3.2.2 Phase 2: Design of Investigation and Methods
Build-up and wash-off are the common pollutant processes associated with road surface pollutants. In order to investigate the relationship between contributing sources and pollutant signatures of build-up and wash-off, a comprehensive data matrix and analysis techniques were required. Phase 2 in the research methodology discusses the approaches adopted to obtain the data matrix. Phase 2 gives the design of investigations and methods which consisted of three sub-components. These are; sample collection, study sites selection for sample collection and laboratory testing. Details of each of these sub-components are discussed below.
a. Collection of build-up pollutants
Justification for collecting build-up pollutants from road surfaces
Past research studies have identified road surfaces as the major contributor of pollutants to stormwater run-off (Bannerman, 1993; Deletic and Orr, 2005; Egodawatta and Goonetilleke, 2007). In this regard, road surfaces act as the key receptors of various forms of pollutants generated via either natural or a range of anthropogenic sources (Al-Chalabi and Hawker, 1996; Al-Khashman, 2004; Christoforidis and Stamatis, 2009). The accumulation of particles from a wide range of urban sources and the importance of stormwater pollution justified the selection of road surfaces as the focus of investigation.
Criteria for study sites selection
Pollutants deposited on road surfaces is a complex mixture (Kreider, et al., 2010). In order to investigate the signatures and potential sources of this complex mixture of pollutants, a set of samples with appreciable variability in source contributions needed to be collected.
It has been reported that varying anthropogenic activities in terms of traffic and land use creates the highest variation in source contributions to urban road surface pollutants. Traffic characteristics including daily traffic volume was considered as the most influential traffic related parameter in related to stormwater pollutants while typical urban land use such as industrial, commercial and residential forms the influential land use factors (Egodawatta and Goonetilleke, 2007; Lim et al., 2005; Shaheen, 1975).
A substantial fraction of road surface pollutants consist of soil originating from adjacent land (Gunawardana, et al., 2012a). Therefore, physical and chemical signatures of roadside soil also play an influential role on the pollutant signatures in build-up. For example, the surface properties of solid particles determine the extent of pollutant adsorption. Additionally, chemical signatures of the roadside soil can vary between different locations which in turn influence signatures of the pollutant build-up.
Considering all of the possible sources and their influential parameters, the following set of criteria were used for site selection:
Road sites from regions with different soil characteristics. In this regard, distance from the coast line was selected as the primary factor.
Road sites in different land use types representing residential, commercial, industrial areas.
Road sites accounting for typical ranges of traffic volume.
Sites with minimal disturbance to road users and ease of access during sample collection.
In addition, skewed pollutant characteristics can occur along a road surface due to the presence of features such as signalised intersections, roundabouts and bottlenecks. Therefore, in this study, the pollutant build-up samples were collected avoiding locations where those specific features are present. This was primarily to maintain the similar influence from traffic sources among all of the selected sites. Additionally, avoiding specific road features reduced the number of variables associated with pollutant build-up.
Justification for multiple build-up sampling episodes
Apart from the signatures relating to contributing sources, road surface pollutant build-up is also dependent on antecedent dry days (Egodawatta and Goonetilleke, 2007; Sartor, et al., 1974). Pollutants are typically accumulated at varied rates depending on the length of antecedent dry conditions. As noted by previous researchers, a relatively high rate of build-up occurs for around the first seven dry days after a rainfall event (Ball, et al., 1998; Egodawatta and Goonetilleke, 2007). After this threshold, rate of build-up reduces and total build up asymptote to an almost constant load. It has also been reported that physical properties such as particle size distribution and the concentrations of associated pollutants are subjected to dynamic changes during antecedent dry conditions (Egodawatta et al. 2013). In order to account for the effect of antecedent dry days on build-up, two sampling episodes were undertaken representing two different antecedent dry conditions. One
of the selected antecedent dry periods was less than seven days (<7) and the other was greater than seven days (>7).
b. Collection of pollutant wash-off from road surfaces
Justification for collecting pollutant wash-off samples from road surfaces
Stormwater quality is dependent on both, build-up and wash-off. However, it is commonly known that the wash-off from a road surface is not complete and a significant amount of pollutants will still remain on the road surface even after a rainfall event (Vaze and Chiew, 2002; Wang, et al., 2011). This confirms the possibility of having different pollutant signatures in wash-off compared to build up. Therefore, it was decided to collect road surface pollutant wash-off samples to investigate their pollutant signatures and potential sources. The pollutant wash-off sample collection was carried out at a limited number of road sites.
