The Effects of Road Transport on Freshwater and Marine Ecosystems

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54 5.4.2 Dispersion Dilution and dispersion results in a decrease in the concentration <strong>of</strong> constituents in the stormwater present at a concentration greater than in the receiving water. <strong>The</strong> degree <strong>of</strong> dilution is a function <strong>of</strong> the stormwater flow, the size and flow <strong>of</strong> the receiving water. Dilution and dispersion in stream, river and lake environments can be modelled and assessed. It should be noted however, that in many urban environments, stormwater flow could make up all <strong>of</strong> the flow in the water body. 5.4.3 Settlement Settlement occurs in two phases. <strong>The</strong> first is the deposition <strong>of</strong> large particles (sand and gravel etc.,) by gravity immediately adjacent to the discharge location. <strong>The</strong> second is the settlement <strong>of</strong> finer particles in depositional environments downstream such as wetlands and slow-flowing areas (in the case <strong>of</strong> streams) to settlement further away from the discharge in the case <strong>of</strong> lakes. <strong>The</strong> amount and nature <strong>of</strong> settlement will depend upon the size distribution <strong>of</strong> suspended and bed load in the stormwater discharge. 5.4.4 Other Processes Once in the receiving environment other processes influence the fate <strong>of</strong> contaminants. <strong>The</strong>se include relatively short term processes (noted in Section 3.3) such as volatilisation across the water-air interface, sorption (a function <strong>of</strong> sediment type, organic carbon content etc.), hydrolysis (reaction with water) and photolysis (reaction with light) and longer term processes such as bio-degradation (bacterial metabolism and detoxification). Particulate contaminants which settle may become re-introduced into the environment via resuspension and bio-turbation (the disturbance and movement <strong>of</strong> sediment by biological activity such as burrowing and feeding). 5.5 Estuarine and Marine Waters 5.5.1 Introduction Stormwater discharged into the estuarine or marine environment is subject to similar processes to those described for freshwaters in the preceding section. Dispersion and dilution are site specific factors that are determined by tides and by coastal geography and oceanography. 5.5.2 Settlement Settlement processes in estuarine and marine environments comprise: Settlement <strong>of</strong> larger particulate material immediately adjacent to the discharge point. Flocculation <strong>of</strong> aggregated fine particles as a result <strong>of</strong> the entry <strong>of</strong> freshwater into saline waters (resulting in a change in the charge associated with fine particles). Dispersion <strong>of</strong> fine particulate matter with tides. As a consequence <strong>of</strong> these processes, there tends to be a halo <strong>of</strong> larger particulate debris on the sediment adjacent to intertidal and sub-tidal stormwater discharge points. Kingett Mitchell Ltd Resource & Environmental Consultants
55 <strong>The</strong>re also tends to be increases in the concentrations <strong>of</strong> contaminants in estuaries, firstly as a result <strong>of</strong> the presence <strong>of</strong> depositional environments (e.g., salt marshes and mangroves) and the location <strong>of</strong> the fresh-salt water interface. As such, these processes result in a stronger gradient <strong>of</strong> particulateassociated contaminants (e.g., metals and PAHs) closer to discharge points entering estuarine and marine waters compared to freshwaters (examples <strong>of</strong> contaminant build-up adjacent to discharge points are provided in Section 6). 5.5.3 Other Processes Metals When particulate matter is deposited in estuarine and coastal environments, the particulate matter undergoes two key processes. <strong>The</strong> first is diagenetic chemical changes in the sediment-water interface that result in the dissolution and precipitation <strong>of</strong> contaminants with a road transport stormwater origin. Metals are involved in pH/eH sulphide interactions in particular and some metals (e.g., Cu) are involved in processes with dissolved organic carbon. <strong>The</strong> chemistry <strong>of</strong> sediments containing contaminants changes over time. Although influenced by factors such as bioturbation, which may alter sediment chemistry, the phase chemistry <strong>of</strong> many metals tends towards greater stability over time. This was shown for example in relation to lead in the sediments <strong>of</strong> the Manukau Harbour (De Mora & Demeke 1990). VOCs In Section 2 it was considered that the concentration <strong>of</strong> VOCs in stormwater were typically low and few were detected. For example, toluene was found in concentrations with a median <strong>of</strong> 0.3 mg/m3 in Long Island stormwater. Wakeham et al. (1983) reviewed the fate <strong>of</strong> toluene in seawater. <strong>The</strong> key pathways for removal <strong>of</strong> toluene were volatilisation and biodegradation (mainly in summer). <strong>The</strong> authors reported a half life for toluene in a marine experimental mesocosm <strong>of</strong> 6 days during a storm to 25-35 days during calmer conditions. <strong>The</strong> mass balance assessments carried out by the authors showed that after 21 days: Volatilisation accounted for 52% <strong>of</strong> the flux in winter and 17% in summer. 30% <strong>of</strong> the toluene remained as dissolved toluene in water in winter and
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iv 7. IDENTIFICATION OF ENVIRONMENT
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vi MW Molecular weight N Nitrogen N
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Page 40 and 41: 30 Metal Cu Cd Pb Zn Table 4.6 - Su
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Page 44 and 45: 34 (1995) provide a general review
Page 46 and 47: 36 Further data can be found in the
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Page 54 and 55: 44 5 PATHWAYS & FATE OF CONTAMINANT
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Page 60 and 61: 50 Figure 5.1 - Percentage
Page 62 and 63: 52 short (e.g., < 1 day). In water,
Page 66 and 67: 56 5.6 Summary The
Page 68 and 69: 58 6.2 Freshwater Ecosystems 6.2.1
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Page 98 and 99: 88 Bioaccumulation can be assessed
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Page 102 and 103: 92 6.5.3 Bioaccumulation of
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Page 106 and 107: 96 7. IDENTIFICATION OF ENVIRONMENT
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Page 110 and 111: 100 trace metals (probably Cu and Z
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104 that “concentrations
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106 A large amount of</stro
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108 A review of se
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110 Samples of man
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112 grazers such as mud flat snails
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114 8. OVERVIEW 8.1 Vehicles as sou
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116 The quality <s
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118 International studies have show
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120 A number of st
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122 construction. Center for Resear
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124 Carey, J.; Cook, P.; Ciesy, J.;
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126 Ellis, J. B. 1986: Pollutional
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128 Harrop D. O. 1984: Stormwater r
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130 Kennedy, P. C.; Kjellstrom, T.
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132 Maltby, L. A.; Boxall, D. M.; F
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134 Novotny, V.; Muehring, D.; Zito
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136 Ruck, J. 1998: Evaluation <stro
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138 Terracciano, S. A.; O’Brien,
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140 Webster, J. G.; Brown, K. L.; W
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142 10. ACKNOWLEDGEMENTS Thanks to
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The Effects of Road Transport on Freshwater and Marine Ecosystems

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