The water quality of Hilo Bay will only get worse as climate change intensifies, according to a recent study by University of Hawaii researchers.

The study, published earlier this year in the journal Water Environment Research, found that sources of freshwater discharge into Hilo Bay coincide with high levels of harmful bacteria, and that those levels are likely to increase as sea levels rise and extreme weather events become more common.

According to the study, certain parts of Hilo Bay contain pathogens such as Staphylococcus aureus (commonly called staph) at concentrations ranging from six to 78 times greater than baseline. Those high concentrations were associated with lower tidal heights, greater freshwater discharge, and the density of nearby sewage disposal systems — cesspools, in other words.

“There’s a lot of factors,” said study co-author and UH-Hilo professor Tracy Wiegner. “There’s the huge concentration of cesspools in the area, all on lava rock, which is very porous and permeable.”

Wiegner said sewage from a Keaukaha cesspool can trickle out into the bay within hours to days.

Steven Colbert, an associate professor of marine science at UH-Hilo, said there are similar conditions throughout the Big Island’s east and southern coastlines.

“So much of the island is just too young for there to be the soil for septic systems to work,” Colbert said. “Instead, we have all these cracks and fissures and lava tubes beneath us that wastewater just flows through.”

As sea levels rise, wastewater will reach the sea even more quickly, and more and more cesspools will be flooded, Colbert said. At the same time, as the ocean gets warmer, Hilo Bay will become a better habitat for bacteria to thrive.

While the bay’s water quality has been a long-known health hazard, Colbert said the study at least has helped confirm where the pathogens are coming from.

“(Fixing the problem) will need a very expensive bullet,” Wiegner said. “That’s why the cesspool conversion process has taken so long.

“Ideally, we’d want to connect homes in Keaukaha to a sewerline and then get the Hilo Wastewater Treatment Plant back up to its maximum capacity … or even higher,” Wiegner went on.

However, the question of who will bear the cost of such a conversion remains an open one. Wiegner said it’s clear that Keaukaha already has borne the brunt of the bay’s wastewater contamination, and most residents can’t afford to convert their cesspools on their own.

A study commissioned by Hawaii County could find some solutions, albeit not for at least two years. In 2023, the county Department of Research and Development applied for federal grant funding for a comprehensive study of the Hilo watershed.

In March, the National Fish and Wildlife Foundation awarded $2 million to the county for that study, and the agreement between the county and NFWF was formally executed Tuesday.

Beth Dykstra, economic development specialist for R&D, acknowledged the project has been slow to progress because of the bureaucratic hurdles inherent with federal funding, but said the two-year study should bear fruit by the end of 2026.

That study, Dykstra said, would build substantially on the UH study and other research, but would be carried out on a larger scale across the entire watershed.

The study also will include potential solutions and, crucially, possible funding sources.

“That’s what the study is really about,” Dykstra said. “So we can go for funding.”

Dykstra said that although the Hilo watershed is the largest in the state, neither the county nor the state Department of Health has sufficient resources to manage its ecology, and the decrepit Hilo Wastewater Treatment Plant is on its last legs and will cost about $300 million to rehabilitate.

Despite its issues, however, Colbert said the watershed’s conditions aren’t too dire yet.

“It doesn’t stop me from going swimming,” Colbert said. “I just have to be careful. I take a shower afterward, and if I have an open wound, I probably won’t go in. If the water’s brown after a storm, you probably shouldn’t go in.”