Fate and transport of pollutants and their impacts on the Ala Wai Canal

Abstract

A water quality model was developed to simulate the fate and transport of nutrients within the Ala Wai Canal and to evaluate the impacts of storm runoff on the canal's biological activity. The hydraulic component of the model was developed by considering the canal as a series of four completely stirred tank reactors (CSTRs), with flushing time as the key hydraulic parameter. Flushing time represents the average time that a particle of freshwater will remain in the estuary. The flushing times were estimated based on salinity data collected by this study and other previous studies. Modeling results indicated that the Total Maximum Daily Load (TMDL) of total nitrogen, which has been set by the state as 25 kg/day, was exceeded by both the wet and dry season averages. This 25 kg/day limit was based on state water quality standards for the canal that state the nitrogen concentration within the canal should not exceed 200μg/l. Using parameters that have been updated since the TMDL study such as the background concentration of nitrogen, flushing time, and volume of the canal, it was also determined that the water quality standard for total nitrogen of 200μg/L was exceeded within certain areas of the canal. Field data of chlorophyll a concentrations were collected over a two month period to provide a base for model testing and calibration. Modeling analysis suggests that the canal is highly productive and that this production increases after increased loading of nutrients following a rainstorm. A final application of this model was discussed that showed its potential to determine the fate of contaminants from possible biological spills within the canal. Field data of diurnal dissolved oxygen and chlorophyll a concentrations were collected to determine if the model's conclusions on the health of the canal were correct. Field data suggested that the canal is highly productive and can be classified as eutrophic. This bioproductivity was shown to greatly increase after storm events.