Mapping and modeling coastal wastewater circulation and dispersion around Boracay Island, Aklan Philippines

Boracay Island, among the world’s top tourist destinations, was recently reopened to public and tourists after its closure in 2018 for rehabilitation due to environmental law violations resulting to poor water quality and degrading environmental health. One of the most common violations is the unregulated construction of discharge pipes from hotel, resort, and restaurants in the island. Thus, this study aims to map the existing discharge pipes in the Island after its reopening and understand the circulation and dispersion of wastewater coming from the pipes. To do this, a seasonal 3d hydrodynamic model with 10 layers of 10% thickness per layer was developed in Delft3d. The layered hydrodynamic model was also coupled with DelftWAQ, one of Delft3d modules that simulates water quality. The initial condition includes velocities in zero (u,v,w=0), salinity is constant at 34 psu and temperature varies linearly from 27 at the surface to 26.4 at the bottom. The sewage was simulated as freshwater discharges at an outflow of 10MLD and released at the bottom. The water level and currents produced by the model is with good agreement with the observed data from ADCP. The model shows that there is longer residence time in the Bulabog area due to weaker currents while there is enhanced mixing of sewage discharge in the northern and southern tips Boracay due to stronger currents. Results also corroborate with E. coli and Total Coliform measurements wherein higher counts were observed in the areas near the end of sewage pipes.