Provision of a Screening Tool to
Identify and Characterise Diffuse Pollution
Pressures: Phase II
WFD19 (230/8050)
January 2006
EXECUTIVE SUMMARY
KEY WORDS:
Water Framework Directive, catchment, diffuse pollution, risk
assessment Funders
Summary
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