Enhancement of stormwater quality in grass swales: Removal and immobilisation of metals

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LICENTIATE T H E S I S

ISSN 1402-1757 ISBN 978-91-7790-697-1 (print)

ISBN 978-91-7790-698-8 (pdf) Luleå University of Technology 2020

Snežana Ga vr ić Enhancement of stor mw ater quality in g rass s w ales

Department of Civil, Environmental and Natural Resources Engineering Division of Architecture and Water

Enhancement of stormwater quality in grass swales:

Removal and immobilisation of metals

Snežana Gavri ć

Urban Water Engineering

131907-LTU-Gavric.indd Alla sidor

131907-LTU-Gavric.indd Alla sidor 2020-11-13 10:092020-11-13 10:09

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Enhancement of stormwater quality in grass swales:

Removal and immobilisation of metals

Snežana Gavrić Luleå, 2020

Urban Water Engineering Division of Architecture and Water

Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology

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Printed by Luleå University of Technology, Graphic Production 2020 ISSN 1402-1757

ISBN: 978-91-7790-697-1 (print) ISBN: 978-91-7790-698-8 (pdf) Luleå 2020

www.ltu.se

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Preface

This licentiate thesis presents a summary of my research work carried out in the Urban Water Engineering research group in the Department of Civil, Environmental and Natural Resources Engineering at Luleå University of Technology. The work was carried out as a part of a research cluster Stormwater&Sewers, a collaboration between Swedish municipalities of Luleå, Skellefteå, Östersund, Boden, municipal water organisations Vakin, MittSverige Vatten & Avfall, VASYD, the Swedish Water & Wastewater Association, and the Urban Water Engineering research group. The research was financed by the Swedish Research Council Formas (Grant no. 2015-778) and DRIZZLE-Center for Stormwater management, funded by the Swedish Governmental Agency for Innovation System (Vinnova), project 2016-05176.

First and foremost, I would like to express my great gratitude to my supervisors Maria Viklander and Günther Leonhardt for their supervision, valuable feedback and encouragement throughout the years. My great gratitude also goes to Jiri Marsalek and Heléne Österlund. Thank you Jiri, for your scientific guidance at some pivotal moments in my studies, for sharing your great knowledge with me and for all the feedback and valuable comments. Heléne, thank you for your advice and support and help with both practical and scientific questions. I also would like to thank Anna-Maria Perttu for her help during my first months as a PhD student and Alexandra, for the help with Swedish translations.

The field study would not be possible without the help of the staff from Luleå municipality who helped me with logistics and provided me with information about the studied catchments. I want to thank Peter Rosander for helping me to make my soil sampler and solve all other practical issues. I want to thank Stefan Marklund for help in contacting people from kommun and Kerstin Nordqvist for her help in the laboratory. I also want to thank all my colleagues in the Urban Water Engineering research group for providing friendly work environment.

Finally, I would like to thank all the people that made my time in Luleå pleasant: my tango group for welcoming me in their community and my friends from other research groups for fun times away from work. I want to thank all my family and friends in Serbia, and especially my parents, Boško and Milica, thank you for all the love and support. To Ivan, for being such a wonderful father, which enabled me to fully concentrate at work.

Thank you for believing in me and helping me manage everything at work and home.

To Luka, my wonderful boy, I love you dearly, thank you for being you.

Snežana Gavrić Luleå, November 2020

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Abstract

Grass swales are common elements of green drainage infrastructure used in urban catchments to provide stormwater quantity and quality control. Concerning stormwater quantity, swales convey runoff and attenuate stormwater volumes and peaks by enhancing hydrological abstractions and providing dynamic storage. Furthermore, grass swales are effective in treating stormwater runoff from trafficked surfaces. Swales are typically designed as long shallow channels with dense grass and permeable soils, and thereby create favourable conditions along the turf-stormwater interface for processes enhancing stormwater quality and reducing pollutant concentrations in swale effluents.

The thesis aim is to advance the knowledge of short-term performance of grass swales in removal of total metals, with respect to such influential factors as concentrations of metals and solids (TSS) in the inflow, swale geometry, and grass-soil characteristics. The literature reviewed showed that solids were the most frequently investigated parameter and the enhancement of stormwater quality by settling gained most attention in the earlier research. On the other hand, studies of swale performance in removal of other stormwater pollutants, such as total and dissolved metals, were limited, as was the understanding of the physical-chemical-biological processes facilitating the removal of other-than-solids pollutants.

Since swales are generally recognized as being effective in removing metals from stormwater through infiltration into swale soils, and the associated metal immobilisation in soils, long-term operation of swales may lead to accumulation of pollutants in, and contamination of, swale soils. Such conditions need to be remedied by relatively costly swale maintenance. A field study was conducted to characterize the soil chemistry of three swales, which serve for stormwater drainage and winter storage of snow cleared from adjacent trafficked areas. The swales studied served three catchments with different land use in the City of Luleå. Swales provided drainage of commercial, downtown, and residential catchments and drained roads with various traffic intensities. The study findings showed that the soil in the oldest swale, next to the road with the highest traffic intensity, contained the highest concentrations of most of the investigated metals. For example, the mean lead (Pb) concentration at this swale was ~70 mg/kg DW, compared to <10 mg/kg DW in the remaining two swales. On the other hand, barium (Ba) showed similar concentrations in all the three swales, 645-699 mg/kg DW. About 90% of the Ba burden was in the residual (immobile) form, which confirmed its mineral origin. Finally, a method for modelling the burdens of three traffic-related metals (Cu, Pb, Zn) in swale soils, with an existing source-based model, and verification of the modelled burdens against the measured soil chemistry data was also presented. The methodology could be used in environmental protection for planning swale maintenance, however, modelling uncertainties and difficulties in accounting for the effects of historical changes in the case of older swales studied in this thesis, increased uncertainties in the estimation of metal accumulations.

