Chapter 4: Spatial distribution, temporal variation, and phase distribution of polycyclic
4.3. Results and discussion
4.3.2. Phase distribution of PAHs
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were higher due to the effect of urban and industrial emission.
72 events, and (c) surface water after the runoff events.
The particulate phase was mainly contributed by the 3-ring (i.e., Flu, Phe, and Ant), 4-ring (i.e., Flt and Pyr), and 6-ring (i.e., Ind and BghiP) PAHs (Figure 4-6). This observation is also in line with those reported in previous studies (Kalmykova et al., 2013; Niu et al., 2018; Park et al., 2012). The particulate phase showed its higher contribution to the surface water after the runoff events. The contributions of rainfall and runoff discharges to the water bodies could induce more solid-bound PAHs in the water layer, resulting in the increase of particulate phase in the surface water after the runoff events (Figure 4-5c). These particulate PAHs could be directly from the runoff discharges and raindrops. In addition, they might be released from bottom sediments into the overlying layer of the water bodies due to turbulence caused by runoffs and rainfall.
Figure 4-6. Contributions of individual PAHs in the (a) dissolved, (b) particulate, and (c) total phases.
4.3.2.1. Spatial variation of the phase distribution
The phase distributions of PAHs were relatively similar among the semi-rural, residential, and industrial sites, with the dominance of dissolved PAHs (Figure 4-5). Moreover, the dissolved PAHs remarkably increased their contributions in the surface water before the runoff events at the industrial site (Figure 4-5b). The greater DOM in the surface water at the industrial site (Figure 4-7) could promote the solubility of PAHs because they have the strong affinity to the DOM (Chiou et al., 1998), leading to the contribution increase of the dissolved PAHs in water layer at the industrial site. Moreover, the water salinity at the industrial site (mean: 11.6 ‰) was higher than that at the semi-rural (mean: 0.06 ‰) and residential sites (mean: 0.15 ‰).
This observation could be because the water sampling site in the industrial area located near
Na Acy Ace Flu Phe Ant FlA Py BaA Chr BbF BkF BaP IcdP DbahA BghiP
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Runoff
Na Acy Ace Flu Phe Ant FlA Py BaA Chr BbF BkF BaP IcdP DbahA BghiP
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Runoff
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Nap Acy Ace Flu Phe Ant Flt Pyr BaA Chr BbF BkF BaP Ind DahA BghiP
(b) Particulate phase (c) Total phase
(a) Dissolved phase
Nap Acy Ace Flu Phe Ant Flt Pyr BaA Chr BbF BkF BaP Ind DahA BghiP Nap Acy Ace Flu Phe Ant Flt Pyr BaA Chr BbF BkF BaP Ind DahA BghiP
Fractions
Runoff Surface water before the runoff Surface water after the runoff
Na Acy Ace Flu Phe Ant FlA Py BaA Chr BbF BkF BaP IcdP DbahA BghiP
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Runoff
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the estuary (Figure 4-1), leading to an effect of seawater on the surface water in this area.
Noticeably, the higher salinity could enhance higher concentrations of ions, which could compete with the PAHs to sorb to the SS (Li et al., 2016). As a consequence, PAHs could be desorbed from the SS and increase their contributions in the dissolved phase.
Contributions of the particulate PAHs in the runoff of industrial and residential sites were higher than that of the semi-rural site (Figure 4-5a). This observation can be explained by that the more impervious surfaces (i.e., parking lots, pavements, and roads) in the industrial and residential areas would induce more particle-bound PAHs deposition. On the other hand, the semi-rural site has more soil and vegetation surfaces; therefore, PAHs could strongly sorb to soil organic matter and lead to the lower contribution of particulate PAHs to the runoff (Parajulee et al., 2017).
Figure 4-7. Concentrations of the dissolved organic carbon (DOC) in the runoff and surface water before and after the runoff events.
4.3.2.2. Temporal variation of the phase distribution
In May, contributions of the particulate PAHs in the runoff and surface water before the runoff events increased noticeably (Figures 4-5a and 4-5b). The large rainfall amount (1131 mm) and high rainfall intensity (3.36 mm/h) two weeks prior to the sampling of surface water (before runoff events) in May (Table 4-4) could induce more SS in the water bodies through runoffs and resuspension from the bottom sediment. The higher concentrations of SS could result in the greater contribution of particulate PAHs as the SS could act as sorbents for organic
Runoff Before runoff After runoff
Semi-rural Residential Industrial April May July September
0 2
Concentration (mg/L)
4 6 8 10
Semi Resi Indus Apr May Jul Sep
0 2 4 6 8 10
Runoff
A
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compounds. Indeed, the positive correlation between the concentrations of TSS and particulate PAHs (R = 0.54) was found. In addition, the SS in the runoffs were expected to be mainly contributed by those depositing on the surfaces and leaching from the soils as mentioned previously. The large rainfall amount (514.5 mm) and rainfall intensity (1.53 mm/h) two weeks prior to the runoff events in May (Table 4-4) might decline the soil infiltration rate. Therefore, the overland runoff could be formed more quickly (Wang et al., 2006), and the small soil sands (e.g., fine and silt sands smaller than 250 µm and 63 µm, respectively) could be preferentially washed off due to their lighter weights. These light soil sands could be more polluted by PAHs (Amellal et al., 2001; Uyttebroek et al., 2006), leading to an elevation of the particulate fraction in the runoff of May. In July, PAHs in the surface water more distributed into the dissolved phase (Figures 4-5b and 4-5c). This result can be explained by the desorption of PAHs from the particulate to the dissolved phase (He et al., 1995), stemming from the higher water temperature during this period.