D. Michael Revitt

The EU Water Framework Directive (WFD) requirement that all surface water bodies achieve good ecological status is still a goal for many regulatory authorities in England and Wales. This paper describes field and laboratory studies designed to identify metal contaminant loadings and their distributions within water bodies located in the Lower Lee catchment (London, UK). Water and sediment samples have been collected from increasingly urbanised sites on the River Lee and its main tributaries over a two-year period with samples analysed for total concentrations of cadmium, copper, lead, mercury, nickel, tin, and zinc. Complimentary batch tests indicate a positive relationship between aqueous metal concentrations and the batch test-derived sediment metal release data, particularly during wet weather events. Field data indicate a dynamic relationship between water and sediment concentrations with both being capable of exceeding relevant environmental quality standards/sediment quality guidelines at all sites. Mean sediment metal concentrations across all sites were found to be highest for Cu (141.1 ± 111.0 µg g⁻¹), Pb (175.7 ± 83.0 µg g⁻¹), and Zn (499.9 ± 264.7 µg g⁻¹) with Zn demonstrating elevated mean water concentrations (17.2 ± 13.8 µg L⁻¹) followed by Ni (15.6 ± 11.4 µg L⁻¹) and Cu (11.1 ± 17.8 µg L⁻¹).

Swales are one type of sustainable drainage system (SuDS) which contribute to the management of water quality in receiving waterbodies. Using a semi-quantitative approach, an impact assessment procedure is applied to the residual water quality that is carried forward to surface waters and groundwaters following treatment within a swale. Both volumetric and pollutant distributions are considered as stormwater passes through the swale system. The pollutant pathways followed by TSS, nitrate, chloride, metals (Cd, Cu, Pb, Zn) and polyaromatic hydrocarbons (PAHs) are determined for a swale receiving highway runoff. For TSS, metals and PAHs between 20% and 29% of the total mean influent pollutant load is predicted to be directed to infiltration through the underlying soils compared to between 4% and 16% of chloride and nitrate. Although surface water impacts are deemed possible, the discharges of swales to groundwaters are assessed to represent a negligible impact for effectively maintained systems.

Polyaromatic hydrocarbons (PAHs) are emitted from a variety of sources and can accumulate on and within surface soil layers. To investigate the level of potential risk posed by surface contaminated soils, vertical soil column experiments were conducted to assess the mobility, when leached with simulated rainwater, of six selected PAHs (naphthalene, phenanthrene, fluoranthene, pyrene, benzo(e)pyrene and benzo(ghi)perylene) with contrasting hydrophobic characteristics and molecular weights/sizes. The only PAH found in the leachate within the experimental period of 26 days was naphthalene. The lack of migration of the other applied PAHs was consistent with their low mobilities within the soil columns which generally paralleled their log K _oc values. Thus, only 2.3 % of fluoranthene, 1.8 % of pyrene, 0.2 % of benzo(e)pyrene and 0.4 % of benzo(ghi)perylene were translocated below the surface layer. The PAH distributions in the soil columns followed decreasing power relationships with 90 % reductions in the starting levels being shown to occur within a maximum average depth of 0.94 cm compared to an average starting depth of 0.5 cm. A simple predictive model identifies the extensive time periods, in excess of 10 years, required to mobilise 50 % of the benzo(e)pyrene and benzo(ghi)perylene from the surface soil layer. Although this reduces to between 2 and 7 years for fluoranthene and pyrene, it is concluded that the possibility of surface-applied PAHs reaching and contaminating a groundwater aquifer is unlikely.

This report describes the development of a methodology to theoretically assess the effectiveness of structural BMPs with regard to their treatment of selected stormwater pollutants (metals, polyaromatic hydrocarbons and herbicides). The result is a prioritisation, in terms of pollutant removal efficiency, of 15 different BMPs which can inform stormwater managers and other stakeholders of the best available options for the treatment of urban runoff pollutants of particular environmental concern. Regardless of the selected pollutant, infiltration basins and sub-surface flow constructed wetlands are predicted to perform most efficiently with lagoons, porous asphalt and sedimentation tanks being the least effective systems for the removal of pollutants. The limitations of the approach in terms of the variabilities in BMP designs and applications are considered.