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Sammanfattning

Svackdiken är ett vanligt inslag i grön infrastruktur, som i urbana områden används för att kontrollera dagvattenflöden gällande både kvantitet och kvalitet. Svackdiken reglerar dagvattenkvantitet genom att transportera avrinning och dämpa både dagvattenvolymer och toppflöden via ökade hydrologiska abstraktioner och tillhandahållande av dynamisk lagring. Dessutom är svackdiken effektiva för att rena dagvattenavrinning från trafikerade ytor. Svackdiken utformas vanligtvis som långa, grunda, kanaler med tätt gräs och permeabel jord och skapar därigenom gynnsamma förhållanden längs med gränssnittet gräs-dagvatten för processer som förbättrar dagvattenkvalitet och reducerar föroreningskoncentrationer i utsläpp från svackdiken.

Avhandlingens mål är att öka kunskapen om kortsiktig prestanda hos svackdiken avseende avlägsnandet av metaller, med hänsyn till påverkande faktorer såsom koncentrationer av metaller och partiklar (TSS) i inflödet, svackdikets geometri samt egenskaper hos gräs och jord. Litteraturstudien visade att partiklar var den mest frekvent studerade parametern och att förbättring av dagvattenkvaliteten genom sedimentation fick mest uppmärksamhet i tidigare forskning. Å andra sidan var studier av svackdikens prestanda med avseende på avlägsnande av andra dagvattenföroreningar, såsom totala och lösta metaller, begränsade, liksom förståelsen av de fysikaliska-kemiska-biologiska processerna som främjar avlägsnandet av andra föroreningar än partiklar.

Eftersom svackdiken allmänt anses vara effektiva för att avlägsna metaller från dagvatten genom infiltration och associerad metallimmobilisering i jord, kan långsiktig användning av svackdiken leda till ackumulation av föroreningar i, och kontamination av, svackdikens jordar. Sådana förhållanden behöver åtgärdas med relativt kostsamt underhåll. En fältstudie genomfördes för att karaktärisera markkemin i tre svackdiken som används för dagvattenavledning och vinterförvaring av snö undanröjd från intilliggande trafikområden. De studerade svackdikena, tjänade tre avrinningsområden med olika markanvändning i Luleå. Svackdikena avleder avrinning från ett handelsområde, innerstad och bostadsområde och med vägar av olika trafikintensitet. Resultaten från studien visade att jorden i det äldsta svackdiket, bredvid vägen med högst trafikintensitet, innehöll högst koncentrationer av de flesta undersökta metallerna. Till exempel var medelkoncentrationen av bly (Pb) ~70 mg/kg DW (torrvikt), jämfört med <10 mg/kg DW i de två andra svackdikena. Barium (Ba), däremot, visade likvärdiga koncentrationer i samtliga svackdiken, 645-699 mg/kg DW. Omkring 90% av Ba-belastningen var i restform (immobil), vilket bekräftade dess mineraliska ursprung. Slutligen presenterades även en metod för modellering av belastningen av tre trafikrelaterade metaller (Cu, Pb, Zn) i svackdikens jordar, med en existerande källbaserad modell och verifiering av modellerad belastning mot uppmätta markkemidata. Metodologin skulle kunna användas inom miljöskydd för planering av underhåll av svackdiken, men modelleringsosäkerheter och svårigheter i att redogöra för historiska förändringar gällande äldre svackdiken som studerats i denna avhandling ökade osäkerheterna i uppskattningen av metallackmuleringen.

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List of papers

Paper I Gavrić, S., Leonhardt, G., Marsalek, J., Viklander, M. (2019).

Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings.

Science of the Total Environment, 669, 431-447.

Paper II Gavrić, S., Larm, T., Österlund, H., Marsalek, J., Wahlsten, A., Viklander, M. (2019).

Measurement and conceptual modelling of retention of metals (Cu, Pb, Zn) in soils of three grass swales

Journal of Hydrology, 574, 1053-1064.

Paper III Gavrić, S., Leonhardt, G., Österlund, H., Marsalek, J., Viklander, M.

Metal enrichment of soils in three urban drainage grass swales used for seasonal snow storage

Submitted to Science of the Total Environment

Assessment of contribution to the above papers

Paper no.

Development of idea

Research study design

Data collection

Data processing and analysis

Data interpretation

Publication process Manuscript

preparation for submission

Responding to reviewers

I Contributed Shared responsibility

Responsible Shared responsibility

Shared responsibility

Shared Responsible II Contributed Shared

responsibility

Shared Responsible

Shared responsibility III Shared

responsibility Shared responsibility

Shared Responsible

N/A

Responsible – developed, consulted (where needed) and implemented a plan for completion of the task.

Shared responsibility – made essential contributions towards the task completion in collaboration with other members in the research team

Contributed – worked on some aspects of the task completion

No contribution – for valid reason, has not contributed to completing the task (e.g.

joining the research project after the task completion) N/A – Not applicable

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1. Introduction

The goal of the EC Water Framework Directive (WFD) (Directive 2000/60/EC, 2000) is to achieve good qualitative and quantitative status of all water bodies in member states, and this goal is further supported by the Environmental Quality Standards Directive (EQSD) (Directive 2008/105/EC, 2008) and the Groundwater Directive (GD) (Directive 2006/118/EC, 2006). Meeting the WFD objectives is particularly challenging in river basins with high concentration of urban areas, which are recognized for multiple pollutant sources impacting on both surface waters and the groundwater. With respect to urban drainage, which is addressed in this thesis, one of the promising pollution prevention and remediation measures is incorporation of green infrastructure (GI) elements into urban and suburban catchments, with the objective of providing local control of polluted stormwater runoff generated mostly on impervious areas. Grass swales represent common GI elements that can be advantageously used, instead of stormwater pipes, to drain runoff from trafficked areas. In addition to runoff conveyance, swales also attenuate stormwater flow volumes and peaks, and reduce pollutant concentrations through the interaction of flow with grass-soil media (Schueler, 1987). In climates with seasonal snow, swales also serve for storage of snow cleared from streets, roads and sidewalks (Backstrom and Viklander, 2000).

Swales ability to remove pollutants during actual rainfall and snowmelt events and irrigation experiments, when runoff is conveyed through the swale, can be referred to as short-term performance. Usually such a performance is estimated by comparing the quality of stormwater before it enters and after it leaves the swale. On the other hand, the performance of swales, that have been operated for many years, in retaining particulate pollutants can be estimated from their soil chemistry. Soil chemical quality is the result of swale long-term operation, i.e., drainage of a series of many runoff/snowmelt events, processes occurring between the events (e.g., evapotranspiration, plant uptake) and even external actions (maintenance of swales, reconstruction). These effects are accounted for in swale long-term performance in immobilizing pollutants in their soils.

