Beyond the design event : sediment pollution movement in SuDS

Sustainable urban Drainage Systems (SuDS) present a ‘blue-green’ method of urban stormwater management that is increasingly implemented in the UK and worldwide. SuDS mimic natural vegetated flow paths and are designed to manage the increase in stormwater quantity and degradation of stormwater quality resulting from development (urbanisation). They have been widely implemented across the UK over the last 15 years, to aid compliance with the EU Water Framework Directive (2000) standards for river water quality. Given the increasing maturity of UK SuDS, there is growing concern over the long-term performance efficiencies of these assets/networks, particularly the variability of treatment efficiency over multiple flow events. Providing the field monitoring evidence base to address this concern forms the aim of the present thesis. Emphasis is placed upon understanding SuDS asset/network sediment detention efficiency, as the majority of urban pollution is adsorbed to sediment material rather than transported and treated as solute. A novel tracer method is, therefore, developed and employed to identify and quantify sediment processes for mature UK case study SuDS. SuDS design manuals (CIRIA 2015, Water by Design 2006, USEPA 2009) present expected or reported sediment detention and pollution mitigation levels for specific SuDS assets. For example, the expected Total Suspended Solids (TSS) removal for a swale has been defined as 90% in the WSUD (Water Sensitive Urban Design) technical manual (Leisenring et al. 2013). Yet, these treatment efficiencies are based on a single ‘design’ rainfall-runoff event through the system; hence, fail to consider the sensitivity of SuDS performance to non-design and multiple repeat events over the long term design life of the SuDS asset. As natural variability in rainfall affects pollutant washoff, shear stress for entrainment, conveyance, deposition, loss of treatment capacity etc., the research presented in the present thesis intensively monitored four established UK SuDS networks for 6-12 month timeframes. Bulk sampling data highlight that TSS treatment is highly variable, ranging from highly effective (>80%) to inefficient (<20%). Similar variability is found in sediment deposition rates (on average: 0.4-1.1 kg/m2/yr), providing insight on temporal and asset dependency of fine sediment detention, including related treatment efficiency and long term loss of capacity. Wetlands illustrate the most effective (mass) sediment detention per area (>1kg/m2) while the swales detain the least (<0.8kg/m2). To advance the volumetric data noted above, source-sink routing of diffuse fine sediment pollution required development of tracer methodology appropriate to use in SuDS. This dictated use of Rare Earth Oxides (REO) as fine sediment tags; although their use in an urban environment is new, it provides long term trace and experimental replicability results without loss of provenance, signature degradation or loss of tag material. Thus, unique time-stamped and source-specific identifiers have been used monitor their movement into and through each SuDS over a 6-12 month period. Use of REO tagged sediment data permits mass balance analysis of fine sediment through the monitored SuDS assets and networks. Data clearly illustrate that sediment is not fully detained (as assumed in SuDS design); rather, sediment is re-entrained and re-deposited multiple times over multiple flow events. Residence time of sediment within a full SuDS network is found to be as short as 12 weeks, raising concern over treatment capability. Reviewing this at finer asset-based resolution, detention efficiency and conveyance rates appear unique to each asset. Generally linear wetland and swale assets demonstrate the greatest (tagged) sediment detention efficiency (>70%) while (the monitored) wetland assets decline to below 50% efficiency over the first 12 months and ponds demonstrated negligible sediment detention efficiency (<10%). As 80% of urban pollution is conveyed adsorbed to fine sediment, the sediment conveyance pattern through SuDS assets has been analysed to define the pollutant concentration levels and trends of detained sediment. Pollutant levels show no consistent trend across SuDS assets. Results illustrate that sediment pollutant contamination shows an influence from particle size distribution and mass deposition as well as asset design. Analysis shows the most numerous significantly elevated sediment pollutant concentrations within the linear wetland, with Fe, Ba, Cr, Cu, Zn, K and P demonstrating average concentrations above contaminated land thresholds. Enrichment and geoaccumulation indexing of pollutants illustrates Fe, Zn, Cr, Ba, Cu and P to be pollutants of concern, with Fe, Pb, Ni, Cr, Mn, Zn, Ba, Cu, Ni and P identified as hotspot pollutants in one or multiple SuDS assets. Cross-correlation of rainfall and flow characteristics with asset/network detention efficiencies were used to define key drivers of multi-event sediment conveyance. Outcomes highlight three variables of strong influence: the number of rainfall and flow occurrences; the antecedent dry days; rainfall clustering characteristics. Weaker correlations are found with flow characteristics (number of flow events, depth and velocity – leading to Fr and Re values) and modal particle size. These influential forcings have then been considered with respect to a selected standard SuDS pollution treatment process (MUSIC k-C*) to identify the compatibility for multiple rainfallrunoff event SuDS fine sediment and pollutant simulation. The research provides multiple event SuDS stormwater treatment efficiencies that can inform improved SuDS design and maintenance planning by engineering consultants, Local Authorities, environmental regulators and SuDS asset managers

