Urban stormwater management

Across the globe, urban water infrastructures are transitioning towards new paradigms that deliver not only drainage functions but – potentially – multiple benefits in relation to broader sustainability challenges. Conceptualised as water-wise, water smart or water sensitive cities,¹ the driver is the need for urban areas with aged and ‘over-capacity’ piped systems to rapidly address the challenges posed by a warming climate (i.e. changes in the intensity, frequency and duration of both rainfall events and their intervening dry periods), increased urbanization and associated uncertainties in the use of current models to address emerging needs. Whilst stormwater management has yet to fully take its place in the circular economy, this transformation from a linear to circular perspective sees stormwater transition from a nuisance/problem to a resource. Stormwater is no longer something that needs to be discharged to avoid/reduce flooding risk and damages in cities, but rather it is a valuable resource that can be (re)used in multiple applications, it replenishes natural waterbodies and contributes to more liveable cities.

While the main focus of the last decade has been the quantitative management of stormwater (climate change adaptation and flood risk reduction, rainwater harvesting, etc.), several issues linked to the management of stormwater quality remain open. These are linked to (i) the high spatial and temporal variability in stormwater pollution levels, (ii) the limited knowledge on pollution control options, and (iii) the need to integrate risks posed by stormwater pollution within an holistic strategy for the management of water in urban areas.

In the rainfall-runoff generation process, water falls from clouds, lands on urban areas, and is subsequently collected and conveyed away by sewer systems, which can be either combined (where the same infrastructure is used for waste- and stormwater) or separated (where dedicated pipe systems collect stormwater and convey it directly into natural waterbodies). During this process, water accumulates a wide range of pollutants from various sources, including traffic and related infrastructure, the atmosphere, building materials and pollutants from other human activities spread across urban areas.

The characteristics of stormwater pollution are therefore highly variable and of heterogeneous quality, closely linked to the spatial and temporal variability of the specific pollutant sources in a given urban area. The research articles brought together in this special issue directly contribute to addressing this need. For example, Zhao and Li et al. (https://doi.org/10.1039/D2EW00813K) showed how the pollution level of urban sediments varies across different surfaces. This stresses the need for accurate catchment characterization to support a better assessment of stormwater quality from a specific area. Zakharova et al. (https://doi.org/10.1039/D2EW00919F) investigated the distribution of heavy metals in highway runoff, showing how pollutant loading is not constant throughout a runoff discharge event.

The complex and heterogeneous mixture of pollutants in stormwater can limit its (re)use and can pose a risk both to human health and to the environment. Mutzner and Spahr et al. (https://doi.org/10.1039/D3EW00160A), based on reviewing 97 stormwater monitoring studies, identified 49 persistent, mobile and toxic, and very persistent, very mobile organic substances, in stormwater which might pose a risk for human consumption. But a lack of monitoring data is still hampering a reliable risk assessment.

Furthermore, rainfall itself is characterized by an intermittent and dynamic nature. All these factors make collection and analysis of extensive data on stormwater pollution a challenging process. The analysis of 50 events collected at 21 sites allowed Bradley et al. (https://doi.org/10.1039/D2EW00933A) to assess the acute toxicity of intermittent stormwater discharges over multiple trophic levels. As pointed out by the authors themselves, further research is still needed to assess the chronic impacts due to multiple discharges, and across a variety of sites (including, last but not least, the effect of different dilution factors when discharged in natural water bodies).

These knowledge gaps can be partially filled by the use of integrated water quality models, representing several interconnected elements of the urban water system (sewer, treatment plants, natural water). These tools allow for a holistic assessment of the impacts from wet-weather discharges (both from combined and separate systems), supporting catchment-wide pollution management strategies. Ianes et al. (https://doi.org/10.1039/D3EW00143A), for example, applied a simple stochastic model to assess the environmental risk posed by different discharges, stressing - once again - the importance of taking the receiving water body into account.

An important fraction of stormwater pollutants is in the particulate form, and these can accumulate along the stormwater infrastructure, resulting in decreased performance (due to e.g. clogging) and potentially creating health and environmental hazards. These can accumulate across the entire urban water infrastructure, including infrastructural elements such as gully pots. Regueiro-Picallo et al. (https://doi.org/10.1039/D2EW00820C) present a novel monitoring methodology which allows estimation of sediment accumulation in combined sewer systems based on temperature readings. This will enable wider data collection and a better understanding of sediment behaviour in the urban drainage network, allowing for a better management of the existing infrastructure and planning of treatment options. Indeed, Pimiento et al. (https://doi.org/10.1039/D2EW00746K) assessed the human health and environmental risk associated with sediment in stormwater facilities in Bogotá. Their results show how sediment can pose an environmental risk and underlines that sediment handling should be included when developing pollution management plans.