Studies investigating short-term swale performance in pollutant removal focused mostly on selected swale characteristics, such as the longitudinal slope, geometry (the length and cross-section), and the grass species and density, in order to develop the “best” swale designs for stormwater quality control. A general analysis of the database derived from 59 swale studies showed that swales were efficient in removing TSS and particulate metals (Zn, Pb, Cd and Cu) from conveyed runoff flows (Fardel et al., 2019). The removal of solids has been investigated extensively, which led to the development of computational methods for the settling of discrete particles (Deletic, 2001; Deletic and Fletcher, 2006).

Long-term exposure to polluted stormwater runoff from roads and parking lots in turn leads to an elevated content of traffic-related metals in roadside soils (Lind and Karro, 1995; Achleitner et al., 2007). Generally, the highest metal concentrations were observed

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in the upper soil layers (typically 0-5 cm) near the point of runoff inflow into the swale (Tedoldi et al., 2017a), from which the pollutant concentrations declined with distance and increasing depth below the soil surface (Tedoldi et al., 2017b). Although the metal enrichment in roadside soils was investigated in some earlier studies, there is a lack of data for swales serving not just for runoff control in warm seasons, but also for snow storage during the winter season.

In spite of a fair number of studies on the role of grass swales in stormwater management, the research of swales is still ongoing, because of the needs to cover the variety of climatic conditions, swale design characteristics and operating conditions in urban catchments.

Swales usually provide a link between impervious surfaces (e.g., highways, streets, parking lots, roofs, etc.) generating stormwater runoff, which partly ingresses into swale soils and partly is conveyed through the swale, and the separate sewer systems. In that sense, swales can be viewed as transport links between the pollutant sources and the receiving environments, including both groundwater aquifers and surface waters. Thus, investigations of both short- and long-term environmental performance of swales are of great importance for creating opportunities for implementing environmental protection by, and establishing maintenance needs of, grass drainage swales.

1.1. Aim and Objectives

The vast majority of literature references reported on the studies, in which grass swales were exposed only to rain-generated runoff and the associated pollution. However, in a climate with seasonal snow addressed in this thesis, swales are also exposed to snowmelt from intermittent melts of fresh snow on trafficked pavements and the melts of polluted snow stored in swales during the winter. Urban snowmelt is generally more polluted than rain runoff, because of winter road maintenance involving salt and grit applications on roads and seasonal activation of other pollution sources (Vijayan, 2020).

This thesis aims to investigate the short- and long-term operation of grass swales serving for stormwater drainage and seasonal snow storage. The specific thesis objectives are as follows:

1. To advance the knowledge of processes affecting total metal concentrations in stormwater runoff passing through grass swales, with respect to influential factors, including inflow pollutant concentrations, swale geometry, and characteristics of grass-soil media (Paper I).

2. To estimate metal enrichment of, and metal burdens in, soils of three urban grass swales serving for stormwater drainage and seasonal snow storage, on the basis of soil chemistry data (Papers II and III).

Much of the discussion of swale environmental performance focuses on traffic-related metals, because of their common occurrence in road runoff at toxic levels and potentially acute effects on biota in the receiving waters.

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3 1.2. Thesis structure

The thesis includes three papers referred to as Papers I-III. Paper I is a review paper, which synthesizes and critically reviews the findings of previous research on the processes affecting pollutant transport with runoff in grass swales and on grass filter strips. Paper II presents a method, which uses soil chemistry data and planning level modelling to estimate the metal burdens in swale soils. Paper III is a field study examining vertical and horizontal profiles of swale soils, in order to advance the understanding of metal distribution and mobility in urban grass swales operated in the cold climate with seasonal snow. A synthesis of these three papers is presented in Figure 1.

The thesis is divided into seven chapters. Chapter 1 introduces the topic of grass swales and their importance in stormwater management, and finishes with the thesis aims and objectives. Chapter 2 presents a background of swale quantity and quality performance in stormwater control, with special focus on the consequences of long-term infiltration of polluted stormwater and snowmelt into swale soils. The investigated field sites and the methods used in the three papers are described in Chapter 3. In Chapter 4, the main results of the licentiate thesis are presented and followed by the discussion of results in Chapter 5. The main study conclusions are presented in Chapter 6. The list of references cited is presented in Chapter 7. Finally, the thesis papers are appended at the end of the thesis.

Figure 1: The relationship among the papers included in the thesis

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2. Background

This chapter describes the role of grass swales in green urban drainage infrastructure and presents an overview of the state-of-the-art knowledge addressing: (i) swale hydrology and design, (ii) swale performance in removing pollutants from conveyed stormwater runoff, (iii) enrichment of swale soils with metals (or other pollutants) entering soils through infiltration, and (iv) modelling stormwater quality processes in grass swales.

2.1. Swale design and characteristics

Progressing urbanisation results in higher runoff peaks and volumes of stormwater of impaired quality travelling faster from the catchment to the receiving waters. One solution for attenuating these negative effects of urbanisation and restoring some measure of balance between the “natural” and “urban” drainage is the incorporation of green infrastructure (GI), such as grass swales, into the landscape of urban catchments. Swales are vegetated shallow drainage channels commonly used along impervious surfaces, such as highways, roads, and parking lots, in order to reduce stormwater runoff volumes and peaks, and remove some stormwater pollutants during runoff conveyance through a dense grass layer and filtration through swale soils. Swales typically fall into three main categories: (i) standard swales, (ii) dry swales, and (iii) wet swales. Standard and dry swales are similar in their appearance, however, the latter feature a filter bed of specially processed soil (an engineered soil) and an under-drain pipe to enhance infiltration (Woods Ballard et al., 2015). On the other hand, wet swales are designed to operate with permanent standing water and wetland vegetation (Woods Ballard et al., 2015). Typically, swales receive runoff through lateral inflows over side slopes, either freely without obstructions or through regular brakes in the curbs, and/or longitudinal inflows from the upstream sources, e.g., from a bridge or a roof. Moreover, swales can be constructed as a part of a stormwater treatment train, in combination with e.g. permeable pavement parking lots, grass filter strips, or bioretention cells, in order to provide sufficient treatment for meeting environmental objectives (WSUD, 2006; Revitt et al., 2017).

