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The proposed stormwater quality treatment in this precinct comprises two or more bio-fitration
systems. A typical arrangement is shown in Figure 8. These systems comprise a layer of
vegetated topsoil overlying a bio-filiration medium such as 5mm single sized gravel, which is
drained by sub-soil perforated plastic drainage pipes. Non-woven geotextile fabric is placed
between layers to prevent the migration of fine particulate matter.

 

There are a number of mechanisms at work within bio-filtration systems; the vertical downward
flow above ground provides enhanced sedimentation processes for the removal of suspended
solids and nutrient removal is implemented by vegetation roots and bio-films formed within the
bio-filtration media. :

The proposed development was modelled using AQUALM, using the “urban” land use for this
precinct. The annual volumetric runoff coefficient for this land use was 0.39, which agrees well
with the value for “urban” land use in the Brisbane City Council Guidelines.

The results of the AQUALM model are summarised in Table 7.4.1

Table 7.4.1 : West Visitor Precinct - Summary of Results of AQUALM Analysis

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Median Existing 1.5 0.35
Pollutant
Concentrations Post-development no 9.0 0.15 0.12
(mg/L) treatment
Post-development with 0.0 0.0 9.00
treatment
80%ile Pollutant Existing 2.2 0.36 0,01
Concentrations
(mg/L) Post-development no 9.0 0.15 0.12
treatment
Post-development with 2.6 0.52 0.02
treatment
Pollutant : Existing 39 4.2 0.05
Exports
({kg/halyr) Posi-development no 663 12.0 1.7
treatment
co Post-development with 107 4.2 0.4
treatment

 

 

 

 

 

 

The pollutant exports (expressed as kg/ha/yr) are approximately 50% higher than the values
indicated in the Brisbane City Council Guidelines (TS = 471 kg/ha/yr, TN = 8.56 kg/ha/yr, TP =
1.203 kg/ha/yr).

The median pollutant concentrations may be compared with Brisbane City Council’s water
quality objectives for freshwater — modified eco-systems (TS = 15mg/L, TN = 0.65mg/L, TP =
0.07mg/L). The median concentrations discharged from the lake are zero, which is considered

"*  — to reflect the extended median retention time within the lake. The AQUALM model allows for
evaporation in the lake, and it was found that there are extended periods when the lake does
not overtop the outlet weir, resulting in these extended retention times.

The 80%ile pollutant concentrations may be compared with ANZECC/ARMCAWNZ trigger values
for slightly to moderately modified eco-systems. Trigger values indicate the upper limits for
sustainable pollutant loads on natural eco-systems. For discharges into tropical wetland
systems, these trigger values are TN = 1.2mg/L and TP = 0.05mg/L. The AQUALM analysis
demonstrates that the trigger values for TN’and TP are not exceeded, which indicates that the
proposed development discharges sustainable pollutant loads into the wetlands.

 

PETRAC LAND HOLDINGS P/L Noosa North Shore Eco-Tourism Portal September 2003
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