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

Abstract

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 two-layered structure of cold, brackish, nutrient-rich waters overlying a warm, low-nutrient, 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 ug-atoms N03 -/liter and 2 ug-atoms PO4 3 -/liter 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.