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

A study of the relationship between point and nonpoint source freshwater di scharges 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 100-300x10(6) m(3) year(-1) of la nd runoff/groundwater seepage and 150x10(6) m3 year(-1) of treated sewage e ffluent enter Mamala Bay and its tributaries, coastal water quality as judg ed by standard chemical and physical parameters is high at virtually all lo cations in the bay. The explanation for the high water quality reflects sev eral important factors. First, much of the nonpoint source discharge enters either estuaries or harbors, which function as buffer Zones by trapping so me of the sediment and nutrients that would otherwise enter the coastal oce an. Second, the principal point source discharges are located in water suff iciently deep that I:heir wastewater plumes are trapped below the surface m ost of the time. When the plumes surface they are sufficiently diluted that their impact on parameters, such as nutrient concentrations, is undetectab le. Third, the coastal current system is greatly diluted by exchange with t he offshore ocean. Based on a simple box model, the degree of mixing with t he offshore ocean is roughly 40 times the rate of input of fresh water from point and nonpoint sources. The offshore wastewater outfalls have no disce rnible effect on water quality at any recreational beach along the shorelin e. The principal impact on water quality at the recreational beaches comes from nonpoint source discharges, and with the exception of one beach locate d directly adjacent to a stream mouth, that impact is on the composition ra ther than the concentration of the plankton. There is a systematic shift fr om 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 delta(15)N of suspended particles due to the hig h delta(15)N bf the biologically available nitrogen in groundwater seepage. (C) 1999 Elsevier Science Ltd. All rights reserved.