The Effects of Road Transport on Freshwater and Marine Ecosystems

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8.5 Freshwater environments
In New Zealand there are few specific studies that have assessed the effect <strong>of</strong> stormwater from roads or
highways in the absence <strong>of</strong> the confounding that could arise from other urban stormwater sources (e.g.,
industrial sites, residential properties, ro<strong>of</strong> run<strong>of</strong>f).
First flush stormwater run<strong>of</strong>f from roads exposes organisms to very high concentrations <strong>of</strong> contaminants.
A range <strong>of</strong> toxicity studies have been undertaken using urban stormwater discharging to freshwater
environments. Tests undertaken using Microtox have shown that for run<strong>of</strong>f parking areas, 50% <strong>of</strong>
samples are toxic and for highways (bridge), 20% <strong>of</strong> samples were severely toxic. Studies involving
freshwater algae have shown that nutrients in run<strong>of</strong>f may stimulate growth but that metals in stormwater
may inhibit growth. Work in New Zealand has shown that urban stormwater has chronic effects on
growth <strong>of</strong> freshwater algae (in toxicity tests).
Toxicity tests have undertaken using a range <strong>of</strong> freshwater organisms and have not demonstrated
consistent toxicity. Work undertaken in the United States using general urban run<strong>of</strong>f has shown toxicity
with some studies indicating greater toxicity in the first flush compared to flow weighted stormwater
samples. This observation reflects data for the concentration <strong>of</strong> key constituents in first flush stormwater
compared to whole storm quality. <strong>The</strong> toxic agent in some US stormwater toxicity studies has been
identified to be organic compounds (in some cases pesticides) but it is likely that dissolved metals are a
contributor or in some cases the causal agent <strong>of</strong> toxicity
Urban stormwater testing has shown that a moderate proportion <strong>of</strong> all stormwater samples demonstrate
toxicity to a fish or invertebrate species. <strong>The</strong> degree <strong>of</strong> toxic effect varies between species with some
studies showing toxicity to species such as Ceratodaphnia dubia and other not. <strong>The</strong> degree <strong>of</strong> variation
is not surprising, as toxicity and the degree <strong>of</strong> toxicity will vary between catchments and locations
depending upon a large number <strong>of</strong> factors. Given the nature <strong>of</strong> stormwater run<strong>of</strong>f from roads, there are
limitations in the representativeness <strong>of</strong> toxicity tests arising from the nature <strong>of</strong> the stormwater sample.
However given the results <strong>of</strong> Microtox testing on parking lot and isolated road run<strong>of</strong>f some toxicity is likely
to be associated with road run<strong>of</strong>f. Several studies have shown moderate to sever toxicity in 20-50% <strong>of</strong>
samples. Freshwater organisms below road stormwater outfalls are exposed to pulses <strong>of</strong> contaminants
following rainfall events. <strong>The</strong>y may be exposed to a first flush containing significant concentrations <strong>of</strong>
some contaminants. This may be followed by lower level prolonged exposure.
Build-up <strong>of</strong> sediment discharged from roadways will occur close to the point <strong>of</strong> discharge (in ponds,
wetlands, streams and lakes) if the discharge point does not have sufficient energy to transport the
contaminants and sediment away. Based upon the composition <strong>of</strong> the suspended sediment and
measurement <strong>of</strong> urban stream sediment quality, it is likely that concentrations <strong>of</strong> contaminants can buildup
to concentrations that could have an adverse effect on freshwater benthic organisms.
It is difficult within urban areas to isolate the effects <strong>of</strong> road transport (via stormwater) to be identified
separately from other urban activities. In remote or rural environments, the tools utilised to assess the
condition <strong>of</strong> the biological community can be applied to assess the effects <strong>of</strong> road transport. In certain
situations an upstream-downstream assessment (<strong>of</strong> a discharge) may provide sufficient information to
assess effects. Given the concentration <strong>of</strong> Cu, Pb and Zn in the suspended sediments <strong>of</strong> stormwater
draining roads, exceedence <strong>of</strong> sediment quality guidelines would be expected. Depending upon the ratio
<strong>of</strong> stormwater sediment to uncontaminated stream sediments, elements such as Zn may exceed
guidelines at which adverse effects (on benthic organisms) could be expected to occur. In urban areas
however, streams may not receive their normal supply <strong>of</strong> sediments and as a result sediments derived
from roadways and other urbanised sources may dominate the sediments in the stream. For lakes, the
sediment concentration will depend on the distribution and build-up <strong>of</strong> contaminants in the lake bed
following stormwater discharge to the lake. Contaminant build-up in small lakes with low natural sediment
budgets would be expected. Available data indicates that contaminant concentrations in lake sediment
adjacent to stormwater discharges from roads can approach and exceed sediment quality guidelines.
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