Impact to Stream Water Quality from Sewage Exfiltration and Legacy On-Site Disposal Systems on the Island of O‘Ahu, Hawaii

Abstract

The concentration of pharmaceutical compounds and nutrients present in perennial streams, springs and a lake on the island of O'ahu, Hawai'i were measured under drought conditions between 2020 and 2022. The combined island-wide daily release of wastewater to the environment on O'ahu from the continued use of legacy On-Site Sewage Disposal Systems (OSDS) and from exfiltration from the 3,400-kilometer network of underground sewer lines has been estimated to be about 80 million liter per day (mld), or around 3.9% of the total island-wide groundwater flux to the ocean. The 36 streams and 11 springs sampled were located down-gradient of areas with varying densities of OSDS and sewage lines while the lake sampled (Lake Wilson) receives direct input from the wastewater treatment plant that serves Central O'ahu. Average pharmaceutical and nutrient levels in streams and springs sampled in areas with high densities of OSDS and sewer lines were slightly higher, but not statistically different than concentration levels measured in streams and springs in areas with low densities of OSDS and low sewer line densities. The average sulfamethoxazole and carbamazepine levels measured in Lake Wilson, the only water body on O'ahu where treated wastewater is discharged into fresh water, are three to four times higher than average levels measured in the island's streams and springs. The presence of elevated concentrations of nitrate and silica in some streams and springs on O'ahu predominately reflects the impact of the historical use of up-gradient lands for sugarcane cultivation rather than wastewater input. The trace levels of pharmaceuticals detected in O'ahu streams and springs under baseflow conditions suggest that the actual combined input of wastewater to the environment from legacy OSDS and exfiltration from sewer lines is less than 20% the wastewater flux previously estimated.