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The differences in the proportion of clay and silt sized particles compared to sand have important implications for the management of catchment stormwater quality. For example, investigations into the quality of road runoff are limited in Australia, with the majority of information for the selection and design of best management practice options predominantly based on research undertaken overseas. Although data are limited, the broad differences in the sediment fractions shown in the figure below highlights the potential fine nature of Australian sediment accumulated on road surfaces.
Compilation of Observed Particle Size Grading of Suspended solids transported in road runoff (Modified after Walker and Wong, 1999).
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Table 1 below gives the fraction of pollutants associated with three different particle size ranges (data from US catchments and it is reasonable to assume that the proportion of contaminants contained in the finer fractions in Australian catchments will be higher). Results show phosphates and pesticides are associated with fine silts and clays. Biological and chemical oxygen demand, volatile solids and nitrogen are largely associated with sand sized particles and heavy metals are equally associated with clay, silt and sand particles. Recent studies by Dempsey et al. (1993) and Colandini and Legret (1996) have shown that when heavy metals are examined as separate ions, a bimodal distribution is evident. Ions of different metals adsorb to different size fractions peaking at the fine sediments (<40 µm) and the fraction coarser than 100 µm.
Table 1. Pollutant fractions associated with particle sizes (Source: Sartor et al., 1974).
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Fraction of total (% by weight)
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Measured Pollutant
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< 43 m
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43-246 m
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>246 m
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Total Solids
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5.9
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37.5
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56.5
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Biological oxygen demand
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24.3
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32.5
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43.2
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Chemical oxygen demand
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22.7
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57.4
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19.9
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Volatile solids
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25.6
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34.0
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40.4
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Phosphates
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56.2
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36.0
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7.8
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Kjeldahl nitrogen
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18.7
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39.8
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41.5
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All heavy metals
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51.2
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48.7
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All pesticides
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73
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