Sealed Judgement

The results demonstrate that the proposed treatment will achieve best practice objectives for
Southeast Queensland specified in the State Planning Policy 2017, namely:

     o   80% Total Suspended Solids reduction;
     o   60% Total Phosphorus reduction;
     o   45% Total Nitrogen reduction; and
     o   90% Gross Pollutants reduction



In this case, it is desirable to minimise the discharge of pollutants to match the pre-development
case. While the modelling indicates that the system will match the pre-development load for the
majority of parameters, the Total Nitrogen load discharged from the Site is greater in the post-
development case than the pre-development case.

Reviewing the results of modelling, it is considered that this could be affected by the default
discharge concentration from bioretention systems. It is also noted that the analysis is conservative
as it does not allow for the potential for exfiltration to occur. Table 4-5 presents the results obtained
with the use of an exfiltration rate of 115 mm/h. This rate was obtained by field testing completed
at the site.

By including exfiltration, the Total Nitrogen load reaches the pre-development load.

Based on this, it is considered that discharge from the Site is acceptable with respect to pre-
development and post-development loads.




                                                                              R.30199.001.03_SMP.docx | 23




                                                                                                             173