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

Studyregion:Keaukaha,Hawai'i.Studyfocus:Onsitesewagedisposalsystems(OSDS)and the Hilo WastewaterTreatmentPlant(HWTP)outfallare potentiallyimpactingKeaukaha's coastalwaters.We used dye tracertests,waterquality,delta 15N macroalgal,delta 15N- and delta 18O-NO3-measurementsalongwith stableisotopemixingmodelsto assessconnectivityof OSDSto coastalwatersand characterizewaterqualityatcoastalOSDS-impactedsprings,coralreefs,and fromthe HWTP.A citizensciencesurveytoolevaluatedthe associationof both the presenceand intensityof sewageodor with weather,ocean,and river conditions.Newhydrologicalinsightsfor the region:All measurementsconfirmedsewagepresencein Keau-kaha's coastalwatersand coral reefs.OSDS-sewagereachedshorelinespringswithin20 h to 3 d.Measuredflow rates rangedfrom 130 to 213 m/d, whichwere 15 timesfasterthan 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 delta 15N 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.