RFI - Attachment 5 - Amended Stormwater Management Plan · MCU22/0094

 31 March 2023


The resulting percentage-based load reductions at the outlet are shown in Table 5 below together
with the Water Quality Objectives for the receiving waters.


                                                                        Residual
                                                     Sources                              % Reduction    Target %
                                                                           Load


 Total Suspended Solids (kg/yr)                         472                94.1                   80.1     80

 Total Phosporous (kg/yr)                               1.21              0.326                   73.1     60

 Total Nitrogen (kg/yr)                                  10                4.42                   55.8     45

 Gross Pollutants (kg/yr)                               71.3               3.25                   95.4     90
                                Table 5 – Percentage Based Load Reduction Results

5.4.3.8 POD Pollutant Reduction

5.4.3.8.1         Litter
Rubbish bins will be located within the on-site bin store, for use by staff and visitors. The carparks
will be cleaned regularly. As a result of this and with the installation of the treatment devices, levels
of litter exiting the site via stormwater are expected to be negligible.

5.4.3.8.2      Sediment
The TSS outflow is identified in Table 5 above. As can be seen from the above table, the pollutant
load reduction achieves the Water Quality Objectives. The TSS level is therefore considered
acceptable.

5.4.3.8.3       Nitrogen and Phosphorous
The TN and TP loads are identified in the Table 5. As can be seen, the TN and TP loads leaving the
site achieve the Water Quality Objectives. The Nitrogen and Phosphorous residual loads are
therefore considered acceptable.

5.4.3.8.4    Hydrocarbons
Hydrocarbons will become trapped within the bioretention filter media.

When there is a build-up of hydrocarbons within the bioretention basin, the top layer of the filter
media can be removed and replaced ensuring that no hydrocarbons are discharged from the site.

Studies have shown that micro-organisms in the organic mulch layer degrade and break down the
petroleum-based hydrocarbons and hence they will not be discharged from site.

5.4.3.9 Surfactants
Washing of cars is not expected on site for the proposed development, as a result surfactants
should not be an issue for the drainage system.

5.4.3.9.1      Heavy Metals
Heavy metals in stormwater runoff generally become attached to fine sediment. The bioretention
basin will remove the majority of this fine sediment. The removal of the fine sediment should
effectively remove most of the heavy metals in the runoff.



 Excellence with Economy in Engineering                                                                             12
 https://mpncnetau.sharepoint.com/sites/administration/shared         documents/9000-9999/9300-
 9399/9307/reports/31-03-23 stormwater management plan_issue d.docx