The Effects of Road Transport on Freshwater and Marine Ecosystems

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A variety <strong>of</strong> physical and chemical processes occur in catchpits during storm events. Morrison et al.
(1988) summarised these as dilution, dispersion, sedimentation, sediment bed build-up and erosion,
washout <strong>of</strong> suspended and dissolved pollutants from the pot liquor and re-aeration <strong>of</strong> the pot liquor.
During stormwater run<strong>of</strong>f events, the stormwater entering the gullypot can cause changes in pH, ionic
strength, dissolved organic carbon and suspended solids concentrations and alterations in these
parameters will influence metal speciation (especially bioavailable and toxic forms) (Morrison et al.,
1984).
Detailed studies <strong>of</strong> roadside gullypots have identified their potential as pollutant sources which can
contribute significantly to the deterioration <strong>of</strong> stormwater run<strong>of</strong>f quality (Fletcher et al., 1978; Mance &
Harman, 1978). Fletcher & Pratt (1981) have shown that the liquid in the catchpit sump may
contribute up to 22% <strong>of</strong> the suspended solids in the total outfall run<strong>of</strong>f for individual storm events. <strong>The</strong>
gullypot contents can undergo significant biogeochemical changes, particularly during prolonged dry
periods. <strong>The</strong> key change that affects geochemical processes in the sump is the generation <strong>of</strong>
anaerobic conditions. Anaerobic conditions in the sump can lead to the release <strong>of</strong> soluble organic
compounds which will influence the distribution <strong>of</strong> metal species between the dissolved and insoluble
phases; changes in the speciation <strong>of</strong> metals and the conversion <strong>of</strong> oxidised forms <strong>of</strong> dissolved
inorganic nitrogen to ammonical-nitrogen. During this time microbial decomposition <strong>of</strong> organic matter
such as leaves in the sump continues leading to increases in the concentration <strong>of</strong> dissolved organic
compounds in the sump liquid. Morrison et al. (1988), Karunaratne (1992) and others have identified
that between storm events, the catchpit sump behaves like a batch reactor. Memon & Butler (2002)
examined processes in catchpit sumps. <strong>The</strong>ir examination <strong>of</strong> catchpit sumps showed that dissolved
oxygen dropped within the sump as biochemical/biological processes used up the oxygen such that by
about 10 days the DO concentrations in winter were 2-3 g/m 3 . During summer, concentrations would
be expected to fall to zero. Ammoniacal nitrogen concentrations are typically higher in summer
compared to winter and rise with time following the cessation <strong>of</strong> the prior storm event. Concentration
differences are greatest for catchpits containing sediment (with organic matter which releases DIN
through biological processes) (Memon & Butler 2002).
Morrison et al. (1988) summarised that although, the dissolved metal concentrations increase in the
supernatant catchpit sump waters between storms, the pattern <strong>of</strong> increase can <strong>of</strong>ten be irregular due
to an increasing tendency as the pH <strong>of</strong> the sump liquid increases over time for metals to adsorb onto
solids in the sump. Morrison et al. (1988) noted that Cd behaviour differs in that dissolved Cd tends to
increase to equilibrium over time. Morrison & Revitt (1987) and Revitt & Morrison (1987) discuss
speciation and chemical changes in the stormwater and catchpit.
Overall, catchpits play a role in the biogeochemical conversions that street surface particulate material
is exposed to. Material trapped in catchpit sumps can be exposed to anaerobic conditions for days
between storms. <strong>The</strong> exposure to anaerobic conditions results in conversion <strong>of</strong> DIN to ammoniacal
nitrogen, release <strong>of</strong> some metals to dissolved phase. <strong>The</strong> flushing <strong>of</strong> catchpit contents during storms
results in a pulse <strong>of</strong> water with different chemical composition entering the downstream stormwater
system and entering the receiving environment. <strong>The</strong> key implication <strong>of</strong> changes in chemistry from the
road to the receiving environment is any potential to transform solid phase non-toxic contaminant
phases into dissolved potentially toxic forms. This is discussed further in the following sections.
5.4 Freshwaters
5.4.1 Introduction
Following transport through the stormwater system, the stormwater may be discharged into a
freshwater stream, river or lake. Following entry into the freshwater environment, the materials carried
in the stormwater are subject to a range <strong>of</strong> physical, biological and chemical processes. <strong>The</strong> two key
processes which influence stormwater in the receiving water are dispersion (and dilution) and
settlement <strong>of</strong> particulate matter.
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