SOURCE CONTROL OF POLLUTION IN SUSTAINABLE DRAINAGE
Background to research
Source control in sustainable drainage systems (SUDS) means the application of the
techniques close to where incident rainfall lands. It can provide the opportunity for the
retention and degradation of individually minor, widely dispersed pollutants close to their
source. The degree to which source control of pollution through SUDS carries a risk to
groundwater has been unclear and there has been insufficient evidence available to allow
safe policy development, despite statutory drivers stemming from the Groundwater
Directive and Water Environment Water Services (Scotland) Act.
This project has delivered a considerable body of evidence on the behaviour of
contaminants in SUDS which have a soil/ vegetation base. Little was known about the
extent to which pollutants might be retained within the soil layer and degraded into less
harmful compounds. Evidence was needed on whether poly-aromatic hydrocarbons
(PAHs) and oil (reported as total petroleum hydrocarbons – TPH) actually degrade in a
swale system, filter drain network or detention basin, and, if so, how this compares to an
end-of-pipe pond solution for a conventional system. Additional questions arose about the
best management of nutrients in urban catchments and whether it would be beneficial for
regulators to require grass/soil source control treatment in sensitive catchments. Suitable
sites were identified and methodologies developed to test the efficacy of a number of soilbased
SUDS techniques and the fate of selected priority pollutants.
The project included field monitoring and artificial dosing experiments. It has
demonstrated the effectiveness of various SUDS techniques in attenuating and degrading
a range of diffuse source pollutants arising from motor vehicles. The project has
established an evidence base to aid future policy development in this area.
Objectives of the research
The objectives of the research were as follows:
- Determine the risk of movement of pollutants through soil into groundwater in soft-engineering SUDS;
- Measure the immobilisation and degradation of priority pollutants and fate of nutrients in soft engineering SUDS;
- Identify the degradation products in a range of SUDS techniques;
- Determine the conditions for the optimal breakdown of oil and PAHs in the range of SUDS investigated;
- Determine the conditions for the optimal nutrient uptake or stabilisation within a range of SUDS techniques;
- Provide monitoring data for existing SUDS facilities, and
conduct experimental work to aid the interpretation of the field site
Outline of the studies undertaken
The project involved four separate but linked studies of different aspects of SUDS
systems addressing the same contaminants in each case. The project has delivered two
A This final report
B Four separate detailed study reports: (now included as Appendices to the final report)
- Nutrients study – desk top study linking nutrients in agricultural and urban areas.
- Lysimeter study – a semi - field study of twelve lysimeters.
- Pollutant Breakdown – a laboratory based degradation study.
- Field studies – four motorway SUDS and one motorway service area.
- The risks to groundwater from passing highway drainage on to
soil based SUDS is low. There is evidence of very low rates of downward
movement of contaminants.
- In general, contamination from the highway runoff in the basin
soils was found to reduce horizontally from the inlet to the outlet.
There was a noticeable difference in the magnitude of horizontal change
between the basins studied, most likely as a function of the variation
in inlet basin design. Where flow could spread across the broad basin,
pollutant concentrations dropped sharply. Where inlet flow was confined
to a narrow channel concentrations remained higher. This points towards
effective attenuation of pollutants in the soil based systems.
- The vast majority of heavy metals, PAHs and petroleum
hydrocarbons (TPH) are retained in the top 10 cm of soil. This
accumulation may impact on soil function with time and has potential
implications for long term maintenance. Pollutant levels in the pond
sediments were generally higher than in the soil.
- The highest TPH and PAH contamination found in the study came
from one of the filter drain catch pits. Average pollutant
concentrations in filter drain sediment were all lower than found in
the upper 10 cm soil samples of the downstream basin suggesting
accumulation in the basin soil over time.
- At the grass filter strip monitored, which was located at a
motorway service station, no sediment had accumulated in the downstream
filter drain at all, implying that it is all being retained on the
- Metals will accumulate in the surface soil layers of
infiltration based SUDS. The tests were on bare soil lysimeters but in
practice there would be a vegetative layer that would take up some of
the pollutants retained in the soil, reducing further the risk of
movement to ground water. The data generated in these experiments would
suggest that infiltration based SUDS represent a low risk to
- Of the eight ponds sampled, the sediment from six would be
classified as having a severe effect on the aquatic environment.
Sediment from all of the ponds would be precluded from inert landfills
because of TPH concentrations, and could potentially be classed as
hazardous waste. There may be waste management issues if sediment which
has been submerged in a pond is required to be transported and disposed
Key words: Sustainable Urban Drainage Systems, SUDS, Groundwater Protection,
Pollutant Breakdown, PAH Studies
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