Just published: UH Hilo-led study documents sewage pollution in Hilo nearshore waters, from Keaukaha to Honoliʻi
Key takeaways from study: High densities of cesspools need to be removed; sewer lines and wastewater treatment plant need to be repaired/upgraded; rivers and streams also transport wastewater to shoreline.
By Susan Enright/UH Hilo Stories.
A study on wastewater pollution along the Hilo shoreline conducted by a group of University of Hawaiʻi at Hilo students and their marine science professors has just been published in a leading peer-reviewed journal.
- See snippets of published study here: Identifying wastewater pollution hotspots in coastal Hawaiian waters with high freshwater inputs (Marine Pollution Bulletin 233, 2026)
The data collected from stations placed in Hilo nearshore waters from Keaukaha to Honoliʻi show sewage pollution all along the coast, relevant and important information to local communities and lawmakers in advancing pollution control measures.
The study was conducted over the course of two years with then-graduate student Joseph “Crispin” Nakoa heading the project. Nakoa, who had already received his bachelor of science in marine science from UH Hilo in 2016, led the pollution study in 2020-2022 as his thesis project for his master of science degree in tropical conservation biology and environmental science; he received his MS in 2022. A 2012 graduate of Kamehameha Schools Hawaiʻi campus, he is now a doctoral candidate at Arizona State University.
“Growing up in Hilo, these aren’t just study sites, they’re places I’ve spent my whole life fishing, surfing, and spending time with family and friends,” says Nakoa about the area covered in the study. “Being able to answer questions that so many of us have had about the health of these waters has been one of the most rewarding parts of this project.”
The research team
Working with Nakoa on his thesis project, co-authors of the study are UH Hilo marine science professors Tracy Wiegner, Steven Colbert, and Karla McDermid, along with then-graduate student Shayla Waiki and alum Devon Aguiar, both of whom have received the same bachelor and master degrees as Nakoa.
Waiki is currently working at the Environmental Office of the State of Hawaiʻi Department of Defense and is a first-year student at UH Mānoa William S. Richardson School of Law. Aguiar now works at the Hawaiʻi Division of Aquatic Resources of the Department of Land and Natural Resources.
Co-authors also include then-undergraduate marine science student Nicolas Storie who is now a doctoral student at UH Mānoa and a water quality specialist at the nonprofit Wastewater Alternatives and Innovations, and Craig Nelson a geochemist from the oceanography department at UH Mānoa, who served as Storie’s advisor.
The work was supported by UH-based program Hawaiʻi Sea Grant and the private grantmaking foundation Hauʻoli Mau Loa Foundation. Wiegner and Colbert served as Nakoa’s thesis advisors; McDermid served as a committee member.
The study: The first to map out wastewater hotspots from King’s Landing to Honoliʻi Beach Parks
Colbert notes Nakoa’s work in this investigation is a companion study to one where research teams, led by graduate student Waiki, used dye to track where sewage from cesspools in Keaukaha and Leleiwi entered the ocean at shoreline springs.
“(Nakoa’s) paper puts those findings of sewage in nearshore waters into a broader context, extending across all of Hilo, from King’s Landing, to Hilo One, to Honoli’i Beach Park,” Colbert explains. King’s Landing is in Keaukaha at the far south point of Hilo Bay; Hilo One is Hilo Bay from Wailuku River to Wailoa River; Honoliʻi is about two miles north of downtown Hilo.
The two studies on ocean pollution in Hilo waters were conducted in 2020-2022, both with Hawaiʻi Sea Grant support that asked the UH Hilo research teams to identify sewage pollution hot spots from cesspools and a sewage treatment plant in Hilo, Hawaiʻi Island. Principal investigators on the overall project were UH Hilo professors Wiegner and Colbert along with Nelson from UH Mānoa’s School of Ocean and Earth Science and Technology.
Though the two studies were conducted simultaneously, Waiki’s investigation focusing on documenting the hydrologic connection between cesspools and shoreline springs as well as wastewater inputs from the wastewater treatment plant to Puhi Bay, was published first in January of last year in the Journal of Hydrology: Regional Studies.
