Trends in Scottish river water quality
Under the Harmonised Monitoring Scheme (a sub-set of our
monitoring network), SEPA monitors and reports on water quality in
56 Scottish rivers. SEPA and its predecessor organisations have
monitored these rivers since the mid 1970s, producing data that
provides an invaluable record of environmental status and change in
Scotland.
Our analysis of trends in these records supports two main
conclusions:
- Climate change is already having an effect on Scottish rivers,
with water temperature increasing in rivers across Scotland and
river flow increasing in some places.
- Stewardship of Scotland’s water environment is working and
improvements in water quality have been delivered through
environmental regulation, cleaner technologies, improved sewage
treatment and changes in agricultural practice.
The maps presented on this website are part of a joint
SEPA/Macaulay Land Use Research Institute initiative to produce a
dossier on water quality trends for Scottish rivers. This website
presents highlights from the full Trends in
Scottish river water quality 
(12.6mb) report.
Introduction
Scotland’s rivers are actively monitored by SEPA for a whole
range of regulatory and environmental management purposes. The
latest status and classification of Scotland’s rivers, which is
based on a three year dataset and is reported under the Water
Framework Directive, can be found at www.sepa.org.uk/water/monitoring_and_classification.aspx.
The Harmonised Monitoring Scheme (HMS) is an important subset of
SEPA’s river monitoring activities. The former Scottish River
Purification Boards started collecting HMS data in the mid 1970s.
SEPA has continued to conduct this work since its inception in
1996. These records, and the on-going sampling, provide an
invaluable record of environmental change in Scotland. Each of the
56 rivers monitored under the HMS is sampled at a location as close
as possible to the tidal limit. Samples are collected monthly and
water temperature and flow are recorded at the time of sampling.
Our scientists analyse these samples for a range of natural and
synthetic substances. Daily river flows are also recorded and used
to estimate fluxes of materials from the rivers to the sea.
Changing flows can make trends in concentration more difficult
to interpret. However, by assessing data on concentrations and
fluxes (loads) together, we can obtain a better understanding of
environmental change. For example, if river flows increase and the
mass of a pollutant entering a river remains the same then the
concentration of that pollutant in the river will inevitably
decline.
Methods
We analysed the data for monthly, seasonal and
annual trends. Experience has shown that it is important to look
for monthly and seasonal trends because annual figures might not
capture all environmental shifts, particularly more subtle shifts.
For example, some rivers show increasing spring flows and
decreasing summer flows but no trend in annual average flows.
A simple statistical tool known as the
Mann-Kendall test was applied to each annual and season time
series, with each of the seasons being treated as a distinct
dataset for the seasonal analysis (ie summer, spring, winter and
autumn data spanning the 30 year monitoring period were each
assessed in isolation). The Mann-Kendall test looks for overall
increases or decreases over time. While the test is robust, it is
not very sophisticated and it does not provide any information
about complex time-related patterns in the data, but rather
identifies broad trends.
We looked for trends in both physical and
chemical parameters. Physical parameters such as river flow, water
temperature and suspended sediments are affected by a changing
climate. Suspended sediment is also affected by changing
agricultural practice and urban sewage treatment processes. For
chemical parameters, we looked at Biochemical Oxygen Demand (BOD)
as a broad measure of water quality because high levels of BOD are
often associated with industrial and sewage pollution. Other
chemical parameters we looked at were ammoniacal nitrogen (a form
of nitrogen which can come from fertilisers, livestock and sewage)
and total phosphorus concentrations. Phosphorus chemistry is quite
complicated and high levels of the element can indicate that a
river is affected by sewage, industrial activity or agricultural
runoff.
Results
Average flow
Water temperature
Suspended solids
Biochemical Oxygen Demand
(BOD)
Ammoniacal
nitrogen
Total phosphorus
Average
flow
River flows are changing across Scotland. Annual mean river flows
are increasing in many rivers and there are no rivers showing
statistically significant declining trends in annual average flow.
This suggests that climate change is making Scotland a wetter
place. The trends towards increasing spring flows are mostly the
result of changing temperatures; warmer springs result in earlier,
more rapid snowmelt. In some rivers, the effect is amplified by
higher rainfall.
Annual
average flow (372kb)
Winter
average flow (369kb)
Water
temperature
Annual average temperatures are increasing in many Scottish rivers.
