FASE II: ESTUDIOS Y DISEÑOS DEL PROYECTO INFRAESTRUCTURA EL DORADO

From the previous sections, it is clear that reported pollutant concentrations and amounts can vary by orders of magnitude from the same land use type. For example, amounts of particulates found in highway runoff range from 0.83 – 740 g/m2 across several studies. This can be caused by a vast range of factors. Whilst the previous sections show variation in both loadings and concentration, it has been demonstrated by Drapper at al. (1999) that the main factors in determining runoff concentration is the surrounding characteristics. This can be interpreted in the way that the pollutant loading of the surrounding area directly affects the concentration of the runoff. Hence, the majority of the factors this section will examine potential causes of variation in pollutant loadings rather than those affecting pollutant concentration in runoff.

2.10.1 Runoff (Wash-off)

Other than surrounding characteristics, the main exception to this is the effect of rainfall, which directly affects runoff, rather than build-up. Rainfall in general can be subdivided into 3 categories: intensity, duration and volume, of which intensity is the most important. This is due to both the kinetic energy of the droplets and also the turbulence caused by the flow of the rainfall, which increases with intensity. Both higher intensity and increased turbulence have both been found to increase particle mobilisation, as described below.

The ability of rainfall to mobilise pollutants is known as Capacity Factor (Cf), and was investigated and quantified in work by Egodawatta (2007). The study examined the effect of different rainfall intensities on the pollutants, and found the values presented in Table 2.13.

Table 2.13: Capacity Factor (Cf) of Selected Rainfall Intensities (Egodawatta, 2007)

Rainfall Intensity (mm/h) Cf (unitless)

< 40 0 – 0.5

40-90 Approx. 0.5

> 90 0.5-1

The results show that up to 40 mm/h, Cf increased until reaching a plateau until 90 mm/h. The plateau was reached due to the inability of remaining pollutants to be mobilised by intensities of less than 90 mm/h. This was evident due to the further increases of mobilised pollutants at this point.

Caution should be taken when using the above results, as they used constant rainfall intensities, whereas in reality rainfall events are seldom linear. Findings may differ without a constant application of rainfall and subsequent energy. Limited further work has since been undertaken by Egodawatta et al. (2007) and Brodie (2011). This study used actual storm events rather than simulated rainfall in the previous study; however findings were much less conclusive. This is the predominant reason for which values attained by studies in countries with different climates, such as Egodawattaet al. (2007) and Wicke et al. (2009) cannot be used accurately. For example, average rainfall intensity was 133 mm/hr for 2 minutes in the tropical climate of the latter of these studies, whereas a realistic UK equivalent would be in the range of 15mm/h for 1 hour.

2.10.2 Build up (Accumulation)

There are a number of factors which affect the loadings of pollutants on highways, all of which are essentially variants within the sources identified in previous sections. The most important is the total solids or particulate matter present on the surface. It is stated (Sartor et al., 1974; Herngren et al., 2006)

that pollutant loadings are directly linked to particulates due to their adsorption potential. In turn, the amount of particulate matter is dependent on the following factors.

2.10.3 Traffic

It has been identified that traffic as a whole, encompassing both vehicles and associated infrastructure, is responsible for a majority of urban pollutants. Therefore, variations in traffic density, calculated as AADT (Average Annual Daily Traffic), are likely to affect pollutants, as a greater number of vehicles would provide a greater number of potential sources of pollutant.

2.10.4 Surface Type

Surface type is another factor which can have influence on the accumulation of pollutants on urban surfaces. This is due to the varying depths and textures occurring between different surface types. These provide different surface areas for pollutants to accumulate on, and deeper surface depths shown in Table 2.14 have been shown to have a higher build up (Gunawardana et al., 2012).

Table 2.14: Average Total Solids from Various Road Texture Depths (Adapted from Gunawardana et al., 2012)

Land Use Road Texture Depth Range (mm)

Average Total Solids Load (g/m2) Residential 0.76-0.92 1.79 0.81 Industrial 0.93-1.14 3.53 7.03 Industrial, Commercial and Residential 0.80-0.91 1.39 0.78 Commercial 0.63-1.11 2.22 1.75 2.10.5 Land Use

Table 2.11 and Table 2.14 detail findings from Herngren et al. (2006) and Gunawardana et al., (2012) respectively. Both show great variation in the loadings of metals and solids with relation to the usage classifications of residential, commercial and industrial. These variations are in part due to the different processes occurring on each, and the associated activities involved. Another point to consider when examining the effect of land use with regards to this study are the similarities and also potential differences between highway and car park pollutants. Touched upon in the opening sections, it has been assumed in several studies, and also within this one, that their pollutant profiles will be of a similar composition.

After examining numerous sources throughout this report, a more detailed opinion can be formed. Taking into account the many complex factors at play, it can safely be assumed that the sources of pollutants will be similar for all usage types. Similarly, factors such as surface type or climatic conditions. Slight

differences may occur in the sense that there is increased residence time on car parks, hence increased periods in which oil or other fluid leaks may occur. Also, lower speeds have been suggested to emit more particles, as has starting the engine of a car. This is particularly relevant, as the engine of cars using a car park must be started, guaranteeing that the car park will be subjected to this source, whereas on a highway, the engine will likely already be running.

2.10.6 Climate

Climatic conditions also have a potentially major impact on pollutant build up. In drier climates, the surface loads of pollutant are washed off less frequently than those where rainfall is frequent. Therefore, when rainfall does occur, pollutant levels will be considerably higher. Work has been done in several studies such as Wicke et al. (2009) to quantify this into a model, with the designation ‘antecedent dry period’.

2.10.7 Surrounding Land Use

Surrounding land has not been identified as a significant contributory factor to urban pollutants, as it can be assumed that the predominant land is built up. Should the land be rural, the potential for airborne particulate matter to be deposited increases, as the ground is not covered/paved as in an urban setting. As a result of increased potential for particulates, the potential for pollutant load also increases, due to adsorption of pollutants to particles.

2.10.8 Timing

The timing of when samples are taken can also affect overall chemical composition. Whilst this could relate to factors such as time of day and associated traffic, or time after a rainfall event, these are covered in other section. This section refers more to the seasonal timing of sampling, with

several studies such as Mangani et al. (2004) and Westerlund (2005) highlighting the potential differences between seasons. Table 2.15 below is a sample from the latter, with great variation in results evident.

Table 2.15: Seasonal Variation of Metal Concentration (Mangani et al., 2004)

2.10.9 Summary

Due to the reasons for variation identified, especially antecedent dry period, topography or climatic factors, when using data for comparison, care should be taken to consider their potential effects.

Additionally, in the variations observed are reasons for possible differences between highway and car park runoff, such as land use and traffic. It is for this reason that many of the studies cannot be used as comparators for this study involving car parks.