Project Report PR-95-11
Nonpoint source pollution within the Anahulu River and Opaeula Stream drainage areas, Kaiaka – Waialua Bay hydrologic unit area, Oahu, Hawaii
Paul A. DeVito, Gordon L. Dugan, Edwin T. Murabayashi, and Henry K. Gee
Kaiaka and Waialua bays, located on the north shore of Oahu, Hawaii, have long-term histories of turbid water conditions occurring after significant rainfall events. The bays are the receptacles of the approximately 70,700-acre Kaiaka – Waialua Bay Hydrologic Unit Area, with Kaiaka Bay receiving runoff from four distinct drainage areas and Waialua Bay from one drainage area. Kaiaka Bay receives runoff from 86% of the hydrologic unitºs surface area, with the remainder flowing to Waialua Bay. The flashy nature of the rainfall events in the hydrologic unit, the short times of concentration, the distances between the five separate drainage areas, and the time required to travel to potential sampling sites precluded the use of grab samples; thus the only practical alternative was the use of stream-height-activated automatic water samplers. With the financial limits dictated by the funding for first phase of the study, coupled with the need for expensive automatic water samplers, only two drainage areas, Anahulu River and Opaeula Stream, which flow to Waialua and Kaiaka bays, respectively, were monitored to ascertain baseline water quality concentrations and constituent loadings. Land use within both subdrainage areas consists of forest reserve and sugarcane agriculture. Grab samples were typically collected during dry weather for a one-year period from two sites on each stream, an upper site located at the lower end of the forest reserve area and a lower site at the lower portion of the agricultural area, just above tidal influence. During the 1992 – 93 “wet season,” traditionally considered to be from approximately November through March, a continuous automatic sampler was installed at each sampling site to continuously monitor streamflow and rainfall and to collect storm runoff samples. Sample analyses of typical water quality and pesticide/toxic chemical constituents revealed several noteworthy relationships. Suspended solids, turbidity, total phosphorus, and nitrate nitrogen were directly related to flow (increase for increase) from upper to lower sampling sites. The nine targeted pesticides and toxins were below the level of analytical detectability for both the dry- and wet-weather samples. The Opaeula drainage basin contributed significantly more pollutants on a per-area basis than did the Anahulu drainage basin, with the exception of filtered phosphorus. Most notable were the suspended solids loading rates, which were four to five times greater for the upper and lower Opaeula sites than for the respectively located Anahulu sites.