Contamination of Detained Sediment in Sustainable Urban Drainage Systems

Adsorption is a key water pollution remediation measure used to achieve stormwater quality improvement in Sustainable urban Drainage Systems (SuDS). The level of contamination of detained sediment within SuDS assets is not well documented, with published investigations limited to specific contaminant occurrence in ponds, wetlands or infiltration devices (bioretention cells) and generally focused on solute or suspended sediment. Guidance on contamination threshold levels and potential deposited sediment contamination information is not included in current UK SuDS design or maintenance guidance, primarily due to a lack of evidence and understanding. There is a need to understand possible deposited sediment contamination levels in SuDS, specifically in relation to sediment removal maintenance activities and potential impact on receiving waterways of conveyed sediment. Thus, the objective of the research presented herein was to identify what major elements and trace metals were observable in (the investigated) SuDS assets detained sediment, the concentration of these major elements and trace metals and whether they met/surpassed ecotoxicity or contaminated land thresholds. The research presented here provides evidence of investigated SuDS sediment major element and trace metal levels to help inform guidance and maintenance needs, and presents a new methodology to identify the general cause (anthropocentric land use) and extent of detained SuDS fine urban sediment contamination through use of a contamination matrix

The short-term influence of cumulative, sequential rainfall-runoff flows on sediment retention and transport in selected SuDS devices

It is necessary to understand Sustainable urban Drainage Systems (SuDS) sediment retention efficiencies to fully comprehend SuDS pollution removal properties and urban sediment movement from source-to sink. This research presents the detention and transport of a single tagged sediment release through four SuDS devices over 12 months, with the aim of quasi-quantifying these selected SuDS devices sediment detention efficiencies. Field monitoring and mass balance analysis of deposited sediment shows that tagged sediment from the single sediment release moves through the monitored SuDS, with deposition declining over the 12-month monitoring period. Initial retention is high (>80% during the first week of monitoring) but falls below 80% after multiple consecutive rainfall-runoff events (events ≤50% ARI). The field monitoring illustrates retention to generally remain above 50%, suggesting that SuDS are highly efficient at retaining urban sediment pollution but that deposition of a single sediment release may resuspend due to cumulative rainfall-runoff events

Urban sediment transport through an established vegetated swale:Long term treatment efficiencies and deposition

Vegetated swales are an accepted and commonly implemented sustainable urban drainage system in the built urban environment. Laboratory and field research has defined the effectiveness of a vegetated swale in sediment detention during a single rainfall-runoff event. Event mean concentrations of suspended and bed load sediment have been calculated using current best analytical practice, providing single runoff event specific sediment conveyance volumes through the swale. However, mass and volume of sediment build up within a swale over time is not yet well defined. This paper presents an effective field sediment tracing methodology and analysis that determines the quantity of sediment deposited within a swale during initial and successive runoff events. The use of the first order decay rate constant, k, as an effective pollutant treatment parameter is considered in detail. Through monitoring tagged sediment deposition within the swale, the quantity of sediment that is re-suspended, conveyed, re-deposited or transported out of the swale as a result of multiple runoff events is illustrated. Sediment is found to continue moving through the vegetated swale after initial deposition, with ongoing discharge resulting from resuspension and conveyance during subsequent runoff events. The majority of sediment initially deposited within a swale is not detained long term or throughout its design life of the swale

