The Effects of Road Transport on Freshwater and Marine Ecosystems

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Deposition <strong>of</strong> airborne particulates and gaseous pollutants onto estuarine and coastal waters.
<strong>The</strong> load or flux <strong>of</strong> a given contaminant is dependent upon the nature <strong>of</strong> the contaminant and the
distance from source. <strong>Transport</strong> is dependent upon particle size, chemical and physical stability and
climatic conditions. A brief overview <strong>of</strong> the fate <strong>of</strong> vehicle emissions is provided to give a context to
the <strong>of</strong>f-site effects that may be associated with non-stormwater contributions.
5.2.2 Particulate Material
<strong>The</strong> size <strong>of</strong> particles emitted by motor vehicles can range from a few tens <strong>of</strong> nanometres to millimetres
in diameter depending upon engine operating conditions, engine condition, fuel type and other factors
(USEPA 1977). Following emission, small particulates will agglomerate to form larger particles. In
general, less than 10% <strong>of</strong> the mass <strong>of</strong> vehicle emissions particles are < 5 µm in size and 98% are <
11.5 µm in size (Ter Haar & Bayard 1971). Typically particles > 300 µm in size will settle very close to
source as a result <strong>of</strong> gravitational settling, however, turbulence can distribute large particles away from
the source. Turbulent deposition can also result in particles (e.g., 5-50 µm in size) settling close to the
roadway (Habibi 1973).
5.2.3 Local Deposition
A number <strong>of</strong> studies in New Zealand have shown that there is a depositional pattern <strong>of</strong> contaminants
adjacent to roads. Fergusson et al. (1980) showed that Pb concentrations decreased rapidly in soil
adjacent to Riccarton <strong>Road</strong> (ranging from ~1200 mg/kg at 4 m to