Heavy Metal Contents of Road-Deposited Sediment along the Urban–Rural Gradient around Beijing and its Potential Contribution to Runoff Pollution

Abstract

Understanding the contribution of road-deposited sediment (RDS) and its washoff process is essential for controlling urban runoff pollution. Ninety-seven RDS samples were collected along the urban–suburban–rural gradient from areas of five administrative units in the Beijing metropolitan region, including central urban (UCA), urban village (UVA), central suburban county (CSA), rural town (RTA), and rural village (RVA) areas. RDS washoff was evaluated with different particle sizes using a rainfall simulator. Heavy metal elements (i.e., Cr, Cu, Ni, Pb, and Zn) were estimated in both RDS and runoff samples. The RDS mass per unit area increased in the order UCA (21 ± 24 g/m2) ≈ CSA (20 ± 16 g/m2) < RTA (59 ± 63 g/m2) < RVA (147 ± 112 g/m2) ≈ UVA (147 ± 198 g/m2). Compared to RDS from the other administrative units, RDS from the UCA and CSA had higher metal concentrations and higher proportions of smaller particles, whereas that from the RVA and UVA had larger quantities of metals per unit area. UCA and CSA had lower potential runoff pollution contributions per unit area. Our findings imply that controlling the first flush in the UCA and CSA, and improving existing street cleaning methods and road surface conditions in the TRA, UVA, and RVA will be appropriate strategies for controlling runoff pollution from RDS.

Understanding the relationship between heavy metals in road-deposited sediments and washoff particles in urban stormwater using simulated rainfall

“…Our RDS samples were collected from each of the administrative divisions in the Beijing metropolitan region. These administrative division units are called a central urban area (UCA) within the urban district, a central suburban county area (CSA) within a suburban county, a rural town area (RTA) within a rural township, a rural village (RVA) area and an urban village area (UVA) [6]. The series UCA, CSA, RTA, RVA and UVA has an urban-rural gradient feature with a gradient variation through different administrative divisions, and differ in characteristics such as population density, average daily traffic, total energy consumption, the method of street cleaning and street cleaning frequency (Table 1).…”

“…Samples with an RDS fraction <250 m (<44, 44-62, 62-105, 105-149, 149-250 m) were obtained from the UCA and CSA sites, while those >250 m (250-450 and 450-1000 m) were collected from the RVA and UCA sites. Further details regarding the experimental design are available elsewhere (Supporting Information; Zhao et al, 2011) [6].…”

“…Each size fraction of RDS in the runoff from the impervious surface was expressed as a percentage of the total RDS mass in the runoff with respect to the initial total mass of RDS on the surface. This was calculated by the following equation [6]:…”

“…Heavy metals washoff from RDS is an increasing problem associated with rapid urbanization that results in urban diffuse pollution [3,4]. Because of the difficulty associated with using natural rainfall events to investigate rigorous RDS washoff processes [5,6], washoff is the least understood among the three processes that occur before materials enter receiving waters (RDS buildup, washoff and transport) [7]. An in-depth understanding of these processes is essential to elucidation of the relationships of pollution between RDS and washoff particles [8,9].…”

“…The urban-suburban-rural gradient pattern is has been strongly affected by rapid urbanization in China, and in turn affects land use zoning, population density, proportion of impervious surfaces, traffic density, energy consumption, street cleaning methods, and spatial distribution of manufacturers [6,22]. Therefore, the urban-rural gradient could affect the amounts and proportions of different chemical fractions of heavy metals in RDS and washoff particles by influencing the particle size of RDS, the metals associated with RDS and the RDS washoff process.…”