There are over 95,000 onsite sewage disposal systems (OSDS) (i.e., cesspools and septic tanks) used in Hawaiʻi, many close to coastlines and streams. These systems have varying levels of treatment capacity for nutrients, bacteria, and other pollutants found in wastewater and may leech into groundwater that flows to the ocean. Excess nutrients can promote rapid algal growth, outcompeting corals and disrupting the natural balance of the ecosystem.