- Learn more about that study here: UH Hilo-led study confirms sewage presence in Keaukaha’s coastal waters and coral reefs (UH Hilo Stories, Jan. 21, 2025)
As both Wiegner and Colbert point out, Nakoa’s investigations in the paper published last week are broader than Waiki’s work and examine the whole Hilo shoreline from King’s Landing to Honoliʻi.
“He mapped out wastewater hotspots at 16 locations, which did include Puhi Bay where Shayla did her work,” says Professor Wiegner. “Crispin measured several different indicators of wastewater, including fecal indicator bacteria, nutrients, and stable isotopes of nitrogen in limu (seaweed) and nitrate.”
Professor Colbert explains the project was conducted during the height of COVID, so students had to work independently and navigate ever-changing regulations.
“I had the good fortune to work with Crispin on an independent research project as an undergraduate and then on this project as graduate student,” says Colbert. “Because of his work ethic and experiences doing research as an undergraduate, I was very glad he was interested in graduate school when we received the funding for this project.”
When this project started, the research team walked the entire shoreline from beyond Leleiwi Point to the Wailuku River, documenting all the groundwater springs.
“It was incredible to see some areas had very little fresh water, while others had springs with freshwater gushing out like a fire hose,” says Colbert. “We used those observations and input from the community to select the 20 stations where we would collect data.”
Data collection and findings: Wastewater detected at all sampling stations
In explaining some background of the project, Wiegner says wastewater from cesspools was a suspected source of water quality pollution in Hilo, threatening both human and environmental health.
“But, there was little to no data documenting the problem, and not for the entire Hilo shoreline,” she says. “Crispin’s study is the first to map out wastewater hotspots from King’s Landing to Honoliʻi Beach Park.”
Sixteen stations were sampled monthly for a variety of traditional wastewater indicators, including fecal indicator bacteria, nutrients, and stable isotopes of nitrogen in macroalgae (limu) and nitrate (a nutrient).
“Hot spots were identified using these data in a wastewater pollution score and stable isotope mixing models,” explains Wiegner. “Wastewater was detected at all stations, although most wastewater indicators had low concentrations.”
The researchers discovered eight to 49% of the nitrate in the water was sourced from wastewater (cesspools and the wastewater treatment plant) depending on the location. However, the dominant source of nitrate was from groundwater. Wiegner explains high groundwater inputs to Hilo’s coastal waters likely diluted the wastewater indicators making their detection challenging.
Here’s more background data from Professor Wiegner: Hilo has high rainfall (2500 mm/yr), of which 76% becomes groundwater. In comparison, across the contiguous U.S. most rainfall is <1800 mm/yr and <10% of runoff flows out as groundwater at the shoreline. Adding cesspool density to the wastewater pollution score (which is based on traditional wastewater indicators), allowed for the wastewater pollution hotspots to be better identified against the high groundwater background.
“Findings from this study support concurrent dye tracer tests [in Waiki’s study] which found that wastewater from homes was rapidly transported to the shoreline [in] hours to days,” says Wiegner.
“This study also demonstrates that in some locations, measurements of traditional wastewater indicators are not enough to document a wastewater pollution issue, especially in areas of high groundwater inputs,” she adds. “In these types of areas, it is important to measure a variety of wastewater indicators and use multiple approaches — dye tracer tests, pollution scoring, stable isotope mixing — to understand the wastewater pollution threat.”
Key takeaways
Professor Colbert says one of the big takeaway messages from this study is that there are some areas along the Hilo shoreline with very high densities of cesspools and close to sewer lines, which should be prioritized for removal.
The other takeaway message is the importance of rivers in transporting wastewater to the shoreline.
“We talk a lot about the importance of removing cesspools from near the shoreline, but it’s also important to remove those near our rivers and streams,” he says.
These observations highlight the relevancy and importance of the study to local communities and lawmakers in advancing pollution control measures.
“This paper documents the sewage pollution found in our nearshore waters,” says Colbert. “My hope is that this kind of documentation helps our county and state leaders get federal infrastructure funds to remove cesspools from near the shoreline and near rivers and upgrade the [wastewater treatment plant] to a higher level of treatment.”
Story by Susan Enright, public information specialist for the Office of the Chancellor and editor of UH Hilo Stories. She received her bachelor of arts in English and certificate in women’s studies from UH Hilo.
