Effects of proposed bioretention system on existing waterbird habitat

As stormwater quality becomes increasingly central to urban development, planning for solutions is necessary. With increased development and finite free space, Toowoomba Regional Council (TRC) has recognised a need for efficient stormwater quality treatment measures. For this reason, this study seeks to anticipate the effectiveness of a stormwater quality treatment system currently being considered by TRC. Upstream of Toowoomba’s existing Waterbird Habitat, TRC has proposed a bioretention system that is hoped will offset a rise in pollutants due to increased development further upstream. Little is known about the current water quality within the Waterbird Habitat, or the impact a bioretention system will have on it. Thus, this project seeks to determine the Waterbird Habitat’s current water quality, and the effects that a bioretention system would have on this water quality. The findings of this report are based on existing literature and measured and modelled water quality results and is hoped to assist TRC in making informed decisions on the implementation and design of the bioretention system. Water samples were taken to obtain an understanding of the current state of the Waterbird Habitat. From here, the process of modelling the effectiveness of the bioretention system included defining the catchment area, defining the areas of future development, defining the basic bioretention properties, and then modelling all of this in a program called MUSIC – Model for Urban Stormwater Improvement Conceptualisation. The sample results gave approximate pollutant levels and were used to calibrate the values used in MUSIC, giving added confidence in the model. These pollutant levels were not found to be excessive or higher than expected. The MUSIC program was used to model the differing states of the Waterbird Habitat – i.e. both as it currently is and what as it will be with the inclusion of future development and a bioretention system. Results from the model provided a comparison of Total Suspended Solids, Total Nitrogen & Total Phosphorus in both the amount being removed and the resulting concentrations within the Waterbird Habitat. It was found that the proposed bioretention system worked to decrease the pollutant levels in the Waterbird Habitat to less than or equal to pre-development levels. This gives some assurance that the bioretention system will help to improve stormwater quality for East Creek. It was also found that the South East Queensland percentage reductions targets of TSS 80%, TP 60% and TN 45% were not met by either the proposed bioretention system or a much larger bioretention system scaled to more than 4 times the originally proposed size. From the results that were produced within this report, it is recommended that the proposed bioretention system will be effective in the purpose of treating future increase in stormwater pollutants, and will increase the quality of the water within the Waterbird Habitat to a level higher than it is currently.