ASSESSMENT FOR THE DEVELOPMENT OF A SCREENING TOOL TO IDENTIFY AND CHARACTERISE DIFFUSE POLLUTION PRESSURES
WFD10

May 2003

EXECUTIVE SUMMARY

Phase I of this project investigated the feasibility of the development of a GIS-based screening tool for diffuse pollution in Scotland. Related on-going work in the UK was examined and the particular pollution issues of concern for Scotland have been identified. Suitable modelling approaches were evaluated for both rural and urban diffuse pollution that could form part of a broad screening tool. The availability and sources of data for use with the potential modelling approaches were assessed.

Screening tools for specific pollutants have been developed in the past in the UK, and a project with similar objectives is also currently under way within the Environment Agency. Relevant models to address individual pollutant pressures in both rural and urban areas have also been developed in the past, but application of a screening tool at such a large scale, and covering both the rural and urban pressures, has not been attempted before. Hence there is limited understanding of total diffuse pollution pressures in Scotland, the relative importance of different diffuse pollution sources in mixed catchments, and the reliability of load and impact estimates obtained at this scale. A screening tool to quantify these pressures will be a significant step forward in the implementation of the Water Framework directive, and provide new insights into diffuse pollution problems that are relevant throughout the UK. This study was the first phase in development of such a tool.

A number of spatial datasets have been identified that are considered indicative of both rural and urban diffuse pollution pressures. These data sets comprised spatial areas (land cover, land use including cropping and livestock numbers, soils, climate and topography), linear spatial datasets of rivers, catchments and features such as roads; and interpretative tabular data such as soil physical properties, agrochemical inputs by crop, manure outputs by livestock type, or pollutants per km of road. Many data requirements were found to be common to a number of pollutant indicators. Examination of data sources for Scotland showed that a sound foundation of data covering the whole of Scotland was available which would allow some form of indicator calculation for all pollutants. In some cases data required transformation or combination with other data to generate an estimate of the required input . In other cases spatial interpolation might be required. Significant work would be necessary to bring all data sets into a common framework. This work would be a priority activity in implementation of the Screening Tool.

A range of model and indicator approaches was assessed. Broadly the simplest modelling approaches were based on runoff modelling combined with typical urban run-off water quality data for urban pollutants; and export coefficient approaches (per unit of land use or livestock) coupled to transport indicators based on climate and physical data for rural diffuse pollutants. More complex approaches existed especially for the major pollutants. Some would be suitable for use within a simple annual-level screening tool but many involved substantial added complexity or data requirements, and incorporating them within a Screening Tool would have major time and cost implications.

It is proposed that the Screening Tool should be based on calculation cells of 1 km2 initially; with an annual or at most monthly time step of calculation using climatic data. Incorporation of finer spatial or temporal scales of calculation as required by some models is for consideration in later Phases of the project, in the light of more detailed assessment of costs and benefits to SEPA.

The Indicators will require evaluation and calibration against summarised river measurement data from a range of catchments with different land use and environmental conditions. This will enable classification of indicator values to match SEPA criteria as developed under the WFD implementation programme. For strongly correlated indicators it may be possible to incorporate the calibration within the indicator calculation so that the output is a direct prediction of the river water quality criterion required. The validation and calibration stage is recommended for both simple and complex indicators, since few have been comprehensively tested against Scottish conditions.

Assembly and processing of the data is the most urgent task. This includes assembly and processing of spatial and interpretative data sets for Scotland to a common standard and spatial scale; decisions on and collation of river vectors and catchment boundaries; determination of river standards; and collation and summarising of selected river water quality data for validation and calibration. Data licensing, although not expected to be a serious problem, will need to be addressed at an early stage too.

Decisions will need to be made on which model or indicator calculation to use for each pollutant. The resources devoted to individual pressures and pollutants will need to be agreed. More than one relevant dataset and modelling approach was identified for many pressures and pollutants.

While it is difficult to prescribe at this stage what the screening tool would look like in detail, its main features and functionality can be outlined. The user will interact with the screening tool via a map display. It should be possible for the user to generate additional unit area estimates using modified data or different modelling approaches or coefficients. There should be various options for the user to aggregate the estimates, for example options to calculate summary loads for the urban and/or rural fraction of a catchment and for the catchment as a whole. Comparison between total loads generated from different land uses or activities might also be considered desirable.

In summary, Phase 1 of the Screening Tool project has shown that the data for such a Screening Tool exist for Scotland; and that suitable Indicators either exist or can be derived for the pollutants of concern under the WFD. It is therefore concluded that creation of a Screening Tool as proposed in the Project is practicable and would be of great use in WFD policy development. A more detailed scoping study of implementation of the Screening Tool in Phases 2 and 3 has been prepared as a separate document.

KEY WORDS
Water Framework Directive, catchment, diffuse pollution, risk

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