Sometimes the swale designs include grass filter strips (GFS) and/or check-berms across the swale bottom section. GFS are considered as pre-treatment devices of swales, however, some studies compared swale performance with and without GFS and concluded that the main pollutant removal still occurs along the bottom of the swale channel (Stagge et al., 2012). Check-berms serve to enhance infiltration into the swale bed by increasing the flow depth. Figure 2 shows typical operating conditions of standard swales.

There are some general design recommendations for swales that may be further modified to meet specific local conditions. Swales are usually constructed to treat stormwater runoff from small catchments, up to 1-2 ha in size, in order that the generated flow depths and velocities allow for stormwater quality processes to occur (WSUD, 2006). The recommended swale cross-sections are trapezoidal or triangular, with a rounded bottom section and a recommended bottom width of 0.5-2.0 m, to allow for easier maintenance

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and prevention of concentrated flows (Woods Ballard et al., 2015). Longitudinal slopes are recommended not to exceed 2%, and inclusion of check-berms is recommended for slopes steeper than 4%, while side slopes should be less than 1:3 (33%) (USEPA, 1999).

To achieve stormwater quality control, swales should perform well in pollutant removal for the majority of events occurring on an annual basis (Woods Ballard et al., 2015). For example, a design event for water quality swales should generate flow depths below the grass height, flow velocities < 0.3 m/s, and adequate times of travel of runoff along the swale (Woods Ballard et al., 2015). Even for major design rain events, with return periods 50 – 100 years, flow velocities within swales should be <2 m/s (WSUD, 2006).

Usual maintenance of swales includes regular visual inspections (e.g., for erosion and blockages of inlets and outlets), mowing grass to a certain height (75–150 mm (Woods Ballard et al., 2015)), and removal of accumulated sediment (when exceeding the depth of 25 mm, (Woods Ballard et al., 2015)) and litter, in order to prevent clogging (Ahmed et al., 2014; Woods Ballard et al., 2015). Occasionally, certain quantities of swale soils can be replaced, if the soil media become saturated with pollutants (Leroy et al., 2016;

Tedoldi et al., 2016; Hanfi et al., 2020). However, the effects of swales operation and maintenance on their long-term performance requires further study (Hunt et al., 2020).

In summary, swale characteristics can be classified as: (i) geometrical (i.e., length, depth, longitudinal slope, side slope, and bottom width), (ii) grass characteristics (the height, type of grass and turf density), and (iii) soil physical characteristics (infiltration capacity, soil texture, and soil compaction). Table 1 summarizes the methods, which were commonly used in previous research to determine these characteristics.

Figure 2: Standard swale built with a native soil and a pre-treatment GFS (Paper I)

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Table 1: Methods used for determining swale characteristics in the previous research

Swale characteristics Method

Ambient environmental and operational conditions: Swale surroundings; channel

erosion and the state of turf cover;

accumulations of debris (litter) and sediment;

maintenance access; functionality of the drainage pipe, condition of the drop inlet,

etc.

Visual observations (during regular inspections)

Swale geometry

Manual measurement at repeated distances along the swale (e.g., 5 m); RTK-GPS survey and building the digital elevation model (DEM) (Rujner et al., 2018) Grass characteristics

Grass type

Processing digital photographs of the grass area with imaging software to estimate the dominant grass species (Ming-Han et al., 2008)

Grass cover

[grass blade/cm²]: Counting the blades within 10 cm² quadrants (Deletic and Fletcher, 2006);

[%]: Processing digital photographs of the grass area with imaging software to estimate grass coverage (Ming-Han et al., 2008; Pan and Shangguan, 2006);

Aerial photographs imported to AutoCAD to draw polygons around areas with bare soils plus visual estimation (Winston et al., 2012)

Grass blade width [cm] Measuring ≈ 50 random grass blades (Deletic and Fletcher, 2006)

Above-ground biomass per unit area (if herbs are more dominant than grasses)^a

An open cylinder (diameter 0.12 m and height 0.1 m) is placed in the centre of two adjacent 0.25 m² quadrants to harvest the contained above-ground biomass, the collected material is oven-dried and weighed (Mazer et al., 2001);

Three plants randomly taken from a 15 cm² quadrant, aerial parts of plants were cut and dried at 35 °C for one week, after which the dry residue was weighed (Leroy et al., 2017)

Soil physical characteristics

Infiltration capacity [cm/h] Double ring infiltrometer test (Deletic and Fletcher, 2006; Rujner et al., 2018; Young et al., 2018)

Soil texture

From wet sieving and hydrometer analysis of two 13 cm long soil cores to determine % clay, % silt and % sand (García-Serrana et al., 2017);

From granulometry of a 30 cm soil layer to determine

% clay, % silt and % sand (Rujner et al., 2018)

Soil compaction

Cone penetrometer (soil was considered compacted if the cone index exceeded 2,070 kPa in the upper 7.6 cm) (Winston et al., 2012)

a Herbs may produce high above-ground biomass despite low plant density (Mazer et al., 2001)

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8 2.2. Swale hydrological performance

Draining trafficked areas requires fast removal of generated runoff from the impervious surfaces, in order to maintain safe road conditions (e.g., avoidance of hydroplaning) and passage of emergency vehicles. Since swales are typically located next to roads and highways, their design needs to provide good hydrological performance, including attenuation of stormwater runoff volumes and peaks, and safe runoff conveyance.

Research has shown that swales are very effective in controlling runoff from small rain events by completely infiltrating the runoff into soils, and avoiding any outflow (Davis et al., 2012, Purvis et al., 2018, Young et al., 2018). This is why some authors noted that small storms did not generate enough runoff in swales to allow for stormwater sampling.

For moderate rain events, swales attenuate runoff volumes and peaks, while for large events, their main function is flow conveyance (Davis et al., 2012).

