UH Hilo researchers discover sewage leaking at surprising rate into waters near Kahaluʻu Beach Park
What surprised the researchers most during the testing on Friday was how fast the groundwater was carrying sewage to the shoreline.
By Susan Enright.
Marine scientists from the University of Hawaiʻi at Hilo conducted a dye test Friday on the coastline of Kahaluʻu Bay located just south of Kailua-Kona in the Keauhou area.
A small amount of the dye fluorescein was added to a sewage disposal system of a residence in the area. By observing whether the dye traveled to the shoreline and how fast, the researchers hoped to learn more about the impacts onsite sewage disposal systems may have on water quality and marine life.
“We added dye to a cesspool to see if wastewater in the cesspool would reach the shoreline, and how long it would take,” says Steve Colbert, professor of marine science and lead on the study. “And, sure enough, the dye emerged at a shoreline spring, telling us that the wastewater from at least one home, and possibly more, is flowing out into Kahaluʻu Bay.”
In an Aug. 7 news release from the County of Hawaiʻi announcing the testing, it’s noted that Kahaluʻu Bay is home to an important and unique coral reef ecosystem and is also one of the most popular snorkeling locations in all of Hawaiʻi. Each year, over 400,000 people visit this shallow, nearshore reef to view its diverse marine life, colorful fish, and coral colonies.
Faster than any flow rate the researchers have measured in Keaukaha or Puakō
What surprised the researchers most during Friday’s testing was how fast the groundwater was carrying sewage to the shoreline.
“We measured groundwater flow rates of 304 yards per day,” says Colbert. “So for homes with cesspools near Aliʻi Drive, sewage can reach the shoreline in less than six hours. Put another way, a toilet flush at high tide may be at the shoreline by the next low tide.”
Colbert notes this is faster than any flow rate the researchers have measured in Keaukaha, Hilo, and twice as fast as the fastest flow at Puakō.
“At this flow rate, for homes within a couple miles of the shoreline, there is not enough time for biological processes to break down the sewage or for dilution of the sewage to occur,” says Colbert.
“Based on a statewide assessment, which didn’t include information from site-specific studies, homes in the Kahaluʻu ahupuaʻa [land division] are set at priority two for cesspool replacement,” says Colbert. “We hope that this data [collected by the research team] can be used by the community, county, and state in modifying those priority rankings so that cesspools with the greatest impact on coastal resources are removed first.”
Investigating impacts of increased sea level due to global warming
The Kahaluʻu Beach Park research is a part of larger project funded by the Pacific Islands Climate Adaptation Science Center, which is a collaboration between the U.S. Geological Survey, UH Mānoa, and UH Hilo. The project is looking at hotspots of sewage pollution between Kailua-Kona and Keauhou Bay, and the impact of sea level rise on sewage infrastructure. Sea level rise is an increase in the level of the world’s oceans due to the effects of global warming.
The research is being conducted, recorded, and analyzed by graduate student Ihilani Kamau who is in the tropical conservation biology and environmental science program. For her thesis, Kamau is looking at how sea level rise will impact sewer infrastructure, cesspools, and future water quality in Kailua-Kona.
- How is sea level rise going to affect wastewater systems in Kailua-Kona? A UH Hilo graduate student investigates (UH Hilo Stories, April 30, 2024)
“The purpose of this project is to assess potential water quality impacts based on current water quality conditions in order to predict how it will be impacted in the future during sea level rise,” Kamau says.
“Another purpose is to document and identify sewage pollution hotspots along the Kailua-Kona shoreline and determine which onsite sewage disposal systems and wastewater treatment infrastructures will be affected by sea level rise.”
The primary partners Kamau is working with are the County of Hawaiʻi; The Kohala Center, which is a community-based nonprofit located in North Kohala that conducts research, education, and stewardship for healthier ecosystems in Hawaiʻi; and the nonprofit Waiwai Ola Waterkeepers and their Kona Coast Waterkeeper group.
In the video below, Kamau presents her research at UH Hilo’s 14th Annual Tropical Conservation Biology and Environmental Science Symposium, April 2024:
Note: This story has been corrected to “cesspools near Aliʻi Drive,” not on Aliʻi Drive.
Story by Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories.
