Identifying locations of sewage pollution within a Hawaiian watershed for coastal water quality management actions

Study region: Puako over bar , Hawai'i Island. Study focus: Locations of sewage pollution in the Puako over bar watershed were identified through measurements of sewage indicators at groundwater wells and within Puako over bar 's and adjacent resorts' shoreline waters. Dye tracer tests, water quality, delta N-15 macroalgal, and delta N-15-and delta O-18-NO3- measurements, along with stable isotope mixing models, were combined to assess water quality impairment caused by different Onsite Sewage Disposal System (OSDS) types, and used to predict water quality improvements from future management actions. New hydrological insights for the region: Sewage indicators were highest within Puako's shoreline waters, including: Enterococcus spp., Clostridium perfringens, human-associated Bacteroides, and delta N-15-NO3-. Mixing model results using delta N-15- and delta O-18-NO3- suggest that sewage was a dominant NO3- source, comprising > 40% at 10 of the 16 shoreline stations. delta N-15 macroalgae measurements confirmed presence of sewage at most stations. In groundwater wells and at adjacent resorts' shoreline waters, sewage indicators were low, and delta(NO3-)-N-15 was indicative of soils and fertilizers. Puako over bar dye tracer tests revealed that sewage reached the shoreline within 5 h to 10 d, and that OSDS type did not affect travel time. Water quality was similar in front of homes with different OSDS. In conclusion, sewage is entering the groundwater at Puako over bar , and the underlying geology, rather than OSDS type, primarily controls the speed at which sewage reaches the shoreline. Our findings highlight the need for improved sewage treatment and collection at Puako.