Sewage pollution from onsite sewage disposal systems and an offshore wastewater treatment plant outfall in coastal waters of Keaukaha, Hawaiʻi Island

Keaukaha, Hawaiʻi. Onsite sewage disposal systems (OSDS) and the Hilo Wastewater Treatment Plant (HWTP) outfall are potentially impacting Keaukaha’s coastal waters. We used dye tracer tests, water quality, δ15N macroalgal, δ15N- and δ18O-NO3- measurements along with stable isotope mixing models to assess connectivity of OSDS to coastal waters and characterize water quality at coastal OSDS-impacted springs, coral reefs, and from the HWTP. A citizen science survey tool evaluated the association of both the presence and intensity of sewage odor with weather, ocean, and river conditions. All measurements confirmed sewage presence in Keaukaha’s coastal waters and coral reefs. OSDS-sewage reached shoreline springs within 20 h to 3 d. Measured flow rates ranged from 130 to 213 m/d, which were 15 times faster than state models for cesspool removal prioritization. Coastal OSDS-impacted springs had higher fecal indicator bacteria and nutrient concentrations than in the HWTP outfall plume. Benthic macroalgal δ15N was indicative of sewage at the outfall and nearby reef, as well as along the shoreline. Reported sewage odors coincided with high bacterial count advisories and HWTP discharge events, demonstrating the effectiveness of human olfaction as a sewage pollution indicator. Our findings suggest that Hilo’s prioritization category for OSDS removal should be elevated to Priority 1, and that the HWTP outfall should be relocated to a less culturally and ecologically sensitive area. [Display omitted] •Sewage indicators confirmed sewage presence in coastal waters of Keaukaha, HI.•OSDS-sewage reached shoreline springs within 20 h to 3 d, flowing 130–213 m/d.•Sewage odors coincided with high bacterial count advisories and WWTP discharge.•Multiple approaches and sewage indicators are needed to assess sewage pollution.