RFI - Attachment F - Engineering Report

summer months had lower suspended solid loading and high gross pollutant loading than
autumn and winter months. Winds are typically stronger in spring and autumn months in
Sydney. This observation has management implications for example. If mechanical
street sweeping is performed to lower suspended solids levels, this should be carried out
more in less windy months, alternatively if manual street sweeping is being performed to
reduce gross pollutants this should be done more frequently in windy months.

A gross pollutant vs monthly rainfall loading curve was developed. This was compared
with work carried out by the CRC for gross pollutant loads in Melbourne. A very similar
relationship was found for gross pollutants in Blackbutt creek catchment as was found in
Melbourne. However gross pollutant loads were only approximately 60% of those
encountered in Melbourne. This suggests that Blackbutt Creek has a low gross pollutant
loading.

Nutrient Analysis
The organic content of the retained material was surprisingly low in the retained material.
Only 10% of the retained material was organic. Overseas research has shown that
between 20% and 50% of suspended solids is organic. Work by the CRC in Melbourne
observed that 66% gross pollutants were organic and 80% of the material caught in a
CDS gross pollutant trap was organic. This may account for the low gross pollutant
loading discussed previously.

October was the month with the highest organic concentration. Generally organic
concentration was highest in particles over 1200 micron in size.

Surprisingly higher concentrations of phosphorous were found in larger particles. This is
contrary to the literature that suggests phosphorous is generally related to fine
particulates. It is suggested that since one of the principle source of phosphorous is from
rock and soil erosion that these particles are eroding into fine particulates as they travel in
the stormwater reticulation system.

Analysing the organic content of the retained material with the concentration of
phosphorous fund there was little relationship between them. Hawkesbury sandstones are
known to have low concentrations of phosphorous therefore it is hypothesised that runoff
and erosion from neighbouring properties are contributing to the phosphorous load in the
runoff from black but creek catchment. 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.

High concentrations of nitrogen were encountered in October. This is associated with the
higher organic loadings during this month which has been attributed to the stronger
winds. Nitrogen concentrations in retained material were found to be related to the
organic content of the retained material.




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