Spatial Distribution and Effects of Sewage in Coastal Hawaiian Waters

Sewage pollution is a worldwide threat to marine ecosystems and human health through the release of pathogens and nutrients into nearshore waters. Goals of this study were to document hydrological connections between cesspools and nearshore waters, detect the presence of sewage through biological and chemical tracers, and determine the spatial extent of sewage offshore. Puakō, located on Hawaiʻi Island, was the focus of this study as most homes have cesspools. Fluorescein dye injected into cesspools was detected at the shoreline in as little as three days. Elevated δ ¹⁵N signatures in macroalgae and high Enterococcus counts further confirmed presence of sewage in nearshore waters. Offshore sampling revealed significant differences among distances from shore for fecal indicator bacteria and δ ¹⁵N signatures in macroalgae. Results indicated distance from shore and stations are important factors of variability. Additionally, nutrient concentrations and macroalgal cover were higher in areas with high groundwater discharge. Surprisingly, δ¹⁵N macroalgal signatures and Enterococcus were not correlated with salinity. These results suggest that possibly the location of cesspools, subsurface geology, and/or nearshore circulation may affect sewage transport to the coastline and offshore. Spatial analysis techniques helped visualize potential hot spots of sewage pollution using δ¹⁵N macroalgal and Enterococcus data. The combination of tools used here to document sewage pollution presence may be useful for communities facing similar environmental problems.