Diffuse Pollution Screening Tool – Stage 3
WFD77
September 2006
EXECUTIVE SUMMARY
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:
-
Simplification and summarisation of the current waterbody database
into a spreadsheet in order to facilitate comparison with other studies
and visualisation through linkages with GIS technologies.
-
Further characterisation of the waterbodies through the update of input
data (waterbodies and their boundaries; point source inventory), taking
account of retention of N and P, exploring an improvement of the
regression models used to predict status and assess suspended sediment
risk for two lower standards.
-
Summarisation of the source apportionment data per waterbody along with
agricultural census data to allow for the contextualisation and
interpretation of these source apportionment summaries.
-
Knowledge transfer through the development of a report and the
provision of a workshop.
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.
Copies of this report are available from the Foundation, in electronic
format on CDRom at £20.00 + VAT or hard copy at
£25.00, less 20% to FWR members.
N.B. The report is available for download from the SNIFFER Website