The impact of phosphorus inputs from small discharges on designated freshwater sites

Nutrient enrichment of freshwater SSSIs is a
widespread problem, with the majority of river and
lake SSSIs in England affected. A range of measures
have been taken to reduce this problem, many of
which are ongoing. These have mainly focussed on
reducing inputs from sewage treatment works and
agriculture. For some time concerns have been
raised about the potential role played by small
domestic discharges, such as septic tanks, but it is
only relatively recently that this issue has received
significant attention.
To help address this issue, Natural England, with a
contribution from the Broads Authority,
commissioned the Centre for Ecology & Hydrology
(CEH) in 2009 to conduct a review of the potential
risk posed by small domestic discharges, such septic
tanks, to freshwater SSSIs. This focussed on the risk
of phosphorus (P) pollution to sites that are
vulnerable to hyper-eutrophication.
The overall aims were to assess whether significant
risks were likely to occur, either at a wide scale or
under local circumstances and, if they were, to
suggest approaches for refining the risk assessment
across the freshwater SSSI series. In addition, CEH
were asked to consider the options available for
reducing the impact of P pollution from septic tanks
where necessary. To this end, the study was divided
into two main components:
• To review appropriate literature relevant to this
issue.
• To conduct two desk-based case studies, in the
River Avon SAC catchment and The Broads, to
estimate the number of dwellings not on mains
sewerage and to assess the possible P pollution
arising from these.
The findings and recommendations from this work
are contained within this report. The majority of the
work was conducted in 2009, with updates in 2013
for certain topic areas such as:
• de-sludging of tanks to reduce P discharges;
• the effectiveness of reedbeds and wetlands to
clean effluent; and
• the current status of registration schemes in
England, Wales and Scotland.
Whilst some aspects of this report may have been
superseded by more recent studies and
developments, it still represents one of the most
thorough reviews undertaken on this subject in the
UK. As such, Natural England considers that it is
worth publishing. However, the case studies in
Chapter 6 make the precautionary assumption that
all of the P estimated to be produced by septic tanks
reaches the watercourse. More recent work
undertaken by CEH for Natural England (NECR171)
strongly suggests that this assumption is over-
precautionary.
The work contained within this report has allowed
Natural England to develop a focused research
strategy to help better understand the risk posed by
septic tanks to freshwater SSSIs. It has been used by
the Environment Agency and Defra as a key source
of information on this topic during a 2012 Public
Consultation on septic tanks.