Stormwater pollution can be addressed by both source control options and end-of-pipe solutions. End-of-pipe solutions have been implemented for decades. Their performance assessments often target “traditional” pollutants (such as nutrients), or pollutant groups (such as heavy metals, PAHs, etc.). Field et al. (https://doi.org/10.1039/D3EW00028A) present a study where heavy metals and per- and polyfluoroalkyl substances (PFAS) are considered together as co-contaminants, investigating their removal in commercially available sorbents. There is the need to investigate how these emerging contaminants behave in existing solution. For example, the mobility of microplastics in stormwater filtration systems is still under investigation and might be affected by natural weather conditions (https://doi.org/10.1039/D2EW00975G). Also, due to the characteristics of these emerging contaminants, new treatment technologies might be necessary, and we can get inspiration from processes applied in wastewater treatment (e.g. biofilm carriers).

Performance assessment of stormwater treatment options is often performed at single-sites and/or for limited periods of time, resulting in a general lack of knowledge over their performance over long time intervals. The use of biochar in biofiltration systems can increase their hydraulic and water treatment performance. In their review on the influence of biochar on biofilters and biofiltration systems, Vithanage et al. (https://doi.org/10.1039/D3EW00054K) show how most available data originates from laboratory studies, with a lack of field studies.

Full scale assessment can be facilitated with the increasing use of online sensors, which allows for the collection of time-resolution data across multiple treatment structures. This development allows for assessing performance of the stormwater infrastructure at the catchment and/or city scale. New opportunities arise both from cheaper and interconnected sensors for traditional indicators, but also from new measuring techniques which allow for (near) real-time measurement of water quality indicators, which before, would require lengthy analysis in the laboratory. For example, Makris et al. (https://doi.org/10.1039/D3EW00141E) demonstrate the use of a near real-time method to monitor bacterial contamination (from e.g. combined sewer overflows) in bathing waters. A widespread deployment of these sensing technologies can increase the recreational use of existing water bodies in urban areas.

This uncertainty on operation and maintenance of these infrastructures can hamper their widespread implementation across urban areas. Furén et al. (https://doi.org/10.1039/D2EW00823H), for example, investigated the accumulation of heavy metals across 29 biofilters which have been in operation for long periods of time (over 7 years). Their results show how heavy metals accumulated on the top layer, stressing the importance of regular maintenance of these treatment facilities. Treatment units should also be adapted to specific climatic conditions in order to maximize their performance. Roy et al. (https://doi.org/10.1039/D3EW00062A) investigated the release of phosphorous from subsurface gravel wetlands in cold climates, showing how engineered soil can result in net phosphorous export, with limited effect on chlorine concentrations.

Source control options target the pollutants as close as possible to their source, by either substituting them, or by providing distributed treatment capacity. In this perspective, solutions like green roofs can also provide an important contribution to pollution reduction. Zhang et al., (https://doi.org/10.1039/D2EW00836J) investigated the use of different additives to enhance pollutant removal potential in extended green roofs. Green roofs, as well as other nature-based solutions are often implemented in a multi-functional perspective, with flow and volume reduction often driving investments in this infrastructure with research by Dong et al., (https://rsc.66557.net/en/content/articlelanding/2023/EW/D3EW00149K) comparing the performance of three green roof models of varying complexities, using the output to identify knowledge gaps. However, there is a need for linking design and planning of water infrastructure for hydraulic purposes, with the need to achieve a stormwater quality level for safe reuse and/or replenishment of natural resources.

Several conclusions from the studies in this themed issue stress the importance of effective governance (including maintenance and operations) of stormwater systems to ensure their effectiveness. Digital solutions can provide a substantial contribution in this field, facilitating e.g. data collection, monitoring of the system performance, planning and implementation of management plans, and risk assessment at the catchment scale. Mason et al. (https://doi.org/10.1039/D3EW00098B) showed how wireless sensors can be used to understand the performance of Green Infrastructure at the catchment scale, creating the basis for better planning and operation of these infrastructures. All these developments would require the interaction between different expertise, enabling holistic management of urban water.

We hope that the articles included in this themed issue will provide new insights for better management of stormwater in urban areas, representing a further step towards more sustainable management of water in our cities.

Notes and references

1. International Water Association, The IWA principles for water wise cities [Internet], 2017, p. 6, Available from: https://iwa-network.org/wp-content/uploads/2016/10/IWA_Brochure_Water_Wise_Communities_SCREEN-1.pdf.

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