A recent study by Arizona State University has mapped locations where sewage-contaminated water is entering the ocean along the west coast of Hawaiʻi Island, raising concerns for coral reefs and public health. The research, published in Frontiers in Marine Science, identifies over 1,000 discharge sites where submarine groundwater carries contaminants from inland cesspools and septic systems directly into coastal waters.
Researchers from ASU’s Center for Global Discovery and Conservation Science used airborne mapping technology and field sampling to collect data from 47 sites across 120 miles of coastline. Their analysis found that 42% of sampled sites had elevated levels of Enterococcus bacteria, a marker for sewage contamination. Of these, nearly a quarter exceeded thresholds considered unsafe for human health.
“Itʻs concerning how many sites we tested were contaminated, but not surprising when we consider statewide, 55 million gallons of wastewater effluent enters the ground each day — much of that within a few hundred yards of the coast,” said Kelly Hondula, first author and associate research scientist at ASU’s Center for Global Discovery and Conservation Science in the Julie Ann Wrigley Global Futures Laboratory.
“The results are clear. The high number of cesspools and faulty wastewater systems, and amount of land development along the coast, are highly predictive of whether the water is contaminated,” she said.
According to state health officials, Hawaiʻi has more than 88,000 cesspools—about 55,000 on the Big Island alone—which pose risks to both water resources and marine ecosystems. Much of the contamination comes through submarine groundwater discharge: groundwater flowing through porous volcanic rock directly into the ocean rather than via rivers or streams.
The study highlights two main contributors to contamination: inland on-site sewage systems such as cesspools and septic tanks, and dense coastal development. Submarine groundwater discharge acts as a pathway delivering polluted water onto vulnerable coral reefs.
“This study provides crucial insight into the impacts of development on our coastal ecosystems that will help us prioritize upgrades to our wastewater infrastructure,” said Kimo Alameda, mayor of Hawaiʻi County. “We are grateful to the ASU research team for their commitment to this important work. Their findings will play an important role in helping us protect our coral reefs for future generations.”
South Kona was identified as especially vulnerable due to its rapid urbanization and highly permeable volcanic substrate allowing faster movement of contaminants underground. This area is significant for both coral reef and fish populations.
ASU’s landscape-scale statistical model can predict contamination risk at specific discharge points based on land cover and known sewage sources. This information may help policymakers target interventions more effectively.
“The findings not only provide tactical information for government- and community-based interventions on land, they indicate where interventions are most needed as part of our reef restoration program here in Hawaiʻi,” said Greg Asner, senior author of the study, professor in the ASU School of Ocean Futures and director of the ʻĀkoʻakoʻa Reef Restoration Program. “Restoration is not just about planting corals, it involves a spectrum of land management, reef interventions and education effort.”
The researchers recommend replacing outdated sewage systems with advanced treatment units and focusing restoration efforts where they will have greatest impact on reef protection and public health.
The Dorrance Family Foundation funded this work; guidance came from members of the ʻĀkoʻakoʻa cultural advisory board.
Arizona State University has received recognition for its innovative approaches across multiple fields and was named No. 1 in innovation by U.S. News & World Report for eight consecutive years.
