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

摘要

A study of the relationship between point and nonpoint source fleshwater discharges and marine water quality were studied during a period of I year in Mamala Bay, a coastal indentation on the south shore of the island of Oahu. Hawaiian Islands. Despite the fact that 100- 300 x 10~6 m~3 year~-1 of land runoff/groundwater seepagte and 150 x 10~6 year~-1 of treated sewage effluent enter Mamala Bay and its tributaries, coastal water quality as judged by stand- ard 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 unde- tectable. 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 waste- water 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 non- point so urce 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 plank- ton. 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 δ~15 N of suspended particles due to the high δ~15N of the biologically available nitrogen in groundwater seepage.