RFI - Attachment F - Engineering Report

                                          TP Concentration vs Organic Content

                              1600
   TP Concentration (mg/kg)




                              1400
                              1200
                              1000
                              800
                              600
                                                                                   y = 2588.3x + 423.54
                              400
                                                                                        R2 = 0.4279
                              200
                                0
                                     0%       5%         10%           15%       20%          25%     30%
                                                            % Organic Content


                                            Figure 15: Total Phosphorous Total Organic Load


Examination of phosphorous in stormwater shows that TP concentrations are related to
suspended solids concentrations (Kinglet Mitchell, CRC). However this is not necessary
organic material. While organic material contains phosphorous, often there majority of
phosphorous is generated by surface erosion. Weaver Et al

Phosphorous is a natural constituent in rock. Often dissolved phosphorus is created by
erosion. In Australia evidence has indicated that phosphorous arises from soil erosion
and is usually transported in episodic events (weaver et al), this was observed in large
storms during the study period. However Hawkesbury sandstones, lower in the
catchment are known to contain very low concentrations of phosphorus. Therefore it is
hypothesised that soil erosion from neighbouring pervious and landscaped garden areas
which drain to the Enviropods are contributing to the phosphorous load in the runoff.
Educational activities could be carried out on neighbouring property owners to encourage
them to minimise erosion on there properties in addition council should also ensure
erosion of soils in not occurring within the road berms.

Overland flow has the capacity to transport significant quantities of phosphorous in
various forms, while some of this is dissolved moving directly with the water, in some
catchments a much larger flux’s of phosphorous can occur via eroded particulate
material. In the Blackbutt Creek the steep slope would affect the overland flow velocity
and therefore the amount transported. This is also observed in Figure 16 where high
loads are experienced in months where there is high peak intensity (June and October). A
high intensity rainfall event will cause higher velocities of overland flow and therefore
greater erosion.




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