J. Bryan Ellis

INCIDENCE OF HEAVY METALS IN STREET SURFACE SEDIMENTS: SOLUBILITY AND GRAIN SIZE STUDIES

Samples of surface and gutter sediment from roads within N.W. London have been analyzed for levels of Cd, Cu, Fe, Mn, Pb, and Zn. Sediment and associated metal grading curves are presented and apart from Mn and Fe which reflect natural background levels, the remaining metals show distributions which relate to road type and traffic conditions. For highways carrying the highest traffic densities the concentrations of Cd and Pb are greatest for grain sizes between 100 and 500 µm and an aggregation process is postulated. Residential side street samples show a marked affinity of Cd, Fe, and Zn for the coarsest grain fractions.Laboratory sorption and desorption studies are described and solution concentrations are obtained for Cd, Cu, Mn, Pb, and Zn over 1 to 30 day time periods. Solubility curves are typically variable with time. The relationship of leachate patterns to particle size and sample site location is discussed and its relative importance to Stormwater loadings is considered. Extraction efficiencies for the five metals are found to be independent of road type and in the order Cd > Zn, Cu > Mn > Pb. The relevance of this hierarchy to Stormwater toxicity is discussed.

Hydrological controls of pollutant removal from highway surfaces

Abstract Sediment and metal loadings to roadside gully-pots (weight/area of catchment/runoff depth) are calculated for two defined sites within a highway catchment and compared with data from other urban studies. The higher metal loadings associated with the more active site are particularly noticeable for Cu but at both sites the relative sizes of the metal loadings reflect the expected metal availabilities. The measured total surface runoff volumes represent between 34 and 83% of the total incoming rainfall volume for the catchment. The possible reasons for these water losses are assessed in terms of surface characteristics, seasonal variations and instrument limitations. Stepwise linear regression analysis of pollutant loadings against five selected hydrological parameters indicates that total runoff volume and storm duration together explain over 90% of the observed variance in Pb, Cd, Mn and sediment loadings. The results demonstrate the lack of importance of antecedent dry period and rainfall intensity in controlling the removal of particulate associated pollutants from this catchment. The derived model is used to predict temporal pollutant removal rates and these are shown to be consistent with those reported in other urban catchment studies.

Transport mechanisms and processes for metal species in a gullypot system

Abstract The measurement of soluble and solid associated Cd, Cu, Pb and Zn species within the immediate environment of a gullypot during storm events has enabled the identification of six major processes which affect metal mobilisation and transport through the gullypot system. These processes are described under the headings: (a) Atmospheric Fallout, (b) Acid Rain Mobilisation from Road Dusts, (c) Constant Metal Concentration Time Zones, (d) pH/Surface Area Dependent Suspended Solid Metal Transport, (e) Metal Increase in the Gullypot Liquor between Storm Events and (f) Contribution of Gullypot Liquor and Sediment to Metals in the Gullypot Outflow. The contribution of each of these processes towards controlling the form of the metal in the gullypot outflow is discussed and a number of theoretical concepts are established.

Seasonal removal of herbicides in urban runoff

The occurrence of two herbicides (diuron and simazine) in receiving surface waters is related to their seasonal applications within an urban catchment. Comparisons of herbicide concentrations within baseflow and during rainfall conditions provide an insight into pollutant transport processes following application to urban surfaces. Two storm events, with different hydrological characteristics, are described. The maximum recorded herbicide concentrations reached 238.4 and 2.23 μg/l for diuron and simazine, respectively, with the former corresponding to an application loss of 45.1% with respect to the diuron recently applied to the urban catchment. The factors influencing the extent of application losses to receiving waters of herbicides applied to urban substrates are discussed.

An impact assessment methodology for urban surface runoff quality following best practice treatment.

The paper develops an easy to apply desk-based semi-quantitative approach for the assessment of residual receiving water quality risks associated with urban surface runoff following its conveyance through best practice sustainable drainage systems (SUDS). The innovative procedure utilises an integrated geographical information system (GIS)-based pollution index approach based on surface area impermeability, runoff concentrations/loadings and individual SUDS treatment performance potential to evaluate the level of risk mitigation achievable by SUDS drainage infrastructure. The residual impact is assessed through comparison of the determined pollution index with regulatory receiving water quality standards and objectives. The methodology provides an original theoretically based procedure which complements the current acute risk assessment approaches being widely applied within pluvial flood risk management.