Atmospheric microplastics : A review on current status and perspectives

Microplastics have recently been detected in the atmosphere of urban, suburban, and even remote areas far away from source regions of microplastics, suggesting the potential long-distance atmospheric transport for microplastics. There still exist questions regarding the occurrence, fate, transport, and effect of atmospheric microplastics. These questions arise due to limited physical analysis and understanding of atmospheric microplastic pollution in conjunction with a lack of standardized sampling and identification methods. This paper reviews the current status of knowledge on atmospheric microplastics, the methods for sample collection, analysis and detection. We review and compare the methods used in the previous studies and provide recommendations for atmospheric microplastic sampling and measurement. Furthermore, we summarize the findings related to atmospheric microplastic characteristics, including abundance, size, shapes, colours, and polymer types. Microplastics occur in the atmosphere from urban to remote areas, with an abundance/deposition spanning 1–3 orders of magnitude across different sites. Fibres and fragments are the most frequently reported shapes and the types of plastic which generally aligns with world plastic demand. We conclude that atmospheric microplastics require further research and greater understanding to identify its global distributions and potential exposure to human health through further field sampling and implementation of standardized analytical protocols

Influence of Sediment on the Hydrological Performance of a Permeable Pavement

Permeable pavements play an essential role in urban drainage systems, making them a subject of great interest to both researchers and practitioners. The majority of studies, however, have demonstrated a significant degree of uncertainty regarding both the operational performance and maintenance requirements of this type of pavement. This paper describes a laboratory-based experimental study investigating the influence of sediment on the hydrological performance of a permeable pavement. The experimental results show that, under sediment and rainfall loading typical for a 10 year period within the UK, partial clogging of the pavement voids with sediment led to a 6·4% decrease in total outflow, a 6·41% decrease in outflow rate, a 9·5% increase in outflow start time, a 20·7% increase in total outflow duration and no significant change in the concentration of suspended solids. However, no surface ponding was observed and it was therefore concluded that an appropriately designed permeable pavement system, exposed to typical UK rainfall and sediment loadings, should be able to operate efficiently for at least 10 years without the need for any post-construction maintenance. Hence, permeable pavements continue to represent an excellent form of source control for both surface runoff and pollutants. </jats:p

Influences and drivers of woody debris movement in urban watercourses

Abstract It is recognised that the blockage of culverts by woody debris can result in an increased risk of infrastructure damage and flooding. To date, debris transport analysis has focused on regional fluvial systems and large woody debris, both in flume and field experiments. Given the social and economic risk associated with urban flooding, and as urban drainage design shifts away from subsurface piped network reliance, there is an increasing need to understand debris movement in urban watercourses. The prediction of urban watercourse small woody debris (SWD) movement, both quantity and risk, has undergone only limited analysis predominantly due to lack of field data. This paper describes the development of a methodology to enable the collection of accurate and meaningful SWD residency and transportation data from watercourses. The presented research examines the limitations and effective function of PIT tag technology to collect SWD transport data in the field appropriate for risk and prediction analysis. Passive integrated transponder (PIT) technology provides a method to collect debris transport data within the urban environment. In this study, the tags are installed within small woody debris and released at known locations into a small urban natural watercourse enabling monitoring of movement and travel time. SWD velocity and detention are collated with solute time of travel, watercourse and point flow characteristics to identify the relationships between these key variables. The work presented tests three hypotheses: firstly, that the potential for unobstructed or un-detained SWD movement increases with flow velocity and water level. Secondly, that SWD travel distance, and the resistance forces along this travel path, influence SWD transport potential. Thirdly, the relationship between SWD and channel dimensions is examined with the aim of advancing representative debris transport prediction modelling.</jats:p

Examination of the ocean as a source for atmospheric microplastics

Global plastic litter pollution has been increasing alongside demand since plastic products gained commercial popularity in the 1930’s. Current plastic pollutant research has generally assumed that once plastics enter the ocean they are there to stay, retained permanently within the ocean currents, biota or sediment until eventual deposition on the sea floor or become washed up onto the beach. In contrast to this, we suggest it appears that some plastic particles could be leaving the sea and entering the atmosphere along with sea salt, bacteria, virus’ and algae. This occurs via the process of bubble burst ejection and wave action, for example from strong wind or sea state turbulence. In this manuscript we review evidence from the existing literature which is relevant to this theory and follow this with a pilot study which analyses microplastics (MP) in sea spray. Here we show first evidence of MP particles, analysed by μRaman, in marine boundary layer air samples on the French Atlantic coast during both onshore (average of 2.9MP/m3) and offshore (average of 9.6MP/m3) winds. Notably, during sampling, the convergence of sea breeze meant our samples were dominated by sea spray, increasing our capacity to sample MPs if they were released from the sea. Our results indicate a potential for MPs to be released from the marine environment into the atmosphere by sea-spray giving a globally extrapolated figure of 136000 ton/yr blowing on shore