Effect of rainfall and antecedent dry periods on heavy metal loading of sediments on urban roads

Abstract

Concentration and mass loadings of heavy metals adsorbed on sediment from an urban road surface in Beijing were investigated. The buildup and washoff processes are discussed. Results showed that masses of sediments smaller than 500 μm in diameter varied from 11.2 to 25.5 g/m². On average, sediments smaller than 125€μm accounted for 42.9% of sediments smaller than 500 μm in mass. Cd, Cr, Cu, and Ni were more likely to attach to smaller sediments than bigger ones, compared to Pb and Zn. Mass loading of the sediments generally increased with antecedent dry periods, but the linear relationship was better for smaller sediments than for bigger ones. Heavy metal concentrations adsorbed on sediments with diameter less than 40€μm generally decreased more obviously than those adsorbed on bigger ones after rainfall events.

Keywords

antecedent dry periods / heavy metals / rainfall events / sediments

Introduction

Dry sediments on urban impervious surfaces are getting more and more attention because they can get into the surrounding aquatic environment with runoff, and the pollutants attached to them will become a serious threat to the water environment (Rice, 1999). With the fast urbanization in China nowadays, the impervious surface area has increased very quickly (Zhao and Chen, 2006). Therefore, it is important to investigate the characteristics of pollutants attached to the sediments on urban surfaces.

Many researchers have investigated the mass distribution of sediments in different size ranges (Lau and Stenstrom, 2002; Aryala et al., 2005). Aryala et al. (2005) found that sediments smaller than 250 μm found on urban impervious surfaces accounted for more than 65% of the total mass in many impervious areas in Switzerland. Du et al. (2002) studied the mass loading of sediments on roads in Xi’an, China and found that most of the sediments were less than 840€μm. For most of the pollutants, higher concentrations were found on smaller sediments (Vikander, 1998; German and Svensson, 2002; Sutherland, 2003).

The buildup process of sediments is very important for characterizing pollutant accumulation on urban impervious surfaces. Together with pollutant distribution on sediments in different size ranges, the buildup of pollutants attached to sediments in different size ranges can be characterized. Some empirical functions between the pollutant loading and antecedent dry periods have been established to simulate the buildup process. Increases of mass loading of sediments with antecedent dry periods and the buildup of mass loadings are influenced by rainfall events and street sweeping (Ball et al., 1998; Vaze and Francis, 2002).

Many studies have focused on the process of washoff during rainfall events (Pagotto et al., 2000; Jennifer and John, 2006; Kim et al., 2007). Vaze and Francis (2002) studied the change in mass loading and particle distribution after rainfall events and street sweeping. The results showed that common storms only removed a small proportion of the total surface pollutant load. However, not many researchers have investigated the differences in mass loading of sediments and heavy metal concentrations together between the beginning and the end of rainfall events, which can represent the end and the beginning of the accumulation of pollutants.

The purpose of this article is to investigate the characteristics of the buildup process for sediments in different size ranges and the characteristics of heavy metals attached to them. The heavy metal concentrations and mass loadings before and after rainfall events are also investigated.

Materials and methods

Site description and sampling methodology

A section of the North 3-Ring Road about 100 m west of the Beitaiping Bridge was selected as the study site to represent a busy road in the metropolitan area of Beijing, as shown in Fig. 1. The average daily traffic is approximately 109500 cars/day.

Samples were collected within 1 m from the curb on the main road with a Panasonic vacuum cleaner (model: MC-CG663). The sampling area was 1 m×5 m, as shown in Fig. 1. Samples were taken between 21∶30 to 22∶30 to avoid heavy traffic.

Pretreatment and chemical analysis

Samples were firstly sieved through a nylon sieve with a mesh opening of 500 μm to remove gravel-sized materials and large plant bodies. Then a nylon sieve with mesh openings of 125 μm was used to study sediment accumulation with time for bigger and smaller sediments. The sampling date, mass of samples less than 500 μm, mass of samples less than 125 μm and antecedent dry periods are described in Table 1.

A series of nylon sieves with mesh openings of 500, 300, 125, 74, 40 μm were used for samples taken on March 20, March 22, March 27, and March 29 to investigate the mass differences of sediments before and after two rainfall events. Sieved sediment within each size range was weighed and mass proportion was calculated.