Rushton (2001) reported a mean runoff volume reduction of 30% in a catchment with swales, compared to a similar catchment without swales. Bäckström et al. (2002) performed field experiments on seven swales (5-10 m long) by pumping water mixed with sediments into the swale at the upstream end (inflow rate 0.5-1.5 L/s) and observed inflow volume reductions of 33-66%. Lucke et al. (2014) studied four field swales (30- 35 m long) by feeding in water with pollutants (TSS, TP and TN) at the upstream end (0.5-2.0 L/s) and observed mean runoff volume reduction of 52%, which depended on the initial soil moisture content. Jiang et al. (2017) monitored two swale sections (5 m long) and reported mean runoff volume reductions of 78-98% for five monitored actual events. Young et al. (2018) studied two swales (210-230 m) draining highway runoff and observed mean volume reduction of 87% for 65 rainfall events.

The swale grass layer further enhances the swale hydraulic function, compared to bare soils. Dense grass increases surface roughness by slowing down the runoff and increasing infiltrated runoff volumes (García-Serrana et al., 2017), which also limits the effect of slope on infiltration rates (Morbidelli et al., 2016).

2.3. Swale performance in treating stormwater runoff

Runoff generated on impervious surfaces of urban catchments contains a variety of pollutants from numerous anthropogenic activities. Draining such a polluted runoff into grass swales can provide local treatment in well-designed swales (e.g., with dense grass turf, mild bottom slopes, good infiltration rates, etc.). Identification of swale characteristics, which are beneficial for stormwater quality control, resulted from studies addressing grass swale performance in enhancing stormwater quality. These studies can be divided into three groups: (i) laboratory and field assessment of simulated inflows, (ii) field assessment of actual rainfall events, and (iii) computer modelling studies.

The first group of studies often aimed to advance the understanding of small-scale processes occurring in swales with respect to influential factors. Typical controlled

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investigations supplied synthesized stormwater runoff at the swale upstream end (Deletic 1999; Bäckström 2002; Deletic and Fletcher 2006; Lucke et al., 2014), but less frequently also over the swale side slopes (Fardel et al., 2020).Developed algorithms for fluxes of pollutants of different types and characteristics, which resulted from such controlled experiments, can be verified using the measured data from the second group of studies.

Field studies of swale short-term performance in pollutant removal during actual rainfall events typically investigated swales next to highways (e.g. Barrett et al., 1998; Winston et al., 2010) or urban roads (e.g. Bäckström et al., 2006), and less frequently parking lots (Rushton, 2001). Moreover, studies of swale field performance for actual rainfall events are very important for collecting high quality data for testing standard urban drainage modelling packages (e.g. SWMM, Mike SHE) and for investigating swale performance in larger-scale systems (i.e., incorporated into the drainage system).

Looking at larger-scale processes is important, because these processes affect the generation and quality of runoff entering the swale. During dry periods, evapotranspiration (ET) restores swale infiltration capacity (Deletic, 2000) and compared to bare soils, evapotranspiration is enhanced by the grass cover (Hino et al., 1987). At the same time, pollutants accumulate on the catchment surfaces (including the swale surface) as a result of dry atmospheric deposition. Even in dry weather, the pollutants accumulated on the contributing drainage surfaces may be transported into grass swales by vehicle induced turbulence, wind, street sweeping and snow clearance from pavements. During wet weather, the accumulated pollutants are washed into the swale via runoff and vehicle- generated splash water (Werkenthin et al., 2014), and additional pollutants enter swales through direct precipitation (i.e., wet atmospheric deposition). Leroy et al. (2016) sampled infiltrated water from a swale section receiving only atmospheric deposition (wet and dry) and compared it to a swale section receiving also road runoff. The authors measured lower concentrations in the former section, but of the same order of magnitude, concluding that atmospheric wet and dry deposition should not be neglected in the conditions of their study (Leroy et al., 2016).

Moreover, specific catchment characteristics, such as, land use type (e.g., highway, secondary road, residential area), percentage of imperviousness, slopes, etc., affect the stormwater runoff pathway. Different pathways of stormwater before reaching the swale will affect the stormwater quality (i.e., the pollutant inflow concentrations reaching the swale). Studies have shown that pollutant removal in grass swales is affected by pollutant inflow concentrations. For example, Stagge et al. (2012) suggested that swales can treat total phosphorus (TP), if its concentration in the inflow exceeds 0.7 mg/L, while Bäckström et al. (2006) suggested that inflow concentrations of TSS >40 mg/L are needed to produce positive removals. Winston et al. (2011) observed that the largest increases in total nitrogen (TN) and TP concentrations, after conveyance through GFS, may be caused by low inflow concentrations. Also, Winston et al. (2010) investigated two wet swales and two standard swales (with GFS) draining highways with asphalt

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pavement with a porous friction course (PFC) overlay. The swale length was 30.5 m and the GFS length was 8 m (Winston et al., 2010). TSS inflow concentrations, to the GFS, were reduced by runoff passage over the PFC to concentrations in the range 10-31 mg/L and negative removals of TSS were observed after conveyance over GFS (Winston et al., 2010). The authors explained such a TSS export by the irreducible TSS concentrations in the influent, ~10 mg/L (Winston et al., 2010). The irreducible concentrations are the minimum (residual) outflow concentrations that cannot be further reduced by stormwater management facilities (Schueler, 2000).

Abundance of solids in urban areas, their release throughout the urban catchments and the role in transporting other pollutants (e.g., adsorbed metals) (Liu et al., 2015), all contribute to the fact that solids are the most investigated quality parameter in studies of grass swale inflows and outflows. In addition, total and dissolved metals, nutrients, traffic- associated hydrocarbons and oxygen-demanding constituents are also often analysed, compared to, e.g., chloride and faecal indicator bacteria, which are studied less frequently.

The pollutant type and characteristics are important to consider, because actual stormwater quality processes causing pollutant removal in overland flow over grass depend on these influential factors. For example, settling and infiltration processes, were found important in removing solids (usually described as total suspended solids (TSS)) (Mendez et al., 1999; Barrett et al., 2004; Stagge et al., 2012), while plant uptake contributed to retention of metals in the roots and the above ground biomass (Leroy et al., 2017).

2.4. Effect of stormwater infiltration on soil media quality

Filtration of stormwater through swale soils is an important process for enhancing stormwater quality, as shown by sampling subsurface flow from a swale underdrain pipe (Purvis et al., 2018; Fardel et al., 2020). Filtration of road runoff through swale soils resulted in a significantly cleaner outflow in the underdrain pipe, compared to untreated road runoff, with respect to TSS, total volatile suspended solids (VSS), enterococcus, E.

coli, and turbidity (Purvis et al., 2018). For example, concentration reductions in the underdrain outflow were 88% (TSS) and 87% (VSS), while reductions in the overflow were substantially lower, 10 and 21%, for TSS and VSS, respectively (Purvis et al., 2018).

