The Effects of Road Transport on Freshwater and Marine Ecosystems

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grazers such as mud flat snails would result in them accumulating sufficient Pb that they should not be
consumed.
7.7 Summary
Stormwater discharges from roads and urban catchments typically discharge particulate material that
settles adjacent to the stormwater outfall. <strong>The</strong> presence and extent <strong>of</strong> the distribution <strong>of</strong> that material is
dependent upon the hydrodynamics <strong>of</strong> the environment at the point <strong>of</strong> discharge. If the physical
environment is depositional in nature then material may accumulate adjacent to the outfall. <strong>The</strong>re is
potential for that material to cause some local change in sediment physical characteristics and hence
habitat. <strong>The</strong> source <strong>of</strong> the material is principally the wear <strong>of</strong> road surface material not motor-vehicles.
<strong>The</strong> discharge <strong>of</strong> urban and roadway stormwater has the potential to cause changes in the quality <strong>of</strong> the
sediment environment immediately adjacent to the point <strong>of</strong> discharge. <strong>The</strong> extent <strong>of</strong> the area is
dependent upon the size <strong>of</strong> the discharge and the nature <strong>of</strong> the discharge environment (e.g., estuary or
open coast). <strong>The</strong> ‘halo’ around the discharge location may be such that it results in a depauperate fauna
consisting <strong>of</strong> hardy or opportunistic species. <strong>The</strong> change in faunal composition appears to be caused by
changes in the physical composition <strong>of</strong> the sediment coupled with changes in the chemistry <strong>of</strong> the
sediments (ammonia, sulphide etc.,). Examination <strong>of</strong> contaminant concentrations immediately around the
discharge points for stormwater run<strong>of</strong>f from roads and urban areas shows that concentrations <strong>of</strong>
contaminants in sediments are typically elevated. Concentrations in both intertidal and subtidal
sediments can rise above sediment quality guidelines and triggers. For most smaller discharge points
haloes <strong>of</strong> metal concentrations are generally restricted to 10-20 m.
In the wider areas <strong>of</strong> urban estuaries and harbours sediments <strong>of</strong>ten contain elevated concentrations <strong>of</strong>
metal and organic contaminants. In locations such as the Waitemata Harbour it appears that all <strong>of</strong> the
surface sediments in harbour are contaminated to some degree. In depositional environments the
concentration <strong>of</strong> elements such as Cu exceed sediment quality guidelines. Within New Zealand such
effects are likely to be restricted principally to depositional environments adjacent to urban areas.
However, stormwater discharges from roads and smaller urban centres to smaller estuaries will have
comparable effects but on a smaller scale. Overall, the contribution that motor vehicles have to sediment
contamination is identifiable where the discharge is derived from an isolated road. In urban areas, the
contribution from vehicles to sediment contamination adjacent to outfalls and within estuaries and
harbours will be dependent upon the ratio <strong>of</strong> the loads contributed by vehicle sources and other urban
sources and activities.
Toxicity testing carried out using stormwater solids and marine organisms in New Zealand has shown that
the toxicity <strong>of</strong> stormwater is variable. <strong>The</strong> testing indicated that some dilution <strong>of</strong> stormwater sediments
would be required to ensure no water column toxicity after discharge. It was noted in that a study that
toxicity was lower than expected. Contaminants other than metals were suggested as having a strong
involvement in toxicity.
Work undertaken in the United States has shown that toxicity to sensitive marine species can occur in
nearshore coastal areas in situations where stormwater discharges are not diluted sufficiently. Studies
have determined that a dilution <strong>of</strong> typically 90% is required to eliminate toxicity to sensitive species. As
such, toxicity may occur in locations where dilution is poor. Such locations include embayments and
estuaries in harbours and the estuaries <strong>of</strong> rivers. Given the relatively small size <strong>of</strong> most New Zealand
road and urban catchments, discharge volumes from open coastal stormwater outfalls are typically small.
Toxicity testing <strong>of</strong> stormwater sediments has shown that adverse effects can be obtained in tests. This
would suggest that adjacent to outfalls where deposited sediment may accumulate toxicity may occur.
<strong>The</strong> toxicity may arise from the elevated concentrations <strong>of</strong> key contaminants such as Zn. Adverse effects
may also be due to elevated concentrations <strong>of</strong> sulphide and ammoniacal nitrogen in those sediments or
other constituents.
In depositional areas such as estuaries and harbours in New Zealand, where sediment contaminant
concentrations are elevated, there is no direct evidence <strong>of</strong> sediment related toxicity. <strong>The</strong>re does not
appear to be any evidence at the present time that elevated concentrations <strong>of</strong> key elements (e.g., Zn that
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