Coastal water quality in Hawaii: the importance of buffer zones and dilution

A study of the relationship between point and nonpoint source freshwater discharges and marine water quality were studied during a period of 1 year in Mamala Bay, a coastal indentation on the south shore of the island of Oahu, Hawaiian Islands. Despite the fact that 100–300×106 m3 year−1 of land runoff/groundwater seepage and 150×106 m3 year−1 of treated sewage effluent enter Mamala Bay and its tributaries, coastal water quality as judged by standard chemical and physical parameters is high at virtually all locations in the bay. The explanation for the high water quality reflects several important factors. First, much of the nonpoint source discharge enters either estuaries or harbors, which function as buffer zones by trapping some of the sediment and nutrients that would otherwise enter the coastal ocean. Second, the principal point source discharges are located in water sufficiently deep that their wastewater plumes are trapped below the surface most of the time. When the plumes surface they are sufficiently diluted that their impact on parameters, such as nutrient concentrations, is undetectable. Third, the coastal current system is greatly diluted by exchange with the offshore ocean. Based on a simple box model, the degree of mixing with the offshore ocean is roughly 40 times the rate of input of fresh water from point and nonpoint sources. The offshore wastewater outfalls have no discernible effect on water quality at any recreational beach along the shoreline. The principal impact on water quality at the recreational beaches comes from nonpoint source discharges, and with the exception of one beach located directly adjacent to a stream mouth, that impact is on the composition rather than the concentration of the plankton. There is a systematic shift from a chlorophyte- to a diatom-dominated phytoplankton community due to the high silicate concentration in groundwater and land runoff, and there is a systematic increase in the δ15N of suspended particles due to the high δ15N of the biologically available nitrogen in groundwater seepage. ©