Modelling the dispersion of treated wastewater in a shallow coastal wind-driven environment, Geographe Bay, Western Australia: implications for environmental management

Abstract

Numerical models are useful for predicting the transport and fate of contaminants in dynamic marine environments, and are increasingly a practical solution to environmental impact assessments. In this study, a three-dimensional hydrodynamic model and field data were used to validate a far-field dispersion model that, in turn, was used to determine the fate of treated wastewater (TWW) discharged to the ocean via a submarine ocean outfall under hypothetical TWW flows. The models were validated with respect to bottom and surface water current speed and direction, and in situ measurements of total nitrogen and faecal coliforms. Variations in surface and bottom currents were accurately predicted by the model as were nutrient and coliform concentrations. Results indicated that the ocean circulation was predominately wind driven, evidenced by relatively small oscillations in the current speeds along the time-scale of the tide, and that dilution mixing zones were orientated in a predominantly north-eastern direction from the outfall and parallel to the coastline. Outputs of the model were used to determine the 'footprint' of the TWW plume under a differing discharge scenario and, particularly, whether the resultant changes in TWW contaminants, total nitrogen and faecal coliforms would meet local environmental quality objectives (EQO) for ecosystem integrity, shellfish harvesting and primary recreation. Modelling provided a practical solution for predicting the dilution of contaminants under a hypothetical discharge scenario and a means for determining the aerial extent of exclusion zones, where the EQOs for shellfish harvesting and primary recreation may not always be met. Results of this study add to the understanding of regional discharge conditions and provide a practical case study for managing impacts to marine environments under a differing TWW discharge scenario, in comparison to an existing scenario.

Reef vision: A citizen science program for monitoring the fish faunas of artificial reefs

“…Tides are semi-diurnal with a low range (usually < 1 m, i.e. microtidal; Tweedley et al, 2016b) and water movement is predominantly wind-driven (Fahrner and Pattiaratchi, 1995;Dunn et al, 2014). The substrate consists of unconsolidated sediments over clay and limestone formations, which are exposed in some areas, and seagrass coverage (predominantly Posidonia sinuosa), is extensive throughout much of the bay (McMahon et al, 1997;Van Niel et al, 2009).…”

Simulation Analysis of the Dispersion of Typical Marine Pollutants by Fusion of Multiple Processes

“…Other studies [14,15] used the fully coupled global climate model CESM to ensure that the oceanic eddy processes in the prediction model are close to the current observation conditions. Ryan et al [16] used a three-dimensional hydrodynamic model and observation data to validate a far-field pollutant diffusion model in Geographe Bay, Western Australia, and determined the effect of treated wastewater on the marine environment under various emission scenarios. Souza et al [17] used the Delft3D far-field model to analyze and evaluate the sewage diffusion situation of a marine discharge outlet in Santos City under various sewage treatment conditions and discharge outlet lengths, providing highly intuitive visualization analysis results.…”

Effect of hydrodynamic conditions on the water quality in urban landscape water

“…Numerical simulation is a useful tool to test hypotheses, simulate various scenarios, analyse the influence of hydrodynamic condition on the water quality, and evaluate the regulation alternatives before project implementation (Dunn et al 2014). Xu et al (2020) used a high-fidelity hydrodynamic-eutrophication-sediment model to investigate spatiotemporal variation in sediment phosphorus retention and release in Danjiangkou Reservoir.…”