Fardel et al., (2020) conducted controlled field experiments to examine Zn, pyrene, phenanthrene and glyphosate removals through standard and filtration swales, and found that chemical removal efficiencies were significantly higher in the subsurface outflow, compared to the overflow. In the same study by Fardel et al. (2020), for all experiments, mass removals of Zn, pyrene, phenanthrene and glyphosate were higher in the filtration swale than in the standard swale. Moreover, Leroy et al. (2015) observed that filtration through the dense root system of grass captured suspended solids (SS) with attached PAHs and limited the transfer of PAHs into deeper soil layers.

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The above discussed research studies acknowledged that, in a long term, filtering polluted stormwater runoff from trafficked surfaces can pose some risk of contamination of swale soils, with contaminants reaching deeper soil layers and even leaching into the groundwater. Monitoring pollutant concentrations in the field can be used to characterize pollutant concentrations, provide spatial or temporal summary of environmental contamination, and demonstrate or enforce the compliance with standards or guidelines (Gilbert, 1987), to name a few examples. Moreover, field and laboratory studies can provide data for studying pollutant transport and quantifying the relationships that control the levels and variability of pollutant concentrations in time and space (Gilbert, 1987). Often, a composite sampling is performed in order to reduce the cost of sample analyses, but at the risk of losing the individual sample information and dilution of the samples (Mason, 1992).

The fate and transport of metals and PAHs in the soils of the infiltration-based Sustainable Urban Drainage Systems (SUDS) has been reviewed by Tedoldi et al. (2016). Metals are often reported in higher concentrations in the topsoil layer, and such concentrations decrease with the soil depth (Tedoldi et al., 2016). The thickness of the “topsoil layer”

differs among the studies. In many studies, the topsoil layer was considered to be 5 cm thick (Lind and Karro 1995; Norrström and Jacks 1998; Achleiter et al., 2007; Ingversten et al., 2012; Rommel et al., 2019), but in other studies (Rushton 2001; Hjortenkrans et al., 2006) a smaller depth was considered (0-3 cm). According to Mason (1992) airborne pollutants and pollutants that are strongly bound to soil particles are found in the top 15 cm, while pollutants from long-term deposition are found in the layers deeper than 15 cm.

There are multiple traffic-related sources of metals, e.g. vehicle operation, tire and brake wear, vehicle washing, and road abrasion that contribute to metal pollution in stormwater runoff (Müller et al., 2020). In many studies, soils were sampled next to the roads with various traffic intensities, in order to assess the contribution of traffic to the metal pollution in roadside soils. Carrero et al. (2013) sampled soils next to: (i) an old secondary road exposed to high traffic (>60 years of service), (ii) a newer highway road (20 years old with 28,200 AADT), and (iii) two roundabouts (1 and 5 years old). PCA analysis showed that samples from the old secondary road clearly differed from the remaining samples by having higher concentrations of traffic related metals (Carrero et al., 2013).

For example, Pb concentrations of 630 mg/kg in the old road case far exceeded the concentrations of Pb <40 mg/kg for the other investigated roads. The authors found a decrease in metal concentrations with depth (the first 20 cm of soil was analysed) and with distance from the road (0, 1 and 3 m distances). Metal concentrations at different depths and distances from the road were not investigated for the old road, because of the high concentrations “near saturation” that would hide the trends (Carrero et al., 2013).

Soil characteristics, such as soil sorption capacity, pH, organic matter and chloride contents can play an important role in immobilization of metals. Exceedance of the soil

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sorption capacity can lead to migration of metals into the deeper layers (Tedoldi et al., 2016). In another case, the high clay content (19.1%) provided high cation exchange capacity enabling metal sorption (Leroy et al., 2016). Moreover, decrease in pH can result in mobilisation of metals (Bäckström et al., 2004), but neutral pH 6-7 reduces the risk of occurrence of metals in the dissolved fraction, as reviewed by Rieuwerts et al. (2015).

Bäckström et al. (2004) sampled the water draining through the soil at a depth of 50 cm below the soil surface, at various distances from the road edge. The sampling was done in the soils next to two roads in mid Sweden during one year and the analytical protocol included pH, electrical conductivity (EC), inorganic carbon (IC), total organic carbon (TOC), chloride, sulphate, and metals (Cd, Cu, Pb, Zn, Na, Ca, Mg, K, Fe and Al). The authors found strong significant correlations between chloride and metals, and electrical conductivity and metals (Bäckström et al., 2004).

Lastly, swales in cold climate regions with seasonal snow have an additional function, i.e., storage of snow cleared from roads and parking lots. This is a very useful swale function for maintaining safe driving conditions during the winter, since snow can be quickly cleared from roads into swales. Comparisons of different snow management scenarios (i.e., transport of snow to, and storage in, central or local snow storage sites, with or without the use of swales) showed that storage of snow in the swales had a favourable impact on costs and long-term traffic-related pollution emissions (Reinosdotter et al., 1998).

Lind and Karro (1995) sampled soils next to two roads (AADT = 11,400-34,000) in southern Sweden, after the first eight years of operation. The authors observed that drainage of stormwater contributed to a metal (Zn, Pb and Cu) enrichment of soils.

Norrström and Jacks (1998) sampled soil next to a 29-year old highway (40,000-50,000 AADT) in southern Sweden. The first 15 cm of soil were sampled by coring at 20 locations along two transect lines at 0.5 and 2.5 m distances from the pavement edge and the cores were divided into 5 cm slices, which were composed for individual transects (Norrström and Jacks, 1998). The authors measured the highest Pb concentration (542 mg/kg) in the top 5 cm at 0.5 m from the highway. Hjortenkrans et al. (2008) sampled two swales draining about 20-year old highways (20,700-22,300 AADT) in the South of Sweden and produced composite samples, comprising at least seven sub-samples, to represent the metal concentrations at different depths and distances from the highway. At one site the highest Pb concentration (200 mg/kg) was measured at the 0.4 m distance, 10 cm below the surface, with the upper layer concentrations being lower (Hjortenkrans et al., 2008). The authors explained this by the annual depositions of sand used in winter road maintenance and the phase-out of Pb from gasoline (Hjortenkrans et al., 2008). The characterisation of soil pollution in cold climate swales is important, because of specific operating conditions, including the drainage from roads serviced by applications of salt and anti-skid materials, and the effects of melting of the stored snow during the winter.

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13 2.5. Conceptual models of grass swales

Computer modelling studies strive to cope with the complexity of urban drainage systems and integration of a large number of drainage elements. Modelling studies of grass swales can be divided in two groups, according to the nature of the models used: (i) Studies based on research models, and (ii) Studies of applications of standard urban drainage modelling packages (e.g., SWMM, Mike SHE, Music, and others).

The first group represents semi-empirical models for computing TSS removal from stormwater flow over grass surfaces, e.g., the Kentucky Method (Tollner et al., 1976) and the Aberdeen equation (Deletic, 2000), which resulted from controlled laboratory experiments. This group of models still attracts a lot of research interest and efforts to verify the original equations for other conditions than those, for which they were developed. For example, the Aberdeen equation was recently tested to predict TSS removal efficiencies in two swales and the modelling results for six rain events were compared to the actual field data (Hunt et al., 2020). The modelled event removal efficiency was a weighted average of the removal efficiencies of each particle size (calculated using Aberdeen method) (Hunt et al., 2020). The maximum difference between the modelled and actual removal efficiencies was 20% and the authors noted that the smallest difference (1-6%) was observed for two events when the flow depth was close to the nominal grass height. There is a research need for more data on other-than- solids pollutant removals and transport in swales, in order to gain more knowledge on influential factors and quantify processes other than settling in swales.

The models in the second group were originally developed for larger (catchment) scales and are continually being refined, in order to simulate small stormwater control facilities, such as grass swales, with sufficient accuracy. For example, Niaizi et al. (2017) reviewed papers on SWMM applications and found only a small number of studies, out of 150 peer-reviewed papers, describing the use of SWMM for modelling stormwater pollutant reductions by GI. One study (Jia et al., 2014) compared the following drainage scenarios:

(i) impervious areas, and (ii) impervious area reduced by incorporating GI features (including a grass swale in a treatment train). However, the pollution reduction by the swale and representation of the stormwater quality processes in the swale was not the focus of the study. A number of recent studies focused on modelling runoff quantity control by grass swales using standard urban drainage modelling packages, Mike SHE, SWMM, and WinSLAMM (Flanagan et al., 2017; Xie et al., 2017; Rujner et al., 2018;

Young et al., 2018; Wadhwa and Kumar, 2020; Broekhuizen et al., 2020). This work is relevant to the studies in this thesis, since reliable quantity simulations are needed to model well the water quality. However, an even bigger obstacle in modelling pollutant reductions in swales is the lack of understanding of physical, chemical and biological processes taking place in grass swales.

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14 2.6. Knowledge gaps and future research

There is a continual need to expand the existing knowledge of design and operation of drainage swales into widely varying and previously unexplored conditions, and assess the underlying limitations of the past research. This assessment was conducted at the start of the thesis project and its findings are briefly summarized below. Recognizing the inherent emphasis of the thesis project on producing new experimental data and physico-chemical concepts, the modelling of swale operation was excluded from the above knowledge gap analysis.

A brief overview of the state of knowledge of urban grass drainage swales in this chapter indicates that the analysed studies were conducted mostly in the temperate climate, without seasonal snow, with the exception of the pioneering work by Bäckström (2003).

To achieve a good control of experimental conditions, laboratory or field research studies mostly considered well-defined but less-complex class of swale layouts:

(i) Land cover/use serviced by drainage swales: mostly adjacent to urban roads or highways, very few studies addressed parking lots, or residential lands

(ii) Generation of runoff inflow – mostly by irrigation water, or by actual rain events;

rarely by snowmelt (from drained surfaces, or stored snow)

(iii) Runoff inflow into swales – in studies applying swale irrigation, the inflow entered at the upstream end only, with a few exceptions of supplementing the longitudinal inflow with lateral inflows as well; in studies of actual rainfall events, both longitudinal and lateral inflows were considered; lateral inflow – typically from one side only

(iv) swale cross-sections – mostly trapezoidal or triangular; many studies addressed treatment in the bottom section only, neglecting treatment/infiltration on side slopes (v) swale surface – turf, natural or synthetic (the latter was used in lab studies); rarely bare earth in lab studies comparing turf with bare earth

(vi) swale soils – investigated in some field studies, within some distance (0-5 m) from the road pavement and typical depths (0-30 cm)

(vii) swale water quality process studied – a vast majority of studies focused on solids settling as the most important quality enhancement process; relatively few studies pursued stormwater filtration through swale turf and soils, or the resulting effects on soil chemistry The licentiate phase of the planned PhD project should strive to reduce or close the above knowledge gaps by focusing on stormwater quality enhancement in urban grass swales providing drainage and snow storage for various land use (for comparative purposes), conveying actual runoff and snowmelt entering the swale at the upstream end as well as on one or both sides, and focusing on the treatment of stormwater by infiltration into swale soils.

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3. Methods

3.1. The literature review

The aim of the critical review paper was to provide a systematic overview of the state- of-the-art knowledge of processes that serve to remove pollutants from stormwater runoff flowing over grass surfaces, with respect to influential factors. The primary sources of information were laboratory, field and modelling studies of stormwater quality processes occurring during stormwater runoff over standard and dry grass swales and grass filter strips (GFS). Literature research focused on peer reviewed articles, academic theses, conference proceeding papers, books, reports and design guidelines in Scopus, Web of Science and Google Scholar databases. The references listed in the reviewed articles were also examined. Searches included a variety of related keywords e.g. “grass swale”,

“vegetative swale”, “grass ditch”, “drainage swale”, “dry swale”, “grass filter strip”, etc.