Study sites selection for pollutant wash-off sample collection
Wash-off is primarily influenced by rainfall and impervious surface characteristics (Egodawatta, et al., 2007; Wang, et al., 2011). However, it was also necessary to consider the following criteria in selecting study sites for wash-off sampling:
An appreciable range of pollutants needed to be available on road surfaces for wash-off.
Site characteristics such as slope, width and length needed to be favourable conditions to conduct wash-off sampling.
Based on the above criteria, a limited number of sites among build-up sampling sites were selected to conduct wash-off sampling. The site selection was done by ranking pollutant build-up sampling sites from the most polluted road site to the least polluted road site. Wash-off sampling was conducted at the selected sites for a varied range of rainfall intensities and durations using simulated rainfall. A rainfall simulator was used to generate the rainfall with predetermined intensities. More details can be found in Section 3.3.2.
c. Potential pollutant source samples collection
Pollutants in build-up and wash-off are a mixture from a range of sources. Therefore, an understanding of the chemical signatures of potential sources of pollutants is important for determining the actual sources of road surface pollutants in build-up and wash-off. Primary potential sources were selected based on an extensive review of literature, as presented in Chapter 2, to test for chemical signatures.
The following are the two key criteria considered in selecting potential pollutant sources to be tested for their chemical signatures:
The sources releasing potentially toxic pollutants such as metals and hydrocarbons into the urban environment.
The sources contributing the major fraction of potentially toxic pollutants to road surface build-up and wash-off.
In an urban environment, traffic related sources, namely; tyre wear, brake wear, asphalt wear, vehicle exhaust, land use related sources (commercial, industrial and residential activities), roadside soil, wearing of building materials and vegetation inputs are typically considered as potential sources. Among them, the potential sources that were considered critical for this study were tyre wear, brake wear, asphalt wear, vehicle exhaust and roadside soil, as these sources generate a high fraction of pollutant loads and potentially toxic pollutants. Due to a range of reasons including difficulty in obtaining representative samples (for sources such as wearing of building materials and vegetation) and minimal metal and hydrocarbon inputs to urban environments, other sources were not considered for sampling.
d. Laboratory testing
The build-up, wash-off and potential pollutant source samples were analysed to identify their physical and chemical signatures. As metals and hydrocarbons are potentially toxic pollutants even at low concentrations (Kelly et al., 2010) and as they can be influenced by the available solids and organic matter (Gunawardana et al., 2011), samples were tested for solids, metals, hydrocarbons and organic matter. Build-up and wash-off samples were also tested for particle size distribution due to linkages between metal and hydrocarbon affinity to solids based on particle size
(Deletic and Orr, 2005; Sartor and Boyd, 1972). In addition, build-up samples were size fractionated to different particle size ranges namely; >425 µm, 300-425 µm, 150-300 µm, 75-150 µm and <75 µm and each fraction was tested for metals, hydrocarbons and organic matter in order to investigate the influence of particle size on metal and hydrocarbon affinity. The laboratory test methods adopted in this study are given in Section 4.3. Road build-up, wash-off and potential source samples were tested for metals and hydrocarbon compounds as outlined in Table 3-1.
Table 3-1 Metals and hydrocarbons compounds Selected for testing
Metals Hydrocarbons Compound Compound ID Compound Compound ID Lithium Li Octane (C8H18) C8 Sodium Na Decane (C10H22) C10 Magnesium Mg Dodecane (C12H26) C12 Aluminium Al Tetradecane (C14H30) C14 Potassium K Hexadecane (C16H34) C16 Calcium Ca Octadecane (C18H38) C18 Titanium Ti Eicosane (C20H42) C20 Vanadium V Docosane (C22H46) C22 Chromium Cr Tetracosane (C24H50) C24 Manganese Mn Hexacosane (C26H54) C26 Iron Fe Octacosane (C28H58) C28 Cobalt Co Triacontane (C30H62) C30 Nickel Ni Dotriacontane (C32H66) C32 Copper Cu Tetratriacontane (C34H70) C34 Zinc Zn Hexatriacontane (C36H74) C36 Molybdenum Mo Octatriacontane (C38H78) C38 Rhodium Rh Tetracontane (C40H82) C40 Palladium Pd Cadmium Cd Tin Sn Antimony Sb Barium Ba Platinum Pt Lead Pb
These compounds are commonly found in the urban environment. Particularly, the selection of compounds was to determine tracer compounds associated with potential sources as an aid to source characterisation of build-up and wash-off. The identification (compound ID) used in Table 3-1 is used in discussions throughout the thesis.