The performance of stormwater quality improvement ,measures, such as wetland systems and detention ponds, in the removal of particulate contaminants is intrinsically related to the characteristics of the suspended solids, particularly sediment, inputs to the system. Sediment-bound pollutants are known to have a higher association with the finer range of sediment sizes and targeting the removal of the appropriate particle size fraction is a necessary design consideration in the appropriate selection and sizing of stormwater quality improvement measures.
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.
Figure 1.
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Compilation of Observed Particle Size Grading of Suspended solids transported in road runoff (Modified after Walker and Wong, 1999)
The band representing other (non-Australian) particle size distributions of suspended solids, shown in the figure above, comes from a variety of sources using a range of different sampling and analytical techniques (See Walker and Wong, 1999, for reference details of overseas distributions). Sampling techniques from these other studies involve a variety of grab sampling and automatic sampling methods, and a variety of particle size analytical methods, including sedimentation analysis, sieving and light diffraction techniques. In spite of the different sampling and analytical techniques, a distinct band of particle size distributions exist. The non-Australian studies that included solids larger than 500 µm in their derived particle size distribution were re-distributed using 500µm as the upper limit to allow a consistent basis of comparison between the data. The adopted 500 µm upper limit assumes that particles larger than 500 µm would be predominantly conveyed as bedload rather than suspended solids in stormwater. Results show less than 25% of the suspended solids collected in Europe and the United States of America were finer than 100 µm whilst 40% or more of suspended solids in Australia were found to be finer than 100 µm.
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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)
