WRRCPR No.94-05 Assessing The Impact of Mokapu Sewage Outfall on the Shoreline Water Quality of Kailua Bay (KB-2)

Abstract

The discharge of secondary treated sewage effluent from the Mokapu Ocean Outfall into Kailua Bay, Oahu, Hawaii, represents a point source of pollution to the bay. Public health considerations are therefore of significant importance due to bodily contact and the possible ingestion of recreational water. The impact of the Mokapu outfall on the shoreline water quality at Kailua Bay was assessed in 1990 and 1991. The concentrations of fecal indicator bacteria (E. coli, enterococci, C. perfringens) were determined in sewage effluent samples and in water samples collected from the zone of mixing (ZOM) sites, offshore sites, nearshore sites, and shoreline sites of Kailua Bay. The indicator bacterial loads discharged from the outfall were on the orders of 106 E. coli/100 ml, 105 enterococci/100 ml, and 104 C. perfringens/ 100 ml. Within the ZOM, some of the sewage surfaced, however most of it was transported submerged and in a northerly direction. The sewage was also transported submerged to the two offshore sites located north and south of the ZOM but preferentially north. The nearshore data also suggested the movement of sewage in a direction north-northwest of the outfall. The absence or recoveries of only very low numbers of bacteria from the nearshore sites closest to the Kailua shoreline did not provide evidence that sewage from the outfall was possibly impacting the quality of the shoreline recreational waters. The geometric means of the seven true shoreline sites all met Hawaii's marine recreational water quality standard. The two other shoreline sites which equaled or exceeded the standard are actually the mouths of land-based fresh water sources which are known to contain high concentrations of indicator bacteria. The overall results suggested that the quality of shoreline water is more likely impacted by land-based sources such as rainfall events which increase surface runoff.