The overall aim of the EU Water Framework Directive (WFD) is the protection and enhancement of all water resources in the European Union. The WFD takes a combined approach to pollution control, limiting pollution at source through emission limit values, and setting water quality objectives for water bodies. The WFD requires that member states estimate and identify significant pollution sources and take action to limit these emissions if they are considered to be harmful. Diffuse urban sources are amongst the most difficult to control, although sustainable urban drainage systems (SUDS; structural source controls and best management practices) are increasingly recognised as appropriate control mechanisms. Most SUDS are currently implemented in new developments, whereas existing urban development provides the majority of diffuse urban loads, hence "retro-fitting" source controls to existing developments must become a SUDS priority. This requires that urban areas contributing the greatest loads are identified, so as to permit more detailed loads assessment (e.g. through detailed monitoring and/or deterministic modelling) and possible SUDS application. This preliminary assessment should preferably be conducted at the basin scale, in accordance with the river basin management planning requirement of the WFD. There are a variety of techniques for conducting this preliminary diffuse urban loads basin assessment. Of these, statistical volume-concentration methods are considered to be most accurate, and remain cost-effective to implement. The volume-concentration methods estimate non-point loads as the product of runoff volume and an emission coefficient, the site event mean concentration (site EMC), and make use of the widely reported observation that EMC is independent of discharge volume. Mean site EMC values characteristic of US urban areas are widely used in the USA, including in the stormwater planning assessments of local government seeking discharge permits under the Clean Water Acts' National Pollution Discharge Elimination Scheme. The volume-concentration method is equally applicable in the European context, but does requires that appropriate runoff estimation techniques and EMC values are used. No recommended EMC values are available for use in Europe, hence the aim of this document is to identify site mean EMC values (pollutant emission coefficients) suitable for application to diffuse pollution screening models applied within northern Europe, and the UK. The objectives are to:

• Assess the validity of the log-normality relationship for European stormwater EMC's, to permit better identification of mean site EMC values and probabilistic modelling;
• Identify any significant differences in pollutant EMC between different urban land-uses, so as to facilitate more spatially resolved stormwater screening assessments;
• Recommend site EMC values suitable for application in UK and northern European diffuse pollution screening models.

The report presents an introduction to urban diffuse pollution and the requirement for preliminary planning (screening) models. The flow weighted pollutant concentration coefficient (EMC) is introduced, and details of its statistical properties and its use in screening models is outlined. The development of a stormwater quality database addressing UK, European and "Global" EMC values is traced. Statistical information on 18 stormwater pollutants including metals, nutrients, oxygen demand, sediment and total hydrocarbons is presented. The analysis concludes with a comparison of results with other non-European meta-analyses, a discussion of the role of land use re EMC's, and the presentation of recommended mean site EMC values for application in northern Europe and the UK.