RFI - Attachment F - Engineering Report

Nutrient Analysis
Organic Matter

Organic concentrations encountered in the Blackbutt catchment were surprisingly low.
On average only 10% of the retained material in the Blackbutt creek was organic. There
has been a significant amount of research on organic content of stormwater contaminants.
Some of the organic concentrations observed in other studies are as follows:
    • Ellis found that 30 to 55% of road sediments are organic,
    • Kinglet Mitchell (ARC 1992) found that approximately 20% of suspended solids
       in stormwater were organic.
    • Lenhart (SMI) found that 50% particles retained in stormwater is organic.
    • Allison found that 66% of the mass of gross pollutants is organic.
    • Allison also found the 80% of the material caught in a CDS in Melbourne was
       organic matter.

The low organic content may account for the low gross pollutant loading discussed
previously.

               Commercial Area               Urban Areas           Bottom of Catchment
             Sub1200     Over 1200      Sub1200       Over 1200   Sub1200      Over 1200
  April         NA            NA            NA             NA         NA           NA
  June         3.5%           2%           6.7%           5.9%       3.7%         3.7%
 August        7.7%          5.5%          6.6%          10.5%       5.5%         11.8%
 October       13.3%         19%          15.9%         24.4%        5.1%          22%
December       5.5%          3.8%          8.2%          8.4%       3.7%          8.5%
 Average       7.50%        7.58%         9.35%         12.30%      4.50%        11.50%

                             Table 5 Organic content of samples


Kinglet Mitchell (ARC 1992) found that there is a general trend of increasing
concentrations of organic carbon in finer sediment in gutter dust. However higher
concentrations are found in coarser particulates in stormwater. This is possibly a
reflection of the floatable nature of plant debris.

Further, Kinglet Mitchell found catchments of low traffic movements have a higher
percentage of organics in coarser particles. This was also encountered in Blackbutt creek
with high organic carbon concentrations in particles over 1200 microns in the urban or
base catchments. This is because large organic particles such as leaves are easily
mechanically broken down by vehicle movement.

Table 5 details the observed percentage of organic material in the retained material
highest percentages were encountered in the month of October. This is assumed to be
because of the stronger winds in his month as discussed previously



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