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

Abstract

Study regionPuakō, Hawai‘i Island. Study focusLocations of sewage pollution in the Puakō watershed were identified through measurements of sewage indicators at groundwater wells and within Puakō’s and adjacent resorts’ shoreline waters. Dye tracer tests, water quality, δ15N macroalgal, and δ15N- and δ18O-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 regionSewage indicators were highest within Puakō’s shoreline waters, including: Enterococcus spp., Clostridium perfringens, human-associated Bacteroides, and δ15N-NO3-. Mixing model results using δ15N- and δ18O-NO3- suggest that sewage was a dominant NO3- source, comprising > 40% at 10 of the 16 shoreline stations. δ15N macroalgae measurements confirmed presence of sewage at most stations. In groundwater wells and at adjacent resorts’ shoreline waters, sewage indicators were low, and δ15NO3-was indicative of soils and fertilizers. Puakō 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 Puakō, 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 Puakō.