Provision of a Screening Tool to Identify and Characterise Diffuse Pollution Pressures: Phase II
WFD19 (230/8050)
January 2006


Water Framework Directive, catchment, diffuse pollution, risk assessment Funders

The first phase of this project investigated the feasibility of developing a Geographic Information System (GIS) based screening tool for diffuse pollution at the national scale, involving a review of available modelling methodologies and datasets.Relevant models to address individual pollutant pressures and appropriate datasets were found to have been developed in the past, but application of a screening tool at such a large scale, covering both rural and urban pressures, and considering all pressures, had not been attempted before. Nevertheless, it was concluded that a basic-level screening tool for Scotland and Northern Ireland was practicable and would be a significant contribution to the characterisation of water body catchments under the Water Framework Directive (WFD).

SEPA’s characterisation report ‘Pressures and Impacts on Scotland’s Water Environment’ and the EHS report ‘Water Framework Directive Summary Report of the Characterisation and Impact Analyses Required by Article 5, Northern Ireland’ published in 2005 assessed the risk of water bodies failing to achieve good ecological status from diffuse pollution. This assessment was based on current surface water monitoring which is predominantly designed to assess the impacts on water bodies from point sources. The primary objective of this second phase of research was therefore to develop, implement and apply a basic-level screening tool for all potential diffuse pollutants for Scotland and Northern Ireland,. The screening tool was to be built upon simple models of pollutant pressures and loads delivered to the water bodies. The secondary objective was to identify those water bodies where additional monitoring or a more complex modelling approach needs to be taken, and to provide guidance on the modelling approach and tools required.

To achieve this, a national Environment Database was first constructed, containing environmental and agricultural statistical data summarised to 1km 2 grid cells suitable for visualisation and querying in a Geographic Information System. The database collates information on specific properties (e.g. land use, agricultural livestock numbers and population counts) controlling pollutant inputs and intrinsic environment properties (e.g. topography, soil physical properties and climate statistics) controlling risk of pollutant mobilisation and delivery. The database covers a land area of 78,770km 2 for Scotland and 14,140km 2 for Northern Ireland, and includes a modelled monthly water balance and index of landscape connectivity. The final database, including summaries of model results, contains more than 80 tables and 1,000 items of data for each 1km 2 cell.

Modelling methodologies to calculate pollutant pressures and loads delivered to surface water bodies and to the base of the soil profile were developed for a) nutrient nitrate and phosphorus; b) heavy metals; c) acidification risk; d) suspended sediment; e) biochemical oxygen demand; f) priority substances and pesticides and g) faecal indicator organisms. The methodologies were developed to work with environmental and agricultural data that were available for the whole of Scotland and Northern Ireland. The methodologies were generally based upon existing indicators of relative pollution risk that are appropriate for application at the regional and national scale, rather than detailed mechanistic modelling. The models also developed approaches that had previously been applied for policy work in the UK, including elements of the NIRAMS (Nitrogen Risk Assessment Model for Scotland) model of nitrate leaching and the Event Mean Concentration (EMC) model of pollutants in urban runoff, or are being developed for this purpose, such as the PSYCHIC model (Phosphorus and Sediment Yield Characterisation in Catchments).

The models and Environment Database were linked to calculate pollutant pressures and loads for each 1km 2 cell across each country. Taken together, the models provided estimates of the diffuse pollutant loads derived from a) agricultural land; b) forestry; c) paved urban areas; d) road infrastructure; e) and septic tank diffuse sources. Additionally, estimates of the pollutant load from point sewage treatment discharges were made by use of per capita export coefficients. Summary statistics were calculated and stored in the national Environment Database, giving data on the proportion of the total pollutant load derived from each diffuse source. The calculated pollutant pressures were summed for the catchments of the river, coastal and lake water bodies defined by SEPA and EHS for reporting under the WFD.

Where monitoring data were available, the outputs from the models were validated against observed loads for selected pollutants. Observed loads were collated for 6 test catchments in Northern Ireland and 13 catchments in Scotland. The risk of failure was assessed by comparing concentrations with UKTAG recommended standards. In most cases, the selected modelling methodologies led to an over-estimate of observed loadings, as they did not take account retention in the receiving water body. However, it is possible that the limited ‘spot sampling’ of pollutant concentrations also resulted in an under-estimate of the true observed loadings.

For nitrate, phosphorus, biochemical oxygen demand and suspended sediment, intermediate empirical statistical models were developed that predicted observed percentile pollutant concentrations from modelled total pollutant load and drainage. These models were then used to predict the likelihood that river water pollutant concentrations were greater than the appropriate standard for unmonitored catchments. The risk of failure due to priority substances and pesticide runoff was assessed by comparison of modelled concentrations in land drainage with standards. The results of these models are included in the Environment Database and allow assessment of the risk of not achieving good ecological status due to diffuse sources only and both point and diffuse sources. These results were combined with an expert assessment of the risk of not achieving good ecological status for acidification, metals and faecal indicator organisms.

The output from the models was used to calculate the relative importance of point and diffuse sources for each of the pollutant pressures, using simple per capita export models to characterise sewage treatment works discharges. Diffuse sources accounted for c. 65% of phosphorus, 85% of nitrate and 85% of sediment losses, but only 30% of faecal coliform inputs to rivers. Roads and urban areas were found to make a significant 10% contribution to the total modelled sediment losses.

Analyses based on modelled nitrate, phosphorus, sediment and biochemical oxygen demand losses determined that only 30.9% of Scotland and 12.4% of Northern Ireland could be demonstrated to be not at risk of achieving good ecological status with confidence. However, when monitoring results were included this figure rose 51.4% with confidence for Scotland. It is necessary to emphasise that this analysis is risk averse. A significant land area could not be proven to be either failing or achieving good ecological status, so was included in the area at risk. This area is significantly higher than that identified in both SEPA’s and EHS’s characterisation report and a priority for future research will be improving confidence in the risk assessment in these areas. In both countries, nitrate and sediment were not a major cause of failure according to current water quality criteria. Phosphorus losses resulted in the greatest land area designated at risk, and were primarily associated with agriculture. Other diffuse pollutant sources, including roads and urban areas, were more critical for losses of priority hazardous substances.

The process of developing the screening tool methodology has identified a need for more extensive monitoring of priority substances, pesticides and metals concentrations in fresh waters throughout Scotland and Northern Ireland. At present, monitoring is largely confined to developed areas on the coast. The screening tool outputs can be used to target additional monitoring in areas at high risk. Also, there is a need to integrate existing hydrological and general water quality datasets to develop observed load estimates for every sub-catchment to facilitate improved validation of the pollutant load models. This will also require an improved and consistent inventory of point source inputs to the water bodies, along with estimates of retention losses, to allow a like for like comparison of modelled and observed loads.

In summary, this research has provided SEPA and EHS with a significant data and model resource for characterisation of river and lake catchments for a wide range of urban and rural diffuse pollutants. The combined outputs will support further catchment scale pollution research, and ultimately the development of targeted and effective catchment management plans to meet the needs of the WFD.

Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 50.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website