The Effects of Road Transport on Freshwater and Marine Ecosystems

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Figure 5.1 - Percentage <strong>of</strong> 0.5 M HCl extractable elements in road surface sediments from
Moana Basin, Hawaii (From Sutherland & Tolosa 2000).
<strong>The</strong> proportions present in the various phases, provides some information about the environmental
mobility under those experimental conditions and probably the broad environmental mobility. <strong>The</strong>
general mobility does not directly relate to the proportion present in the dissolved phase. Examination
<strong>of</strong> the dissolved data presented earlier suggests that for Pb and Cd, the exchangeable fraction may
control much <strong>of</strong> the dissolution process for these elements with some contribution from the carbonate
phase. For Zn the exchangeable phase does not contribute much <strong>of</strong> the Zn seen in storm events and
it is likely that this is sourced from carbonate and oxide phases. For Cu, the dissolved copper is likely
to come from both organic and oxide phases in street surface particulates. Revitt & Morrison (1987)
concluded following an assessment <strong>of</strong> speciation <strong>of</strong> key metals through the stormwater system that
the proportion considered bioavailable were 59%, 53% 38% and 5% for Cd, Zn, Cu and Pb. <strong>The</strong>
implications <strong>of</strong> the relative bioavailability, is discussed further in the following sections on effects in
freshwaters and marine waters.
Overall, geochemical processes on the road surface affect the behaviour and form <strong>of</strong> inorganic
materials, compounds and metals on the road surface before and during storm events. <strong>The</strong> key factor
is the affect <strong>of</strong> low pH (acidic) rainwater and the dissolution processes that move metals from solid
phase to dissolved phases in stormwater. Although the exact processes are not known for all key
metal contaminants, available information indicates that while on the road surface metals undergo
phase re-distribution.
Organic Compounds
VOCs will be present in rainfall in concentrations that reflect their concentration in air and ambient
temperature. VOCs will undergo processes whilst in urban air through reactions with other
atmospheric constituents, such as hydroxyl radicals, which result in degradation. Volatilisation <strong>of</strong>
VOCs from street surfaces and particulates is temperature dependent. In a reversal <strong>of</strong> this physical
process, the atmosphere may also contribute VOCs to water, especially in cold weather. Air/water
partitioning <strong>of</strong> VOCs is described by their Henrys Law constant (H) which identifies the ratio <strong>of</strong> the
partial pressure <strong>of</strong> the compound in the gas phase to the concentration in the water that is in
equilibrium with the atmosphere. A second dimensionless constant (H/RT) is derived using H, gas
constant and temperature which provides the ratio <strong>of</strong> air to water concentrations at equilibrium. A
VOC with an H/RT value <strong>of</strong> 0.05 or larger will be volatile when in water (e.g., BTEX in Table 3.2).
Compounds such as MTBE which have lower H/RT values will tend to partition from the gas phase
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