Experimental Design, Monitoring, and Assessment of Bioretention Systems for Urban Stormwater Management

Abstract

Climate change and urbanisation exacerbate urban flooding and stormwater pollution, causing significant environmental and socio-economic impacts. Bioretention systems provide decentralised solutions to these challenges; however, their effectiveness and longevity are dependent on optimised design and proactive maintenance, both of which are hindered by a lack of performance and monitoring data, particularly in a UK context. This research provides the first empirical evidence to support the design of UK-specific bioretention configurations through performance evaluation, and to inform maintenance strategies through the analysis of contaminant accumulation. The research comprised integrated laboratory and field studies. A series of column experiments was conducted in this study to evaluate the influence of two key design variables: vegetation and biochar amendments, on bioretention performance under simulated rainfall conditions representative of Cardiff, UK, with accelerated heavy metal loading. All designs consistently achieved high removal efficiencies (80-99%) for suspended solids and heavy metals. In contrast, phosphorus removal was more variable, ranging from 53% removal to significant net leaching, depending on the specific design configuration. Vegetation was critical for sustaining hydraulic function, effectively preventing clogging observed in non-vegetated systems, while providing secondary treatment benefits. Performance was species-dependent, with Carex pendula identified as the most effective for combined treatment and hydraulic performance. Biochar amendments, while beneficial for dissolved zinc removal, reduced suspended solids and particulate lead retention and were a net source of dissolved phosphorus, leaching up to 1.36 mg/L. The results emphasise that biochar amendments must be selectively optimised and validated for specific stormwater treatment objectives. Analysis of filter media profiles revealed that, the majority of heavy metals were captured in the top 0-3 cm layer, reaching potentially toxic concentrations. The investigation into heavy metal accumulation was further advanced through a field-scale study at two established bioretention sites in Cardiff. Traditional sampling and portable X-ray fluorescence (pXRF) were employed to map the spatial distribution of heavy metals and identify contamination hotspots. Concentrations in the surface layer (0-3 cm) ranged as follows: Cu: 15-69, Pb: 18-340, Zn: 69-583, and Cr: 13-95 mg/kg, with accumulation levels increasing with system age and decreasing with depth. While most metal concentrations fell well below screening levels, centralised inlets created hotspots approaching these limits for Pb and Cr. Therefore, prioritising diffuse inlets in design to promote a more uniform distribution, complemented by pXRF monitoring, enables targeted maintenance to keep all concentrations below screening levels indefinitely.