Lian N. L. Scholes

The treatment of metals in urban runoff by constructed wetlands

Abstract The use of constructed wetlands for the treatment of domestic wastewater is now well established in the UK and their ability to treat a range of industrial wastewaters is now being investigated. However, their ability to treat urban runoff is relatively untested despite the fact that this application could have important environmental and operational benefits, in both industrial and developing countries. In response to this, the Environment Agency have developed constructed wetland treatment systems at two selected sites in south-east England, both of which receive large volumes of urban runoff. The sites are located at Brentwood and Dagenham and were completed in April 1995. Water and sediment samples have been collected at bi-monthly intervals at each site since October 1995 and analysed for a range of parameters including the total concentrations of six trace metals — cadmium, copper, nickel, chromium, lead and zinc. Similar analysis has been carried out on plants collected from both sites in the spring of 1997. Results show a wide variation in pollutant levels, reflecting the highly variable quality characteristics of urban runoff. Mean removal efficiencies of metals in the water vary between sites in dry weather conditions, with maximum removal efficiencies being recorded at the Dagenham wetland during a storm event. Analysis of plant tissues indicates that the reeds bioaccumulate trace metals and that metal uptake is greatest in the roots. Sediment metal concentrations are typical of a site receiving urban runoff. At both sites the highest sediment concentrations are consistently recorded in samples collected from the settlement tanks.

Heavy metal concentrations and toxicity in water and sediment from stormwater ponds and sedimentation tanks.

Sedimentation is a widely used technique in structural best management practices to remove pollutants from stormwater. However, concerns have been expressed about the environmental impacts that may be exerted by the trapped pollutants. This study has concentrated on stormwater ponds and sedimentation tanks and reports on the accumulated metal concentrations (Cd, Cr, Ni, Pb, and Zn) and the associated toxicity to the bacteria Vibrio fischeri. The metal concentrations are compared with guidelines and the toxicity results are assessed in relation to samples for which metal concentrations either exceed or conform to these values. The water phase metal concentrations were highest in the ponds whereas the sedimentation tanks exhibited a distinct decrease towards the outlet. However, none of the water samples demonstrated toxicity even though the concentrations of Cu, Pb, and Zn exceeded the threshold values for the compared guidelines. The facilities with higher traffic intensities had elevated sediment concentrations of Cr, Cu, Ni, and Zn which increased towards the outlet for the sedimentation tanks in agreement with the highest percentage of fine particles. The sediments in both treatment facilities exhibited the expected toxic responses in line with their affinity for heavy metals but the role of organic carbon content is highlighted.

The design of wetland systems for the treatment of urban run off

The results of studies of the pollution removal performance of constructed wetlands for urban runoff treatment are discussed. The design of urban runoff wetlands including primary treatment components, substrate composition and flow rate regulation are considered. The use of front-end treatment systems such as oil interceptors, spillage containment facilities and wetland forebays or lagoons are recommended to provide an initial pollutant removal capacity as well as attenuating the impact of high discharges. Design criteria which will ensure inlet flow velocities do not exceed 0.7m/s and which will achieve optimum retention times (10-15 hours) for maximum pollutant removal are described. The relative advantages of different soil types are discussed and an overall minimum substrate depth of 0.6m is recommended for sub-surface systems. Two case studies of urban runoff wetlands in the UK which are currently being monitored are described.

Effects of temperature on metal tolerance and the accumulation of Zn and Pb by metal-tolerant fungi isolated from urban runoff treatment wetlands

Aims:  To investigate the ability of two fungi to accumulate Zn and Pb, the effect of temperature on their metal tolerance and possible mechanisms involved in metal accumulation.

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.

Identifying and Classifying the Sources and Uses of Xenobiotics in Urban Environments

The sources and uses of xenobiotics in urban environments are very diverse, making structured approaches to source and use classification a fundamental requirement for effective pollution management. This chapter provides a general introduction to the topic of substance source and use identification, highlighting the key differences between different types of sources (e.g. processes vs. commodities; natural vs. anthropogenic etc.) and different types of uses (e.g. active vs. passive; dispersive vs. non-dispersive, etc.). Examples of relevant classification systems and their applications are also given, and the diversity of potential xenobiotic sources and uses is clearly demonstrated through the description of a series of ‘archetypes’ (i.e. model examples). The chapter concludes with an overview of useful source tracking approaches (e.g. database mining, marketing surveys, forensic approaches etc.).

Assessment of storm water ecotoxicity using a battery of biotests

As part of the European Union 5th Framework Programme (EU FP5) sustainable storm water management project DayWater, an international monitoring programme was established to investigate the ecotoxicity of highway and urban runoff samples. This involved the collection of samples from a total of 35 storm events from sites in Sweden (Lulea and Stockholm), Germany (Wuppertal), and France (Nantes). To enable both chronic and acute end points to be addressed, the ecotoxicity tests were performed using rotifers (Brachionus calyciflorus), bacteria (Vibrio ficherii), and algae (Pseudokirchneriella subcapitata). Samples collected at the Stockholm site were additionally analysed for a range of water quality parameters enabling this site to be considered from a combined physico-chemical and ecotoxicological perspective. This paper provides an overview of the results of the whole monitoring programme and demonstrates that storm water frequently exerts a toxic effect. Although the levels of ecotoxicity detected were found to vary greatly in relation to sites, storm events and test organisms, further analysis demonstrated a moderately strong correlation between the responses of algae and rotifer tests. Microtox was generally found to be the most responsive test in terms of both frequency of detection of toxicity and level at which a toxic impact could be detected.

An investigation of urban water and sediment ecotoxicity in relation to metal concentrations

The majority of urban rivers and watercourses routinely receive pollution inputs from a wide variety of sources such as separately sewered stormwater and combined surface water outfalls (CSOs). Although the physico-chemical characteristics of urban runoff have been widely reported, the ecotoxicological impacts of these discharges are less certain. Implemen-tation of the EU Water Framework Directive, with its over-arching aim of protecting, restor-ing and enhancing the ecological status of aquatic ecosystems and its specific reference to the need to control diffuse pollution, has refocused attention on the impact of non-point source discharges on receiving water bodies. However, the implementation of this Directive has also raised several key and complex questions such as how should the ecological status of various water body types be determined. Should the proposed all-encompassing Environmental Qual-ity Standards (EQS) be developed in relation to water, sediment or biotic components? And, once determined, how should compliance with EQS be assessed? In order to contribute to the current and considerable debate surrounding these issues, the ecotoxicity of urban water and sediments was investigated in relation to concentrations of se-lected heavy metals in these environments. The Microtox test was used to evaluate the eco-toxicity of urban stream water, sediment porewater and water samples produced by resus-pending urban sediments to simulate the impact of storm event conditions. Total concentrations of Zn, Cu and Cd were then determined in these same water fractions, as well as in whole sediment samples. The results of this monitoring programme provide an insight into the metal pathways in urban rivers during different conditions and are interpreted in terms of the usefulness of this approach in contributing to the evaluation of the overall eco-logical status of urban water bodies.