The Effects of Road Transport on Freshwater and Marine Ecosystems

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Catchpits
Between the street surface and the stormwater system are catchpits (<strong>of</strong>ten referred to as gully pots or
traps). <strong>The</strong> catchpits <strong>of</strong> which there are very large numbers in the urban stormwater drainage system
function is to trap particulate debris and prevent it from entering the stormwater system. <strong>The</strong> base <strong>of</strong>
the catchpit usually remains filled with water between storm events.
<strong>The</strong> quality <strong>of</strong> the catchpit water has the potential to affect stormwater quality. Memon & Butler (2002)
provide an overview <strong>of</strong> catchpit water quality dynamics (particularly as it relates to water quality
management). As catchpits retain stormwater between storm events, their chemistry changes over
time as a result <strong>of</strong> changes in the redox chemistry (i.e., the availability <strong>of</strong> dissolved oxygen) and the
biological activity within the catchpit (e.g., decomposition <strong>of</strong> organic matter). <strong>The</strong> key changes in
catchpit water quality are decreases in dissolved oxygen concentrations, increases in dissolved solids,
BOD 5 (e.g., 13.7 mg/kg), chemical oxygen demand (COD), nitrate (e.g., 9.6 g/m 3 ) and ammonia (e.g.,
1.39 g/m 3 ) (see Fletcher et al. 1978; Mance & Harman 1978).
Changes in the concentration <strong>of</strong> dissolved metals would also be expected. <strong>The</strong> relative change
between particulate and dissolved metals in catchpit waters will depend upon the pH and redox
conditions in the catchpit (Fletcher et al. 1978). As the catchpit water is replaced by incoming
stormwater during the storm event, the quality <strong>of</strong> the catchpit water has the potential to influence the
quality <strong>of</strong> the initial volume <strong>of</strong> stormwater discharged to the receiving water. <strong>The</strong> actual change in
stormwater quality is dependent upon the ratio <strong>of</strong> catchpit water to stormwater. It is likely for a small
storm event that the catchpit is an important volume <strong>of</strong> the overall storm (e.g., 10% or greater).
First Flush
First flush is a phenomenon that may occur during the early phase <strong>of</strong> stormwater events when the first
passage <strong>of</strong> water carries material in the “wave” through the stormwater system. <strong>The</strong> first flush can result
in substantial rises in the concentration to a peak in the early phase <strong>of</strong> the storm event. <strong>The</strong> peak in
concentration is however highly variable between contaminants, between storm events and between
catchments as the first flush is dependent upon the availability <strong>of</strong> materials and the size <strong>of</strong> the rain fall
event (Bertrand et al. 1998, Deletic 1998, Lee & Bang 2000, Lee et al. 2002). First flush characteristics
can also be influenced by catchpit contents (that may be flushed from the catchpit) and material
accumulated in stormwater pipes (that may be eroded).
<strong>The</strong> first flush is a characteristic <strong>of</strong> storm events derived from roadways as the road surface accumulates
particulates and contaminants that are subsequently transported through the sewer system. It is a
feature <strong>of</strong> all impervious surfaces. <strong>The</strong> first flush becomes important to road transport management if
vehicles contribute to the presence <strong>of</strong> contaminants that become an environmental issue in the first flush.
This is discussed further in Section 6 <strong>of</strong> this report.
In summary, the quality <strong>of</strong> catchpit water differs from that <strong>of</strong> stormwater running <strong>of</strong>f roadways. <strong>The</strong>
differences are attributable to processes that occur in the catchpit between storms. <strong>The</strong>se water
quality changes have the potential to influence receiving water quality when catchpits are flushed
during a storm.
4.8 Summary
<strong>The</strong> quality <strong>of</strong> stormwater generated in urban areas and motorways is a function <strong>of</strong> a number <strong>of</strong> factors
the principal influences being rainfall composition, road surface composition, gutter dust accumulation
and chemistry.
Most road surfaces are constructed using bitumen. Some contribution to metal concentrations is
likely. PAH concentrations appear low. Little data appears available on the hydrocarbon losses from
newly constructed bitumen roads.
Kingett Mitchell Ltd
Resource & Environmental Consultants