This chapter primarily provides an understanding of the variability of pollutant build- up and wash-off from road surfaces and chemical signatures of the potential pollutant contributing sources. Following are the important conclusions derived from the analysis.
Solids build-up load on road surfaces is influenced by the pre-existing solids load, which in turn is influenced by the previous rainfall received. Furthermore, pollutant build-up is highly site specific and is attributed to the variable contribution from different sources at a particular location.
Over 60% of the particulates in build-up comprise of particles less than 150 µm.
The highest amount of organic matter content was reported at Clearview Estate (residential road sites). This is attributed to the presence of vegetation in surrounding areas when compared to other study sites.
Metals namely, Na, Mg, Al, K, Ca and Fe were found in abundance in build-up. Rh, Pd and Pt were at minimum level while Ti, V, Cr, Co, Ni, Cu, Zn, Mo, Cd, Sn, Sb, Ba and Pb were in moderate levels of loading in road surface build-up.
The metal distribution cannot be described according to the land use type as there was a significant deviation in different sites even within the same land use. However, there was consistency in metal distribution in residential land use sites compared to other land uses.
Metal build-up on road surfaces follows a similar pattern of variation with solids build-up with the antecedent dry periods, thus confirming metal affinity for solid particles.
The highest total metal load was reported in the <75 µm particle size fraction. However, the other particle size fractions are also important as there was over 11% of the total metal load attached to the >425 µm fraction.
In commercial road sites, all the hydrocarbons compounds which were analysed (C8-C40) were detected and found in abundance. This could be due to the frequent vehicular traffic activities in commercial areas which release these hydrocarbons into the environment. A comparatively low hydrocarbon loading was reported in residential areas. The analysis outcome confirmed that the hydrocarbons loading in build-up is also site- specific.
The amount of solid wash-off in the first intervals of a rainfall event is high irrespective of the rainfall intensity and the road site. However, solids wash-off occurs throughout a rainfall event.
Similar to metal build-up, namely, Na, Mg, Al, K, Ca and Fe were found
in abundance in wash-off. Rh, Pd and Pt were at minimum levels and less frequently detected in wash-off.
Even though solids wash-off occurs throughout a rainfall event, metal wash-off is dominant in the first intervals of a rainfall event. Additionally, it is important to note that only a fraction of metals in build-up are removed with wash-off during a rainfall event.
Wash-off from commercial sites contain a high load of hydrocarbon compounds compared to residential sites.
As rainfall continues, the hydrocarbon loading in wash-off increases. Pollutant sources
Roadside soil samples indicated elevated concentrations of elements, namely, Al, Ca and Fe followed by Na, Mg, K, Ti, Mn, Ba and Pb. Potential toxic metals such as Cr, Cu, Zn and Cd were also present in soils in minor quantities.
Since road build-up had the elevated metal concentrations compared to roadside soil, it was concluded that the elevated presence of metal elements in road build-up is influenced by anthropogenic activities.
Moreover, Pt, Pd and Rh were not detected in soil samples, confirming their anthropogenic origin.
Less than 1 mg/kg of aliphatic hydrocarbon compounds ranging from C8 to C40 were detected in soil. Road build-up consists of elevated levels of hydrocarbons compared to soil, which requires further investigation to determine the sources responsible for the elevated concentrations.
Zn is a reliable marker element for tyre wear. As far as other metals are concerned, the concentrations are one order of magnitude lower than that of Zn, although significant amounts of Na, Ca, Al, K, Ti and Fe were reported. High molecular weight hydrocarbon compounds (>C32) can also be used as marker compounds to identify tyre wear from a mixture of sources.
Ca, Ti, Fe, Cu and Ba are dominant in the brake dust sample analysed in this study while other elements are also present in significant amounts. Hydrocarbon marker compounds for brake dust could not be identified.
Na, Mg, Al, K, Ca, Ti and Fe are the major elements detected in the asphalt sample. V, Cr, Co, Ni, Cu, Zn and Pb are also present in the asphalt sample with comparatively low concentrations. High molecular weight hydrocarbon compounds were dominant in the asphalt sample (> C32) while C8, C14 – C16 were present in minor quantities.
Source Apportionment of Road
The build-up collected from road surfaces is an agglomeration of particles from a wide range of sources. Virtually any anthropogenic or natural source can contribute to the complex road build-up mixture. The build-up accumulated on road surfaces consists of potentially toxic pollutants, which consequently influence urban waterway ecology due to wash-off with stormwater runoff (Göbel et al., 2007). In this context, assessment of sources of road surface pollutant build-up and wash-off is vital in order to define control measures to manage pollution. Current research literature has qualitatively assessed the sources of build-up and/or wash-off (Brown and Peake, 2006; Chang, et al., 2009; Lu et al., 2010; Pitt and Bonzeman, 1982; Rogge, et al., 1993; Wang et al., 2002). However, quantitative assessment of sources of road surface build-up and wash-off is limited and is more valuable for effective mitigation design.
Quantitative assessment of pollutant sources requires extensive field data sets. The data set used for the analysis in this chapter is derived from the experimental methodology outlined in Chapter 4. The analysis is presented in Chapter 5 primarily revealed that the distributions of road surface pollutants such as metals and hydrocarbons are affected by the variable contributions from a wide range of anthropogenic sources. This chapter focuses on the investigation of the exact sources of metals and hydrocarbons which are incorporated road surface build-up and wash- off, and then quantifies source contributions.
The sources of metals in road build-up and wash-off were initially identified using a range of different data analysis techniques. With respect to the identified sources, quantification was done using Principal component analysis/Absolute principal component scores (PCA/APCS) receptor model. The relative importance of the sources was also determined by estimating the percentage of contributions of individual metal elements from each of the identified sources. The same analytical
procedure was adopted for hydrocarbon source identification and quantification. Source identification and quantification of pollutant wash-off (metals and hydrocarbons) were conducted based on the two data sets available for Via Roma Drive (C4) and Yarrimbah Drive (R4) separately.
6.2 SOURCE APPORTIONMENT OF METALS IN ROAD SURFACE POLLUTANTS