Innovative road runoff control systems, which combined flow-balancing basins with constructed wetlands, on the recently constructed Newbury Bypass were monitored over 18 months. The results have shown that despite variability in the pollutant loadings, the system generally promoted removal and accumulation of sediments and associated metals. The correlation structure of the data set indicated possible removal processes, and enabled interpretation of the behaviour of these pollutants. Correlation of metal content in settling solids between successive upstream and downstream parts of the wetland gave indications of potential regression models for metal behaviour, despite variability in flows, pollutant loadings, and other conditions. Other findings suggested that the combined flow balancing and pollutant removal functions could be improved by avoiding transport of sediments, fluctuations in conditions and by the promotion of processes favouring metal retention by bed deposits.

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