A pollutant removal prediction tool for stormwater derived diffuse pollution

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.

Polyaromatic hydrocarbon distributions in sediments of an urban catchment

Abstract The levels of six selected polyaromatic hydrocarbons (PAHs), anthracene, fluoranthrene, pyrene benzo(b)and(k)fluoranthenes, benza(a)pyrene and benzo(ghi)perylene, within the sediments of a 5.3 km2 urban catchment in N.W. London have been investigated over a 12 month period. The variations in levels throughout the catchment are discussed in terms of the influence of established hydrocarbon-sediment associations on both the contributions of different source inputs and on the controlling effects of on-line sedimentation ponds and the terminal receiving basin. These factors cause local variations within the overall pattern which demonstrates an increase in sedimentary PAH concentrations downstream as the percentage urban surface area increases. The PAH distribution is essentially hydrauli-cally controlled through discontinuous resuspension of the mobile surface sediment layer during turbulent storm flow conditions.

Comparison of tracer techniques for monitoring sewer losses

Two different experimental techniques employing conservative tracers to determine exfiltration losses in a sewer system are reported. Both techniques compare the in-sewer responses measured along designated indicator and reference sections to assess the level of exfiltration although with different degrees of reliability. The experimental and interpretation errors associated with a pulse dosing technique, in which Rhodamine WT is used as the single tracer, are described. The estimation of exfiltration requires the comparison of indicator and reference peaks which need to be well resolved and reproducible in order to provide realistic results. A more consistent technique involves the continuous dosing of two different tracers with the objective of producing simultaneous downstream peak tracer concentrations. Any errors due to flow variations are eliminated and the longer measurement period allows both peak and background concentrations to be more accurately determined. Rhodamine WT, Li and Br were used in different combinations over the approximately 20 min duration of each continuous dosing experiment and were found to demonstrate conservative behaviours with measured recoveries of 97.6-100.6%, 97.3-101.4% and 108.4-109.2%, respectively. The analytical problems associated with the determination of the different tracers in sewage are described and are shown to be responsible for the reduced accuracy of the results associated with the use of Br. The preferred combination of tracers is Rhodamine WT as the indicator and Li as the reference and these are shown to be capable of estimating sewer losses to an accuracy of +/-1.0% using the continuous dosing approach.

Urban surface water pollution problems arising from misconnections

The impacts of misconnections on the organic and nutrient loadings to surface waters are assessed using specific household appliance data for two urban sub-catchments located in the London metropolitan region and the city of Swansea. Potential loadings of biochemical oxygen demand (BOD), soluble reactive phosphorus (PO4-P) and ammoniacal nitrogen (NH4-N) due to misconnections are calculated for three different scenarios based on the measured daily flows from specific appliances and either measured daily pollutant concentrations or average pollutant concentrations for relevant greywater and black water sources obtained from an extensive review of the literature. Downstream receiving water concentrations, together with the associated uncertainties, are predicted from derived misconnection discharge concentrations and compared to existing freshwater standards for comparable river types. Consideration of dilution ratios indicates that these would need to be of the order of 50-100:1 to maintain high water quality with respect to BOD and NH4-N following typical misconnection discharges but only poor quality for PO4-P is likely to be achievable. The main pollutant loading contributions to misconnections arise from toilets (NH4-N and BOD), kitchen sinks (BOD and PO4-P) washing machines (PO4-P and BOD) and, to a lesser extent, dishwashers (PO4-P). By completely eliminating toilet misconnections and ensuring misconnections from all other appliances do not exceed 2%, the potential pollution problems due to BOD and NH4-N discharges would be alleviated but this would not be the case for PO4-P. In the event of a treatment option being preferred to solve the misconnection problem, it is shown that for an area the size of metropolitan Greater London, a sewage treatment plant with a Population Equivalent value approaching 900,000 would be required to efficiently remove BOD and NH4-N to safely dischargeable levels but such a plant is unlikely to have the capacity to deal satisfactorily with incoming PO4-P loads from misconnections.

Assessing the impact of swales on receiving water quality

Abstract 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.