The Effects of Road Transport on Freshwater and Marine Ecosystems

119
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 that no water column toxicity after discharge. It was noted in that 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 road and urban catchments, discharges 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
are derived in part from vehicle emissions being a causal agent in toxicity or changes in benthic biological
community composition.
<strong>The</strong>re are currently no specific tools for isolating the effects <strong>of</strong> roadway stormwater discharge on the
marine environment. However, in the case <strong>of</strong> isolated roadway discharges, traditional quantitative
sampling and assessment tools that assess abundance, diversity and other metrics are capable <strong>of</strong>
identifying effects <strong>of</strong> discharges (variability permitting).
Information in New Zealand has shown that build-up <strong>of</strong> contaminants occurs adjacent to urban
stormwater outfalls discharging to the marine environment and in intertidal areas adjacent to roads.
Most studies carried out to-date in New Zealand have dealt with the identification <strong>of</strong> possible effects <strong>of</strong>
urban run-<strong>of</strong>f on coastal ecosystems. <strong>The</strong> studies have indicated that urbanisation as a whole results in
adverse changes to the diversity <strong>of</strong> the biological communities present. Further specific research would
need to be carried out to isolate the potential effects <strong>of</strong> road and highway run-<strong>of</strong>f by carrying out effects
studies in locations where run<strong>of</strong>f from roads and highways was isolated from the confounding effects <strong>of</strong>
general urban run<strong>of</strong>f. If the adverse effects arising from urban run<strong>of</strong>f are attributable to elements such as
Zn, then it is likely that roads and motor vehicles will contribute their proportional share <strong>of</strong> the adverse
effect observed. <strong>The</strong> proportion <strong>of</strong> that effect will vary from location to location. At this stage the
proportion <strong>of</strong> any observed effect is not known (even when identified on a contributed load basis).
Kingett Mitchell Ltd
Resource & Environmental Consultants