2.3 Pollutant processes
2.3.1 Pollutant build-up
Pollutant built-up is the process which describes the accumulation of pollutants on impervious surfaces. It is a dynamic process where deposition and removal due to natural and anthropogenic activities influence the net accumulation at a given time. Pollutant build-up on road surfaces is due to atmospheric deposition and direct deposition through anthropogenic activities such as vehicular traffic under dry weather conditions. A number of studies have been conducted to understand the factors influencing build-up load and variability associated with build-up composition (Miguntanna et al., 2010; Shaheen, 1975). Similarly a number of
approaches are available to predict the build-up load on road surfaces (Ball et al., 1998; Bradford, 1977; Egodawatta et al., 2007; EWGCC, 2000). The primary factors which influence the build-up on road surfaces are,
Land use characteristics;
Road maintenance practices (street sweeping);
Climatic characteristics (antecedent dry days, wind turbulence) (Ball, et al., 1998; Bradford, 1977; EWGCC, 2000; HEC, 2007).
A number of studies have evaluated the influence of vehicular traffic on build-up (Al-Chalabi and Hawker, 1996; Mahbub et al., 2010; Rogge et al., 1993; Shaheen, 1975). Mahbub, et al. (2010) found that heavy metals such as Ni, Cr, Pb, Cd, Cu, Sb are strongly correlated with vehicular traffic congestion. Al-Chalabi and Hawker (1996) found that significant difference in Pb concentrations in different roads in Brisbane, Australia. They attributed this difference to variations in traffic volumes and mix. As they noted small petrol driven vehicles release high amount of Pb. Therefore, this reflects the significance of vehicular traffic characteristics on road surface build-up. However, it should be noted that Pb was phased out in Australia more than a decade ago. A recent study conducted by Egodawatta et al. (2013) noted that still significant amount of Pb can be found in urban road build-up. They noted that a portion of Pb accumulated on road build-up is due to the past usage of leaded fuel and its accumulation on geogenic sources (soil). In addition to that, traffic related sources such as tyre wear and brake wear can contribute to Pb accumulation on road surfaces (Egodawatta, et al., 2013) indicating the influence of traffic on road build-up.
Land use also influence pollutant build-up. For example, Miguntanna, et al. (2010) investigated build-up solids load in different land uses such as residential, commercial and industrial and found that land use is a highly influential factor in build-up. Egodawatta and Goonetilleke (2007) investigated build-up for different urban forms within one land use (residential) which is representative of varying population density. They noted that relatively high pollutant build-up loads and rates can be associated with areas with high population density. Therefore, it can be
concluded that build-up variation is not only due to land use variation but also due to variation within the same land use.
In addition, Sartor and Boyd (1972) found relationships between street sweeping practices and pollutant build-up. They noted that there was 0.34 ton/km of pollutant load on road surfaces in residential areas in the USA which was higher than that for commercial areas with a pollutant loading rate of 0.08 ton/km. It was reported that the higher pollutant loading rate in residential areas was due to less frequent street sweeping compared to commercial areas. Further, street sweepers removed 15% of the materials finer than 45 µm and 48% of the materials finer than 246 µm. However, Vaze and Chiew (2002) have questioned the efficiency of sweeping practices noting that they release attached materials on road surfaces and if the suction is not powerful enough to extract the released materials, these can be readily available for wash-off.
Climatic characteristics influence the pollutant build-up load and composition on road surfaces. For example, Zhang et al. (2004) found that the concentration of total suspended solids in atmospheric aerosols in spring is approximately three times greater than in summer and autumn in North China. It is evident that, pollutant build- up through atmospheric deposition may vary seasonally causing variations in build- up. However, these effects are different from one region to another or country to another. Strehlow and Baritrop (1987) noted that Pb concentration in road build-up in London was 353-5430 µgg-1 while Fergusson and Ryan (1984) found that in Jamaica it was 817-909 µgg-1. These concentration variations may be due to factors such as difference in traffic flows, location and climatic conditions.
The effect of wind may alter the build-up. Wind can transport accumulated pollutants long distances and/or redistribute them in the immediate vicinity of the road surfaces. In addition, past studies have shown that there is no uniform distribution of pollutants over the entire road surface since wind and traffic generated turbulence can shift particles towards barriers such as the median strip and kerb (Deletic and Orr, 2005; Novotny et al., 1985). Deletic and Orr (2005) noted that 66% of build-up accumulates within a 0.5m strip next to the kerb.
A range of mathematical replications for pollutant build-up can be found in research literature (Ball, et al., 1998; Egodawatta, 2007; Vaze and Chiew, 2002; Wang et al., 2011) while two concepts are more prominent. The replication adopted in most water quality models conceptualise that the surface pollutant load builds up from zero over the antecedent dry days. The second replication conceptualise that storm events only remove a small amount of pollutants and consequent build-up occurs relatively fast. Vaze and Chiew (2002) introduced these two concepts as ‘source limiting’ and ‘transport limiting’, respectively (Figure 2.1). Wang, et al. (2011) evaluated the use of source and transport limiting concepts in modelling and noted that there is no consensus on what concept is most appropriate. Source limiting concept is appropriate where pollutants are easily washed off by runoff and thereby little or no build-up remains after rainfall. The transport limiting concept performs well in areas where rainfall events have great temporal variation and a considerable amount of build-up remains after the rainfall event.
Vaze and Chiew (2002) stated that build-up over dry days occurs relatively quickly after rainfall, but slows down after several days as redistribution occurs. Egodawatta (2007) confirmed that the pollutant accumulation rate is significantly reduced after seven antecedent dry days. The surface pollutant load, through disintegration, also becomes finer over the dry days.
Sartor et al. (1974) stated that pollutant accumulation is a function of antecedent dry days and can be mathematically replicated using an exponential function. Egodawatta (2007) and Ball et al. (1998) found that the power function is the best replication of build-up compared to linear, exponential, hyperbolic and reciprocal functions. As Sartor et al. (1974) conducted research in the USA and Egodawatta (2007) and Ball et al. (1998) conducted studies on Australian urban road surfaces, the influence of climate and location may affect build-up replication. However, both of these approaches state that build-up can be mathematically replicated by a decreasing rate increasing function of antecedent dry days.
Figure 2-1 Representation of build-up and wash-off of road surface pollutant loads (a) source limited and (b) transport limited case
(Adapted from Vaze and Chiew (2002))
Although Sartor et al. (1974) replicated the build-up load with respect to different land uses such as industrial, commercial and residential (Figure 2-2), it is clear that pollutant build-up is significantly influenced by the land use as well as the site specific characteristics within each land use (Liu et al., 2011). Therefore, validity of available models for a given surface is questionable and it reflects the necessity of an alternative approach to replicate build-up instead of deriving models for different land uses.
Figure 2-2 Pollutant build-up rates for different land uses (a) Industrial (b) Residential (c) Commercial (d) All land use
(Adapted from Sartor, et al. (1974))
Po llu ta n t b u ild -u p lo a d ( lb /ke rb mi le )