, Haleiwa, Mokuleia, Pupukea and Laie — all areas that were hit with heavy flooding during the recent storms. Waimanalo in Windward Oahu, which was also hit particularly hard, has a dense concentration of high-priority cesspools, as do parts of Makaha and Waianae in West Oahu.

Maui County faced similar flooding in its high-priority cesspool zones, with heavy flooding from the central to west areas on Molokai’s south shore and in South and West Maui.

DOH said in an email to the Star-Advertiser that “it’s reasonable to think the water quality in areas with more cesspools or cesspools nearer to shore will be most greatly impacted” by heavy rainfall, “but the biggest impact on water quality and storm runoff is simply the volume of rainfall and the extent of flooding.”

The agency said flooding overflows all wastewater systems, regardless if they are cesspools or centralized collection systems, adding that runoff also carries mud, debris and other pollution. Cesspools and sewer lines both can be overwhelmed by groundwater during storms, which then leeches raw sewage into the environment, while treatment plants may become overfilled and require discharges of sewage into the ocean.

Heavy rain also can cause cesspools to back up, pushing raw sewage to the surface.

A risk to reefs

According to a DOH news release April 8, samples from floodwaters and mud in Waialua and Central Oahu, where North Shore floodwaters drained, following the two Kona-low storms, confirmed the presence of several pathogens commonly found after storms due to runoff and wastewater contamination, including E. coli, staphylococcus aureua and salmonella bacteria.

Shellie Habel, a researcher in UH’s School of Ocean and Earth Science and Technology, said cesspools are essentially cinder-block-lined holes in the ground where raw sewage is pumped. Fifteen feet of space between the base of the cesspool and the groundwater table is required to be safe and effective, but in coastal zones that distance is much tighter.

Habel said she’s seen cases where residents can see the level go up and down in their cesspools with ocean tides, even in sunny conditions. Essentially any time the groundwater table is high and comes in contact with a cesspool, it picks up sewage and transfers it directly to the ocean.

This creates a “turbo-charge” of raw sewage flow during heavy rainstorms, said Greg Asner, director of the Center for Global Discovery and Conservation Science at Arizona State University, who is based on Hawaii island. He added that other products in human waste, such as pharmaceuticals, can be “very detrimental to coral reproduction, metabolism and growth.”

Phosphate and nitrogen are key nutrients that can be expected at elevated levels after heavy storms, said Nyssa Silbiger, associate director of the Uehiro Center for the Advancement of Oceanography at UH. Phosphate can be found in fertilizer and sewage, while nitrogen in the form of ammonium is a byproduct of human waste, and nitrate, another form of nitrogen, is indicative of fertilizer, she explained.

Nitrogen helps algae grow, and while reefs need it to survive, excess amounts cause algae overgrowth and bring the ecosystem out of balance, Silbiger said. Invasive algae, especially, thrives when excess nitrogen enters the reef. Coral then competes with algae for space, she said, and some algae release chemicals that can be toxic to coral.

UH assistant professor Andrea Kealoha added that increased algae also can increase “bio-erosion” by attracting more organisms that eat and weaken reefs, making them more susceptible to erosion from waves or currents.

General storm runoff is also an issue, Silbiger said. Mud and sediment can block the sun, interfering with corals’ ability to photosynthesize, while sudden surges of freshwater can shock coral and cause bleaching or death.

These effects are exacerbated even more by rising ocean temperatures.

“We have studies that have shown that areas with higher sediment loads and higher nutrient loads, those corals are more sensitive to temperature,” Silbiger said. “When we have issues that are happening with coral bleaching because of the warming of our oceans, if we also in addition to that have issues with runoff and nutrient loading and sedimentation, it’s like the reefs getting a double whammy.”

Costly conversions

Silbiger, Kealoha and early career research fellow Sara Kahanamoku teamed up for one of the first studies on how acute storm events impact coral reefs in Hawaii.

They gathered 769 seawater samples from across the state after the March storms to test for salinity, nitrates, ammonium, phosphate and two compounds indicative of sediment known as silicate and fluorescent dissolved organic matter. Test results are posted periodically on the project’s website as results return in batches, though it is too early to make any conclusions.

As of Saturday, 135 samples returned salinity results and 87 returned nutrient results.

One of the questions the team wants to answer is whether these high-priority cesspool zones receive acutely elevated nutrient loads, Kealoha said. They also hope to identify areas that have more trouble flushing out pollutants and are more vulnerable to storm effects.

Stuart Coleman, executive director of the nonprofit Wastewater Alternatives and Innovations, said he hopes the severe flooding from the recent storms and its impacts push policymakers to prioritize cesspool conversions. He noted that the current conversion rate — a few hundred per year — is nowhere near close enough to meet the 2050 deadline to convert more than 83,000 cesspools statewide as required by Act 125 in 2017.

“These storms revealed the urgency of the situation,” Coleman said. “We cannot wait any longer.”

He said it’s important to consider innovative approaches to what wastewater treatment could look like, which may be smaller neighborhood systems or septic tanks that can remove nitrogen.

Septic tanks can work in some areas, he said, but they can still be inundated with groundwater during storms and leak sewage if conditions permit.

The biggest hurdle in converting cesspools is the high cost to homeowners. Coleman said conversions can range from $25,000 to $45,000, with certain properties presenting more challenges for engineers and higher prices for homeowners.

House Bill 1618, which would create a cesspool loan fund and financing program to help make conversions more affordable, was kicked back to the Senate Friday after House lawmakers disagreed with amendments made by the Senate. Before the bill passed its third reading Tuesday, state Sen. Mike Gabbard (D-Kapolei-Makakilo-Kalaeloa) emphasized the state needed to take action.

“These cesspools spill approximately 53 million gallons of untreated sewage into our groundwater and ocean every day,” he said. “It comes down to this: We can set deadlines all we want, but if we don’t help people get there, it’s not going to happen.”