Modeling fate of pathogenic organisms in coastal waters of Oahu, Hawaii

A comprehensive modeling study was undertaken to examine the transport and fate of pathogenic organisms in the coastal waters of Mamala Bay, the southern shore of the island of Oahu, Hawaii. Specifically, two mathematical models were developed, calibrated, and validated in an attempt to understand both the relative contributions of various outfall and shoreline sources to organism counts throughout Mamala Bay and the benefits achieved by various source control options. A three-dimensional hydrodynamic model was constructed to simulate the advective and dispersive processes observed in the bay. A near field plume model was coupled with the hydrodynamic model to realistically incorporate initial mixing dynamics. Results of the hydrodynamic model were then used by a pathogen fate model to predict the distributions of fecal contamination indicator organisms and specific pathogens at several locations throughout the bay. Two sources were identified as primary contributors of contamination within Mamala Bay: the outfall from the Sand Island wastewater treatment plant and the Ala Wai Canal. The relative importance of the sources was dependent on the indicator organism used to assess fecal contamination. Using the fate model results and assuming a seven consecutive day exposure, the maximum risk of infection at Waikiki Beach was estimated to be 1.3/100 for virus and 3.1/10,000 for Giardia. The upgrade of the Sand Island treatment plant to chemically-enhanced primary treatment reduced organism counts throughout the bay by about a factor of two. Additional reductions were achieved by adding disinfection or upgrading the plant to secondary treatment. The implementation of the evaluated source control options would reduce the risks of infection at Waikiki Beach.