Heavy Metals Concentrations

The results from the analysis of heavy metals are shown in Table 4.2 below, with the outputs from the statistical test in the top half, and interpretations of results below. An ANOVA test was applied to the concentration of metals in each of the samples. Variation with intensity was shown by 5 of the 6 metals (highlighted red in Table 5.2:), and variation with time shown by Pb alone (p = 0.0300). Cu did not vary with either intensity or time.

Post hoc, a Fishers LSD Test was applied to the 5 metals showing variation with intensity. Of these, the 24 mm/h event was shown to give the lowest concentration in every case. In 4 of the 5 cases, the results from the 54 mm/h event were shown to be not significantly different to both the 24 mm/h event, and the 54 mm/h event, whereas for Pb, all intensities were shown to give significantly different results. No interaction effects between time and intensity were shown for any of the metals.

Table 4.2: Statistical Summary of Metal Concentrations

Concentrations in mg/kg, Cd μg/kg. Cu Ni Pb Zn Cd Cr 173.84 26.93 81.99 554.11 446.96 42.98 55.56 8.68 17.75 158.20 132.34 11.98 0.761 0.024 0.000 0.000 0.004 0.002 0.572 0.902 0.030 0.264 0.429 0.901 0.925 0.599 0.164 0.720 0.867 0.547 24 a a a a a a 39 a b b b b b 54 a ab c ab ab ab Metal Group (LSD Test) Average C.I P Intensity P Time P (Interaction)

4.4 Discussion

The methodology for the study was based on that of Herngren (2005), which has been successfully used repeatedly. There were, however, several constraints that prevented this study following the methodology as closely as desired. The field rainfall simulator used was primarily designed for experiments on soil erosion in storm events, and as such was designed for higher rainfall intensities. The pump and nozzle method of controlling intensity means that intensities were affected by limited nozzle sizes, and pressure requirement to form an even spread.

This meant there were problems achieving the lower intensities that are more representative of storms in the UK. It may be possible to attain these intensities using specifically designed apparatus, with lower pressure requirements, or other distribution methods such as capillary tubes. This would give more options as to the range of intensities investigated.

4.4.1 D50

The lack of variation in d50 values over time or with rainfall intensity suggested that here were no particular particle sizes are preferentially washed off. It may be expected that higher intensities may mobilise larger particles, as a result of the greater kinetic energy in higher intensities providing more mobilisation, however this was not found. It is possible that this scenario would occur with larger or lower intensities than those included in this study, but the energy in the rainfall intensities in this are sufficiently similar to one another that they mobilise sediment with the same physical characteristics.

The average d50 value was 100 µm, which is similar to several studies that analysed the runoff from highways, such as Deletic and Orr (2005) and Poleto et al. (2009), which ranged from 26 - 122 µm and 86 – 369 µm respectively. Given the differing uses, and the many different sources of sediment, it is interesting that the physical characteristics of the sediment in

4.4.2 Runoff Volume

The volume of runoff generated from the rainfall events was shown not to vary with time. This is most likely due to the relatively shallow storage of the car park surface. As a result of this, even the lower intensity exceeds the storage capacity, and therefore there is no detectable delay or other variation exhibited. It was shown that the runoff volume varied with intensity, with the least for the 24mm/h intensity was shown to be the lowest, as by definition involved the least amount of ‘rain’ for the duration, however no significant difference was shown between the 39 and 54 mm/h intensities. It would be expected that the higher the intensity, the greater the runoff volume. The reason for this not being displayed it likely due to the runoff coefficient of the site, a dimensionless coefficient which relates the rainfall volume, to the volume of resulting runoff. In the case of car parks, the factors affecting this include ‘pore’ spaces on the surface, allowing for storage of water, surface material, and topography of the sample space. This may be exacerbated by the small sample size.

4.4.3 Sediment Mass

The mass of sediment was shown to vary with intensity, but not time. This variation was shown not to be as a result of volume, as the correlation between volume and mass was very weak. Although it would be expected that the higher the intensity, the greater the mass, the difference between the 39 and 54 mm/h intensities was not significant.

It is suggested that the kinetic energy of the rain droplets is the reason, with higher intensities having higher kinetic energy, and thus mobilising more of the sediment build-up. The 39 and 54 mm/h intensities not being significantly different suggests that there is a threshold between 24 mm/h and 39mm/h where mobilisation changes. However, it was also shown that there was no significant difference between the results from the 24 mm/h intensity and the 54 mm/h intensity. This overlap tends to suggest that natural variation in sediment mobilisation is greater than the effect of the intensity.

Another possibility for the non-linear increase is the nature of rainfall simulation apparatus. As described in the methodology section, the intensity is a function of nozzle size and pressure. In order to achieve the desired intensities, the 39 and 54 mm/h used the same nozzle, with differing pressures (39 mm/h = 14.5 psi, 54 mm/h = 21.8 psi). It is possible that the increased pressure for the 54 mm/h intensity led to finer ‘raindrops’ compared to 39 mm/h. Subsequently, there may have been more kinetic energy in the 39 mm/h event, leading to different mobilisation. Specialist equipment not available for this study could be used in further investigations to determine the kinetic energy in the rain drops of each of the intensities.