February 2008
EXECUTIVE SUMMARY 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:

  1. Determine the risk of movement of pollutants through soil into groundwater in soft-engineering SUDS;
  2. Measure the immobilisation and degradation of priority pollutants and fate of nutrients in soft engineering SUDS;
  3. Identify the degradation products in a range of SUDS techniques;
  4. Determine the conditions for the optimal breakdown of oil and PAHs in the range of SUDS investigated;
  5. Determine the conditions for the optimal nutrient uptake or stabilisation within a range of SUDS techniques;
  6. Provide monitoring data for existing SUDS facilities, and conduct experimental work to aid the interpretation of the field site data.

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 separate reports;

A This final report
B Four separate detailed study reports: (now included as Appendices to the final report)

Key findings

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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 grass strip.

  6. 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 groundwater.

  7. 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 off site.

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