UH Hilo-led study confirms sewage presence in Keaukaha’s coastal waters and coral reefs
Data collection for the study involved putting dye into nearby cesspools and a septic tank. The researchers found the dye emerged at the Keaukaha shoreline every time. And it moved fast, a couple of football field lengths each day.

By Susan Enright.
Research conducted by group of marine scientists and their students that identifies sewage hotspots in waters off Keaukaha, Hilo, has just been published in a leading hydrology journal.

“The work identifies sewage hotspots along Hilo’s coastline, examining inputs from onsite sewage disposal systems [such as cesspools] and the Hilo wastewater treatment plant,” says Tracy Wiegner, a professor of marine science at the University of Hawaiʻi at Hilo and a co-author of the study.
- Sewage pollution from onsite sewage disposal systems and an offshore wastewater treatment plant outfall in coastal waters of Keaukaha, Hawaiʻi Island (Journal of Hydrology: Regional Studies, Feb. 2025).
“This project developed out of discussions at the Keaukaha Community Association meetings where community members complained about the stink [smell] associated with the Hilo wastewater plant that frequently impacted their community,” says Wiegner, “as well as potential human and ecosystem impacts from the wastewater treatment plant outfall in Puhi Bay,” which Wiegner refers to as the community’s “refrigerator.”
The data collection took place from July 2020 to May 2022. The results are concerning: All measurements confirm sewage presence in Keaukaha’s coastal waters and coral reefs.
While the researchers discovered that fecal indicator bacteria and nutrient concentration measurements were relatively low along the Keaukaha shoreline in comparison to other areas impacted by sewage pollution on Hawaiʻi Island and elsewhere, the group’s dye tracer tests confirm that sewage from onsite sewage disposal systems in Keaukaha reach the shorelines within 20 hours to three days.
“This is a faster travel time than previously reported in other areas of Hawaiʻi, which are ranked as a higher priority by Hawaiʻi State for cesspool conversion,” write the researchers. “Our results suggest that the Hilo region, which includes Keaukaha, should become a Priority 1 area for cesspool conversion.”
Pilau hotspots: the nose knows
The project’s goal was to identify sewage hotspots along Hilo’s coastline, and conduct more detailed measurements in Keaukaha to be able to distinguish contributions of sewage from the county sewage treatment plant and nearby cesspools. The objectives were to determine the connectivity between cesspool and adjacent coastal waters through dye tracer studies, and evaluate the presence of sewage in nearshore waters through measurements of well-established and novel sewage indicators in areas with cesspools and the sewage treatment plant outfall.


Lead author of the paper is Shayla Waiki, who at the time of the study was a graduate student in UH Hilo’s tropical conservation biology and environmental science program. Already holding a bachelor of science in marine science from UH Hilo, Waiki pursued her master of science degree researching water quality, sewage pollution, groundwater springs, and the movement of freshwater, in the places where she grew up on Hawaiʻi Island. Her graduate thesis focused specifically on her investigation into sewage pollution in Keaukaha and nearby shorelines. Waiki is currently employed by the Department of Defense Army National Guard as a natural resources management specialist. She is planning to apply to law school at UH Mānoa this year.
Waiki’s primary thesis advisor and lead on the Puhi Bay portion of the Keaukaha research project was Steve Colbert, associate professor of marine science and also a co-author of the study, who says the investigation was launched when community members asked the researchers if they could document bad smells that occur at Puhi Bay. The bay is located at Keaukaha Beach Park at the southern side of Hilo Bay.
The researchers created the Pilau-meter, a web-based form anyone could fill out when they visited Puhi Bay. Pilau is the Hawaiian word for stink, rotten, foul.
“A couple times, people smelled sewage, and we later determined these were times when there were high fecal indicator bacteria counts or a wastewater discharge there,” says Colbert.
“So, if something doesn’t smell right, it’s probably not right,” he adds. “The nose knows.”
Data collection involved putting dye into three cesspools and one septic tank. The researchers say dye tracer tests provide irrefutable evidence of the connection between sewage sources and the coastal waters of interest.
“We found dye emerge at the shoreline every time,” Colbert says. “And the water moved fast, one to two football fields [in length], including end zones, each day.”
The source of the problem? Cesspools and sewage treatment plant
The project proposal summary states that cesspools are used more widely in Hawaiʻi than any other state.
“They are concerning because many homes are close to the water on highly permeable substrate, with sewage quickly contaminating groundwater and posing an immediate human health risk,” write the researchers. “Because of this and ecosystem concerns, Hawaiʻi recently banned installment of new cesspools.”
The majority of cesspools are located on Hawaiʻi Island, and some communities are more affected by sewage pollution than others. Keaukaha, a Hilo neighborhood with fringing reefs, is one of them. This community houses Hilo’s county sewage treatment plant (STP) and its outfall; the majority of homes still have cesspools within 1 km of the shoreline. A tremendous amount of groundwater discharges along this coast, making it likely that untreated sewage quickly reaches the water.

