The Effects of Road Transport on Freshwater and Marine Ecosystems

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Cations/Anions
Common ionic constituents <strong>of</strong> stormwater will be derived from the interaction <strong>of</strong> rainfall with inorganic
material on road surfaces and the road surface and from rainfall. Common ionic constituents in
stormwater from highways have been measured in a number <strong>of</strong> studies (Bellinger et al. 1982, Harrison
& Wilson 1985). Concentrations <strong>of</strong> common cations (Ca, Mg, Na, K, Fe, Mn) and anions (Cl, SO 4 ) are
dependent on the geochemistry <strong>of</strong> soils contributing to particulates on road surfaces and the quality <strong>of</strong>
rainfall. <strong>Road</strong> surfaces may also contribute and as noted by Mosley & Peake (2001), the changes
contributed by surfaces contacted by rainwater will depend upon the time for interaction between the
rainwater and the surface. Sadecki et al. (1996) showed that concrete would be expected to produce
run<strong>of</strong>f with higher conductivity, higher (alkaline) pH, higher alkalinity and higher K and Na than asphalt.
Environmentally significant changes in run<strong>of</strong>f chemistry typically only occur as a result <strong>of</strong> the use <strong>of</strong>
de-icing salts. Although de-icing salts are used in New Zealand, they do not appear to be used in
urban areas in New Zealand and as such are not discussed further in this report. Overall, general
chemistry <strong>of</strong> run<strong>of</strong>f is likely to be site specific and very dependent upon the availability <strong>of</strong>
geochemically reactive surfaces.
Mosley & Peake (1999) examined the ionic composition <strong>of</strong> stormwater in Dunedin. <strong>The</strong>ir work has
shown that the ionic composition <strong>of</strong> stormwater tends towards that <strong>of</strong> rainfall during the storm event.
pH tended to decrease during the event. <strong>The</strong> ionic composition <strong>of</strong> stormwater is important because
the toxicity exerted by free ions is influenced by the stormwater hardness. Hardness (calculated from
calcium and magnesium concentration) will be greatest during the initial phase <strong>of</strong> storm events and
decrease over time from 80 to 30 g/m 3 (as CaCO 3 ) over the duration <strong>of</strong> the storm event sampled
(Mosley & Peake 1999).
Nitrogen
ARC (1998, 1992) and Williamson (1993) describe the nitrogen content <strong>of</strong> stormwater. Table 4.12
summarises <strong>of</strong> nitrate and ammonia nitrogen stormwater data for New Zealand urban and overseas
motorway areas. Williamson (1998) reports that total nitrogen is dominated by organic nitrogen and
nitrate-nitrogen. Only a small proportion <strong>of</strong> the dissolved nitrogen is normally present as ammonia
(e.g., see Macaskill et al. (2003) for Rotorua stormwater). However, in catchpits, anaerobic conditions
can result in a greater proportion being present as ammonia (refer following section). As un-ionised
ammonia is toxic, this is discussed further later in this report. <strong>The</strong> data in Table 4.12 indicates that the
concentration <strong>of</strong> dissolved inorganic nitrogen in motorway and other roadway run<strong>of</strong>f can be elevated.
Given the concentration <strong>of</strong> nitrogen in rainfall, the actual contribution <strong>of</strong> motor vehicles to stormwater
nitrogen concentrations does not appear to have been identified.
Although motor-vehicles emit nitrogen (e.g., ammonia in exhaust emissions and nitrogen oxides), there is
insufficient information to determined whether vehicle emissions are a significant contributor <strong>of</strong> nitrogen in
urban road run<strong>of</strong>f. It is however, generally acknowledged that vehicles are not an important source <strong>of</strong>
nitrogen in run<strong>of</strong>f (e.g., Macaskill 2001).
Phosphorus
ARC (1998, 1992) and Williamson (1993) summarise international and New Zealand data for
phosphorus in urban stormwater. Some New Zealand data and a range <strong>of</strong> data for motorways is
presented in Table 4.12. Phosphorus has been identified as being present in bitumen. <strong>The</strong>
contribution <strong>of</strong> bitumen and bitumen wear to dissolved and total phosphorus in urban run<strong>of</strong>f is not
known. It likely, that the overall contribution is small. Kenendy & Gadd (2000) reported that tyres,
brake pads and brake pad dust contained low concentrations <strong>of</strong> total phosphorus (medians 22.5-172.5
mg/kg). Raw bitumen was found to contain no detectable phosphorus (