This may have important implications for river ecology and salmon
behaviour. While some of the trend towards warmer water might be
the result of increased urbanization, most is thought to be due to
warmer air temperatures. This effect is most pronounced in the
winter, when widespread warming has been observed. A cooling trend
was observed in the River Wick in the far north of the country, and
we are working on understanding what might be causing that.
Annual
temperature (372kb)
Winter
temperature (369kb)
Suspended
solids
The amount of suspended solids in water is an important
measure of water quality. High levels of suspended solids are
associated with soil erosion and sewage effluent. Changes in soil
erosion can be caused by changing rainfall, agricultural practice
or increased construction activity. Improvements in sewage
treatment often lead to reductions in suspended solids
concentrations in rivers which run through cities.
Two trends can be seen in the annual
concentration of suspended solids in Scotland: concentrations are
declining in the centre and south of the country, probably
resulting from improvements in sewage treatment and industrial
processes, but are typically increasing in the north east, possibly
related to increased erosion. This pattern is more obvious when
looking at trends in spring concentrations where there are
increasing trends in many rural and agricultural catchments.
Annual
suspended solids concentration (372kb)
Spring
suspended solids concentration (372kb)
Biochemical Oxygen Demand
(BOD)
Biochemical oxygen demand (BOD) is a measure of
the oxygen consumption of a water sample. Typically, the test is
run over five days and results are reported as milligrams of oxygen
consumed per litre of water. High levels of BOD are an indication
of poor water quality and are often associated with urban or
industrial pollution.
In Scotland, BOD has declined in almost all
rivers. However, this study identified possible increasing trends
in BOD concentrations in the Water of Leith and the River Tyne, the
underlying causes of which will be investigated further. A
similarly positive picture is apparent when looking at trends in
BOD flux. The absolute amount of BOD is declining in most Scottish
rivers but there are some exceptions, most likely due to changes
both in flow and in BOD concentration.
Annual BOD
concentration (372kb)
Annual BOD
load (370kb)
Ammoniacal
nitrogen
Ammoniacal nitrogen is associated with
sewage and agricultural fertiliser. Concentrations of ammoniacal
nitrogen are declining in rivers across Scotland. The Don was the
only river identified as having an increasing trend and is the
subject of ongoing investigations.
A slightly different picture appears when
looking at fluxes: the trends toward increasing flows in many
rivers offset some of the reductions in concentration and thus
fewer rivers show statistically significant declines in fluxes than
is seen for concentrations.
Annual
ammonia concentration (372kb)
Annual
ammonia load (369kb)
Total phosphorus
Phosphorus is an
essential macro-nutrient and so is applied as a major component in
fertiliser. When phosphorus levels are high, the resulting
excessive growth of aquatic plants (phytoplankton and macrophytes)
in rivers and lakes can lead to water quality problems such as a
large daily variation in dissolved oxygen. Total phosphorus
measures the combined concentration of dissolved and
particulate-bound phosphorus (e.g. phosphorus bound to suspended
sediments).
Total phosphorus concentrations are very low
in the north of the country but are higher in agricultural and
urban areas. High total phosphorus concentrations in summer can be
linked to sewage effluent while high concentrations in the winter
and spring are often associated with sediment losses from
agricultural catchments. This investigation revealed that
concentrations of total phosphorus are declining in many urbanised
rivers, an improvement in water quality which appears to have been
achieved by improved sewage treatment and reduced use of
phosphate-containing detergents. However, the total phosphorus
concentration in some northern rivers appears to have
increased.
Increases in summer total phosphorus flux were
noted in some catchments. The reasons for the increased phosphorus
concentrations and flux could be linked to historical patterns of
fertiliser use, to increases in the total volume of sewage
discharges in northern areas or to climate change.
Annual total
phosphorus concentration (372kb)
Summer total
phosphorus load (373kb)
Next Steps
This assessment demonstrates the great value of
long-term environmental monitoring in identifying and
characterising the state and changing state of the environment. It
is crucial that we maintain and develop such monitoring and that we
do so in conjunction with other science and research providers
(such as the Macaulay Institute) and other stakeholders as part of
a shared approach to environmental stewardship.
Web Links
Harmonised Monitoring Scheme
(HMS)
www.defra.gov.uk/environment/statistics/inlwater/iwhmsdb.htm 
Trends in
Scottish river water quality (12.6mb)