Index models to evaluate the potential metal pollution contribution from washoff of road-deposited sediment

• An innovated road-deposited sediment (RDS) index was developed.
• The RDS characteristics were quantified in the RDS index models.
• Both source and transport factors were considered in the RDS index models.
• The RDS index models can be used to calculate the pollutant load and strength.
• The RDS index models can be used to evaluate potential risk of runoff pollution.

The proper evaluation of pollutant strength and loads associated with road-deposited sediment (RDS) is crucial for controlling diffuse pollution in urban areas. A new index model, which combines source and transport factors and is called the RDS index, was developed using RDS characteristics (e.g., the amount, grain size, mobility, and metal concentrations) and used in a case study in the Beijing region. The observed and weighted RDS characteristics along an urban–rural gradient, which included central urban (UCA), urban village (UVA), central suburban county (CSA), rural town (RTA), and rural village (RVA) areas, were used to calculate the RDS index for the pollutant load (RDSindex,load) and the pollutant strength (RDSindex,strength). Our results demonstrated that the RDSindex,load and RDSindex,strength values both changed significantly along the urban–rural gradient. RDSindex,strength increased along the urban–rural gradient and the RDSindex,load value along the main roads decreased in the order RVA > UCA > CSA > RTA. The method offers a new way of assessing metal pollution in RDS and provides an important scientific basis for controlling pollution caused by RDS washoff.

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