The researchers explain that while Hilo has been identified as a priority area for cesspool upgrades as part of the state’s mandate to replace all cesspools by 2050, very little water quality data are available. Data from the Puhi Bay research project will inform the community’s and county’s decisions regarding cesspool replacement and possible STP upgrades.
Currently, Keaukaha’s primary concern is sewage from the STP, and less about from cesspools, even though cesspool effluent contains human pathogens which are rarer in treated sewage. The researchers say the people of Keaukaha are worried about the number of waterborne infections in their community, particularly methicillin-resistant Staphylococcus aureus or MRSA, that are likely related to sewage pollution.
The researchers say that although there is a “tremendous amount of community concern regarding sewage pollution, no study to date has documented it along this shoreline, where people recreate, subsistence fish and gather, and perform cultural practices, except at a few Hawaiʻi Department of Health monitoring stations.”
The UH research team says their study clearly shows that dye tracer tests in conjunction with water quality measurements are necessary for documenting nearshore sewage pollution. “Together, they provide irrefutable evidence of the environmental problem that poses a human and ecosystem health threat,” write the authors of the study.
Collaborative research team, collaborative support
In addition to Waiki, Wiegner, and Colbert, the research team also includes former UH Hilo graduate students Joseph Nakoa and Devon Aguiar, and two marine science undergraduate students Nicolas Storie and Ashlynn Overly. Nakoa is now a doctoral student in the environmental life science program at Arizona State University. Storie is a doctoral student in the biological oceanography program at UH Mānoa. Aguiar and Overly are now working at the Hawaiʻi Division of Aquatic Resources Hawaiʻi Coral Reef Initiative on Hawaiʻi Island.

Other students involved were marine science undergraduates Darienne Kealoha, Kaitlin Villafuerte, Finn Reil, and Brooke Enright, and tropical conservation and environmental science graduate student Walter Boger.
Karla McDermid, a UH Hilo professor of marine science who is an expert in limu (seaweeds) is also a co-author of the study. Two co-authors are from UH Mānoa: Noe Puniwai, who did her post-doctoral work at UH Hilo’s Department of Geography and Environmental Science and is now an associate professor at the Kamakakūokalani Center for Hawaiian Studies, and aquatic biologist Craig Nelson who is a research professor in the Department of Oceanography at the School of Ocean and Earth Science and Technology.
The Puhi Bay investigation was done in collaboration with many community members and officials from the County of Hawaiʻi as well as the Hawaiʻi Division of Aquatic Resources, which is a state agency led by the Department of Land and Natural Resources.
Funding for this project was provided by state, federal, and community programs. Supportive UH Hilo programs included the Pacific Internship Program for Exploring Science (PIPES), the Students of Hawaiʻi Advanced Research Program (SHARP, a program funded by the National Institute of Health, Award No. R25GM11347, now completed), and the marine science and environmental science/geography departments. Additional support was provided by the UH Sea Grant College Program, the USGS Pacific Island-Climate Adaptation Science Center, the Hauʻoli Mau Loa Foundation, and Kamehameha Schools.
This type of collaborative research is extremely important to island communities.
“Results from this research have been shared with the community and Hawaiʻi county, and are being used by the county to help prioritize locations for sewer line expansion in Hilo as part of the statewide effort to transition away from cesspools,” says Wiegner.
Story by Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories.