Water Quality Characteristics of Honokohau Harbor: A Subtropical Embayment Affected by Groundwater Intrusion!

This study describes the water quality characteristics of a subtropical embayment that is markedly affected by the infiltration of cold, nutrient-rich groundwater. The spatial, vertical, and tidal variations of physicochemical characteristics (e.g., temperature, salinity, oxygen, turbidity) and nutrients (e.g., nitrate, phosphate, ammonium) are depicted and show conditions of pronounced stratification. The harbor supports an unusual twolayered structure of cold, brackish, nutrient-rich waters overlying a warm, lownutrient, oceanic layer. Temperature and salinity range from 20.5 to 24.5°C and from 18.1 to 35%0 at the surface and bottom (5.5 m), respectively. High nutrient levels in the surface layer (about 30 Ilg-atoms N03 /liter and 2 Ilgatoms PO4 3 /Iiter and the close correlation with thermohaline parameters identify groundwater intrusion as the major nutrient source. The prolific (1.5-2 million gallons per day) and continual groundwater influx produces persistent flow out of the harbor irrespective of the tidal condition and produces harbor flushing rates six to ten times those calculated for tidal flushing alone. The potential eutrophying effects of the groundwater nutrients are avoided as a result of the rapid harbor flushing. This study details the potential impact of groundwater nutrients on the aesthetic and water quality character of island coastal developments, indicates that consideration of terrestrial features (e.g., land slope, rainfall) cannot be used to predict the likelihood or extent of groundwater effects, and describes the importance of infrastructure design to optimize flushing as a critical criterion in maintaining good water quality in such embayments. THE HONOKOHAU SMALL BOAT HARBOR was created in 1970 by blasting and excavation of approximately 10 acres of shoreline in Honokohau Bay to provide a haven and launch site for boating activities in the area. The resulting origination of pelagic and benthic marine habitats offered a unique opportunity to investigate resulting trends in water quality conditions and to study the seral stages of community succession in this subtropical coastal environment. In addition to delineating valuable ecological cycles regard1 Funding for this work was provided by the U.S. Army Corps of Engineers and by the State of Hawaii Department of Transportation, Harbors Division. Manuscript accepted 21 February 1980. 2 Oceanic Institute, Makapuu Point, Waimanalo, Hawaii 96795. ing succession on substrate that had never before been colonized, the study describes . several important phenomena that may influence to some degree the marine character of other coastal developments of this sort in the islands. The harbor's isolation from other affecting forces, such as run-off, river/stream, or domestic/industrial sewage inputs, enhances the predictive value of witnessed trends for other comparable coastal situations. The purposes of this research, spanning 5 years of investigation, were to monitor community development within the newly created basin, to determine resulting water quality conditions, to use this information to predict the impact of harbor completion (via fivefold expansion of water area), and to direct such expansion so as to mmimize