WaterSubject

The emphasis placed on diffuse pollution under Article I of the Water Framework Directive (WFD) is of particular relevance to the problem of nonpoint urban surface water drainage. Although the Directive does not define diffuse pollution, it does specify within Articles II.3 (h) and Article II the need to identify and quantify diffuse sources, with Annex IV and VII requiring estimates and a programme of measures for monitoring and control of such diffuse sources within future River Basin Management Plans (RBMPs). The CIWEM report on diffuse pollution indicates that nonpoint urban runoff is responsible for over 11% of total polluted Scottish rivers (and 31% of seriously polluted rivers) and for the downgrading of at least 4–5% of rivers in England and Wales. The situation may be more serious than this, given that existing regulatory authority monitoring systems for urban watercourses consistently fail to identify the true extent of diffuse pollution resulting from episodic urban surface water drainage. At least one third of all oil pollution incidents within the Thames Region of the Environment Agency can be attributed to impermeable urban surface runoff, and urban drainage comprises the third major source of receiving water pollution in Scotland. However, such diffuse urban pollution cannot be addressed readily by the regulatory authorities given the difficulties of source tracing and the episodic nature of the polluting events.

The proposed changes to the UK planning system and associated urban development approaches recommended in the Barker Report, combined with the legislative context of the WFD, offer considerable opportunities to review the adequacy of current measures for controlling and managing diffuse urban pollution and therefore to either modify them or seek new powers. There are already pressures for developers to consider sustainable drainage systems (SUDS) on new brownfield and greenfield sites and local authorities are increasingly including statements on sustainable drainage in their Local and Unitary Development Plans [or Local Development Documents (LDDs) as they will be termed under the new 2004 planning legislation]. Such statements typically advocate that source control options should be considered at the earliest stages of masterplanning for all new development. At the national level, the recently issued Policy & Planning Guidance Note (PPG25, 2001) on ‘Development and Flood Risk’ advocates that all development plans should promote the use of SUDS drainage and that developers should be required to implement appropriate drainage systems to prevent an increase in flood risk. PPG25 indicates that local authorities should work closely with the Environment Agency, sewerage undertaker, navigation authorities and developers to coordinate surface water runoff control ‘as near to the source as possible through the use of sustainable drainage systems.’ The Environment Agency has also issued policy guidelines and implementation plans (EAS/0102/1/3; August 2002) to promote SUDS as a technique to manage surface and groundwater regimes in a sustainable manner. The Scottish Environment Protection Agency (SEPA) has also issued a policy document (Policy No. 15, May 2001) ‘Regulation of Urban Drainage’ recommending SUDS as the preferred solution for drainage of surface water runoff.

One requirement of the WFD is the identification and quantification of diffuse pollution sources in terms of the generating land use activities and associated discharge loadings. There is already a reasonable database available for diffuse urban runoff pollution, although it is undoubtedly incomplete and variable in terms of quality control and land use coverage. There is, for example, very little data available on some WFD priority pollutants such as arsenic, cyanide or herbicides. The range of pollutant concentrations and loadings associated with diffuse urban runoff indicates that such surface water discharges can be highly variable in quality with event mean concentrations (EMCs) frequently close to, if not exceeding, the minimum NOEL (no observable effects limit) value and thus potentially presenting a problem to receiving water ecology.

It provides a summary of the range of the drainage quality associated with specific types of urban land use types and surfaces (including gully pot liquors), which suggests that even roof runoff can contribute significant loadings of inert solids, nitrogen and bacteria to the storm drainage system. The principal sources of the most significant urban pollutants are identified based on a search of data found in the UK literature. Achievement of Article II objectives of the WFD would be considerably underpinned by a national database for diffuse urban pollution similar to that of the US Environmental Protection Agency sponsored ‘National Stormwater Best Management Practices Database’ developed by the Urban Water Resources Research Council in collaboration with URS Greiner Woodward Clyde. The structural template of this database includes ‘required’ or mandatory data for entry to the main database, e.g. catchment, storm event and sampling detail; ‘essential’ primary data covering specified pollutant parameters and their analysis; and ‘useful-to-have’ data, providing additional site information, e.g. O&M, costings, amenity issues, etc. The Environment Agency in collaboration with HR Wallingford are exploring a protocol structure for SUDS data monitoring, capture and entry, which could form the basis for a UK national standardised database, although the funding and future support for this database development are not clear. A separate database of Scottish SUDS performance is being supported by SEPA through a SUDS Monitoring Programme.

Although such databases are focused on the performance of SUDS structures, they nevertheless provide information on the varying quality of influent stormwater runoff into best-practice control devices, and thus reflect the condition and polluting potential of the associated urban catchment and its land use activities as required under the terms of the WFD. However, relatively little data are available to identify any post-SUDS benefits accruing in terms of receiving water quality or habitat enhancement.

Through literature review, Mitchell has developed a database relating urban runoff quality (given as EMC values) with catchment characteristics, including land use, residential density and vehicle flows. The database addresses 18 common pollutants monitored across 678 urban catchments worldwide, of which 242 are in Northern Europe and 71 in the UK. The database has been used to recommend land use-specific EMC values for use in diffuse pollution load estimation for the UK and Northern Europe.