LATE RFI Attachment 6 - Stormwater Management Plan

In summary, Table 1 shows there is a high degree of compliance with SQIDEP v1.3. It is
considered unlikely that the rainfall gauge was calibrated a second time during the analysis
however the Independent Evaluators have assessed the significance and risk of a non
conformance as low. In addition, using historical rain radar records we have verified that
field test pluviograph rainfall records generally match radar records.

A number of other checks on the data have been performed and are reported below.

Comparison of Inflow Concentrations
Influent concentrations are impacted by a range of factors including antecedent conditions
and catchment activity. Antecedent conditions allow accumulation of pollutants between
events and it is possible to examine reported influent concentrations to identify indicative
trends.

The inflow concentrations from this study were compared to previous studies of road
catchments for cross-reference. In particular, the pollutant concentrations of TSS, TP and
TN were extracted from Duncan (1999) which examined 42 (road) sites across Australia. A
follow-up study, and one that is in close proximity to Sippy Downs was conducted by
Drapper and Lucke (2015) for catchments within the South-East Queensland region. The
pollutants concentrations from both studies are summarised below along side the inflow
concentrations found at Sippy Downs (Table 1). Full graphs are shown in Appendix A.

The most noticeable point between the studies is the pollutant concentration range. Drapper
and Lucke (2015) cited that the inflow concentrations observed in that study were
significantly different to results of Duncan (1999). And similarly, the Sippy Downs
concentration ranges vary differently to those of the comparison studies, however they are
still considered realistic. This highlights the difficulty of quantifying pollutant runoff
parameters, and consequently, modelling inflows. It is noted that Sippy Downs appears to
be on the low end of the spectrum which would yield a conservative result. Any MUSIC
generic node developed from this Application would be applicable to both clean and dirty
sites.

We also note mean TSS influent concentrations, at 147mg/L are about 50% of default
MUSIC road EMC values but not untypical for a new well sealed road, mean TN
concentrations at 1.72 mg/L are not far off typical MUSIC default values at 2.2 mg/L while
the TP loads were considered to be about 33% of default MUSIC values for a sealed road,
i.e. low.

Table 1: Typical pollutant concentrations for road catchments

                           Duncan (1999)        Drapper and Lucke         Current study –
                              study                (2015) study            Sippy Downs

 TSS (mg/L)                60 – 700 (n=42)      1.45 – 5800 (n=325)       16 – 1130 (n=19)

 TP (mg/L)                 0.1 – 0.8 (n=25)       0.08 – 26 (n=325)        0.03 – 1 (n=19)

 TN (mg/L)                   1 – 9 (n=17)         0.38 - 8.5 (n=325)       0.2-8.9 (n=19)