Heavy metal concentrations were measured using the ICP-MS method for every sieved sub-samples. Chromium (Cr), copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb) and zinc (Zn) are reported in this paper.

Results and discussions

Heavy metal loading analysis

The mass per unit sampling area of the sieved samples is shown in Table 1. The masses of samples less than 500 μm varied from 11.2 to 25.5 g/m². The masses of samples less than 125 μm varied from 4.32 to 11.53 g/m², which accounted for around 42.9% of the sediments less than 500 μm in diameter on average.

Combined with the heavy metal concentration on sieved sub-samples, the heavy metal loadings on the sub-samples were calculated. Then the proportion of heavy metal mass on sediments less than 125 μm in diameter to the heavy metal mass on sediments less than 500 μm in diameter were plotted using the boxplot in Fig. 2.

Mass loading of sediments less than 125 μm only accounted for about 42.9% of the mass of sediments less than 500 μm, as shown in Table 1. The heavy metal loading less than 125 μm accounted for much more than 45% of the heavy metal mass on sediments less than 500 μm for most of the samples, as shown in Fig. 2. This is due to the higher concentration of heavy metals in sediments less than 125 μm than in sediments 125-500 μm. Smaller particles with higher heavy metal concentrations are easily washed off and enter surrounding water bodies, which pose a high threat to the water environment (German and Svensson, 2002; Sutherland, 2003; Kim and Sansalone, 2008).

Figure 2 demonstrates that Cd, Cr, Cu, and Ni are more likely to attach to small sediments than big ones, compared to Pb and Zn. The heavy metal loading proportion varied substantially from 29% to 73%, as shown in Fig. 2, while the mass proportion ranged from 36.9% to 49.8%, as shown in Table 1. This is due to the large variance in heavy metal concentrations on the sieved sub-samples.

Effect of antecedent dry periods on mass loading of sediments

Figure 3 demonstrates the change in mass loading with antecedent dry periods for sediments less than 125 μm and sediments between 125 and 500 μm. It shows that the mass loading of the sediments generally increases with the antecedent dry period. The linear relationship is better for smaller sediments than the bigger ones. This might be caused by the stable input sources of small sediments, which include tire and road surface abrasion, atmospheric deposition, particles from wearing off of different parts of vehicles and so on.

Washoff effect on heavy metal loading

Two rainfall events happened during the sampling periods on March 21 and March 28. The rainfall event on March 21 lasted for about 11 hours and the limited amount of rainfall distributed evenly over the whole period with very little runoff. The second rainfall event on March 28 lasted for about 4 hours with lots of runoff.

Table 2 shows the proportion of mass of sediments that remained on the road one day after a rainfall event to that before the rain for sediments in different size ranges. Results show that after the first rain, the mass of smaller sediments decreased more sharply than that of the bigger sediments. After the second rainfall, small and big sediments decreased at about similar proportions. This might be caused by different rainfall intensities for the two events. The rainfall intensity of the first event was not high enough to wash off bigger sediments. The first event with longer rainfall duration washed off more small sediments than the second event.

Figures 4 and 5 compare heavy metal concentrations in sediments on the road before a rainfall event and sediments on the road one day after the rainfall event for sediments in different size ranges. Results show that the heavy metal concentrations of small sediments less than 40 μm in diameter decreased 6.1% to 55.4% after the rainfall for Cd, Cr, Cu, Ni, Pb, and Zn. A similar trend did not exist for larger sediments.

Conclusions

Sediment samples were collected from the 3-Ring Road in Beijing, China. Heavy metal concentrations and loadings on sediments in different size ranges were studied. Results show that:

1) A mass of sediments less than 500 μm in diameter varies from 11.2 to 25.5 g/m², and a mass of sediments less than 125 μm accounts for 42.9% of the total mass of sediments less than 500 μm on average. The heavy metal loading on sediments less than 125 μm accounts for more than 45% of the heavy metal mass in sediments less than 500 μm. Cd, Cr, Cu, and Ni tend to attach to smaller sediments other than the bigger ones, compared to Pb and Zn.

2) Mass loading of the sediments generally increases with the antecedent dry period. The linear relationship is better for smaller sediments than for bigger ones.

3) Heavy metal concentrations of small sediments less than 40 μm in diameter generally decrease more obviously than the bigger ones after rainfall.

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