Diffuse Pollution Screening Tool – Stage 3
September 2006


Background to research

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).

Phase II of the project (WFD19, 230/8050) was therefore commissioned to develop, implement and apply a basic screening tool for all potential diffuse pollutants in order to assess the risk of individual waterbodies failing to meet good ecological status required under the WFD. The screening tool was built on simple models of pollutant pressures and loads delivered to these waterbodies. To achieve this, a national Environment Database was first constructed, containing environmental and agricultural statistical data summarised to 1km2 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 78,770km2 for Scotland and 14,140km2 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 1km2 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; g) and 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 1km2 cell across each country. Taken together, the models provided estimates of the diffuse pollutant 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 were made by use of per capita export coefficients. Summary statistics were calculated andstored 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 the 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. In the absence of ecological criteria for good water status, the risk of failure was re-defined as the risk of failing to meet existing chemical water quality standards that were based on pollutant concentrations. These included the Environment Quality Standards (EQS) set for Priority Substances, the standards set for nitrate and pesticides under the Nitrates and Drinking Water Directives, and the SEPA Operational Standard for soluble phosphorus in freshwaters. In most cases, the selected modelling methodologies led to an over-estimate of risk, as they did not take account of dilution in the receiving water body or of retention.

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 failure due to diffuse sources only, point and diffuse sources. These results were combined with an expert assessment of the risk of failure for acidification, metals and faecal indicator organisms.

Objectives of research

The user-friendliness of the Environment Database produced during phase II of the project was thought to be restricting the wider dissemination of the valuable data contained therein. In addition new datasets and improved methodologies offered the opportunity to address some the weaknesses inherent in the modelling undertaken in phase II of the project. These were just some of the factors that drove Phase III of this project. The specific objectives were:
Key findings and recommendations

The development of the Summary Database Tools for Scotland and Northern Ireland facilitate access to  the wealth of data that are housed within version 2 of the Environment Database in a logical and userfriendly manner. Importantly the tool allows comparison with WFD Article 5 Pressures and Impacts summary data, simple source apportionment, land cover and land use at a water body scale. In addition, associated tools and utilities allow the user to investigate the impacts of changing Environmental Quality Standards thresholds and associated impacts. The Summary Database Tool was used to undertake a preliminary interpretation of the version 2 Environment Database.

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 in Scotland and actual monitored discharges in Northern Ireland. Diffuse sources in Scotland accounted for c. 65% of phosphorus, 83% of nitrate and 99% of sediment losses, but only 32%of faecal coliform inputs to rivers. Roads and urban areas were found to make a significant ca 9% contribution to the total modelled sediment losses. In Northern Ireland diffuse sources accounted for c. 77% of phosphorus, 87% of nitrate and 99% of sediment losses, but only 10% of faecal coliform inputs to rivers. Roads and urban areas were found to make a significant ca 12% contribution to the total modeled sediment losses.

Analyses based on modelled nitrate, phosphorus, sediment and biochemical oxygen demand losses from agriculture and forestry determined that only 36% of Scotland and 39% of Northern Ireland could be demonstrated to be not at risk of failing to achieve good ecological status with confidence. 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. 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 confined to certain areas, for example developed areas on the coast in Scotland. 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. This increased ease of access to the Environment Database will allow further and a more thorough exploration of the dataset to inform WFD policy implementation and mitigation.

Key words: Water Framework Directive, Diffuse Pollution, Screening Tool, Summary Database Tool, Scotland, Northern Ireland.

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