3.2. Study sites

In this thesis project, three grass swales serving for stormwater drainage and seasonal snow storage were selected for study using such selection criteria as: (i) Well-functioning swales with clearly delineated inflows (on one or both sides) and outflows, (b) Coverage of a variety of sites with various soils, land use and traffic intensity, and (c) A general suitability with respect to the site proximity, access and field crew safety. Three sites meeting these conditions were selected in the City of Luleå, Sweden (Paper II and III). The climate at the study location is a cool temperate climate, characterized by long winters, with the snow season starting in October-November and snow remaining on the ground until April. The mean annual temperature is 1.4 ºC. The studied sites represent different land use types, i.e., a swale in a commercial catchment (swale L1), a swale next to the busiest road in the city in the downtown area (swale L2), and a swale in the residential catchment (swale L3). The first swale (L1) receives runoff from a parking lot (408 m²), a small part of a building roof (5 m²), and a single-lane road (241 m²) with the average daily traffic (ADT) of ~ 2,750. The second swale (L2) receives lateral stormwater runoff from a two- lane road (728 m²) with the highest traffic intensity (ADT ~ 11,650) among the studied locations. This swale receives road runoff only from one side, because the other side features a continuous curb preventing any stormwater runoff discharge into the swale (further called the no-runoff (NR) side). The third swale (L3) receives runoff from a parking lot (287 m²), a roof (812 m²), a grassed area (726 m²), and a two-lane road (520 m²) with ADT ~ 2,500. The age of the three studied swales was estimated from the years of construction of the roads next to the swales; thus, the swales years of operation at the time of the soil sampling campaign was 57 years for swale L2, and 38 years for swales L1 and L3. There are uncertainties concerning the years of swale construction, which depended on when the road was completed and possible swales modifications in the following years, resulting from road reconstruction or other building activities in the catchments. All the three swales are used for snow storage during winter road

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maintenance, which includes the clearance of snow from the roads and parking lots adjacent to the swales and applications of anti-skid materials (grit). Road salt (NaCl) is applied only as an additive to grit material, to prevent its freezing and formation of clumps in the grit material. Such a salt/grit mixture is applied only in early or late winter, when temperatures are above -6° C and salt is effective in melting the ice layer formed on grit particles. In early spring, after the winter season (end of April-beginning of May), the residual grit is brushed off the roads and parking lots and collected for disposal. As an example, Figure 2 shows swale L1 in the commercial catchment, before and after the winter. It can be seen from Figure 2 that the stored snow may remain on the swale ground even after the sweeping and removal of the residual grit from the roads and parking lots.

Regular maintenance of the three studied swales includes: (i) regular mowing of grass in the summer, and (ii) removal of gravel accumulations from the swales, which is done once a year in early spring, after snow melted away and swales became dry, but before the grass layer was established.

3.3. Soil sampling

In each swale studied, a 20 m long section was selected for soil sampling, which was done in October 2017, using a stainless-steel core sampler with a 5 cm diameter and the length of 30 cm. The section received only direct lateral runoff from the adjacent road and/or parking lot, and the measured soil chemistry was used to examine if there were differences in metal concentrations in soils draining different land covers. Because of soil characteristics variation along the swale, samples were collected at three cross-sections 10 m apart to allow for statistical analysis. In order to investigate the metal concentrations along the runoff flow path, at each cross-section, samples were collected at the distances Figure 2: Swale L1 in the commercial catchment. The picture on the left side shows the swale before the soil sampling campaign (September 2017) and the picture on the right shows the same swale after the winter (April 2018).

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of 40 and 80 cm from the edge of the pavement and at the deepest point of the cross- section, in the swale bottom section. Using a stainless-steel knife, each soil core was divided into 5 cm slices representing individual samples, which were placed in a plastic bag, refrigerated and kept in cold storage (up to 7 days) until analysed. At all three swales, top three soil layer samples (0-5, 5-10 and 10-15 cm) and the deepest layer sample from the swale bottom section were analysed, while for the swale sides there were some exceptions:

(i) In swale L3, no soil samples could be collected from the swale side draining the parking lot, because of the presence of gravel from the parking lot construction.

(ii) Only the top layer (0-5 cm) samples from the side draining road (L3) and parking lot (L1) were analysed, because some deeper soil layers, at a 40 cm distance from the pavement edge, were highly compacted and did not allow sample extraction.

(iii) In swale L2, only the top layer (0-5 cm) samples from the no-runoff side were analysed.

Figure 3 shows an example of sample distribution at three cross-sections of swale L1;

the black coloured symbols identify, which samples were analysed. The same sampling pattern was applied in swales L2 and L3, with minor exceptions listed above. In total, 96 individual soil samples were collected and analysed.

Figure 3: Distribution of 30 cm deep soil cores collected at the three studied swales (obtained from Paper III). Black colour indicates, which samples were subject to the analysis at swale L1. All lengths are in cm, unless indicated otherwise.

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In order to investigate the swale topography and runoff contributing areas, location data (x-y-z coordinates) were collected at numerous points along the swale using a real-time kinematic-GPS device (model GeoMax Zenith35 Pro TAG) with the precision of 1.5 cm (for x and y) and 2 cm for z. The location data was used to build the TIN (Triangular irregular networks) surface in AutoCAD Civil 3D software.

3.4. Infiltration measurements

In order to investigate the swale infiltration capacity, field measurements were performed in September 2018 using the Modified Phillip Dunne (MPD) infiltrometer (ASTM, 2018). Infiltration measurements were performed at undisturbed sites, which were covered with turf, along the three sampled cross-sections (Figure 3), within ~ 30 cm of the corresponding sampling points, and at two additional points at 120 and 200 cm distances from the pavement edge. The saturated hydraulic conductivity was calculated according to the method developed by Upstream Technology Co. following the ASTM standard (ASTM, 2018). The best-fit values of saturated hydraulic conductivity (Kf,best_fit) were calculated using the method developed by Weiss and Gulliver (2015):

K_f,best_fit= 0.32(K_f,arit) + 0.68 (K_f,geo) (1)

where,

Kf,arit and Kf,geo represent the arithmetic and geometric means of the saturated hydraulic conductivity, respectively.

The measured data was also compared to the literature data on infiltration capacities of soils of various textures to inform about the soil texture of the studied sites.

3.5. Laboratory analysis 3.5.1. Soil parameters

Soil samples were prepared according to the standard ISO 11464 (2006) with minor changes, and analysed for electrical conductivity (EC) and pH in the university laboratory. The samples were air dried and dry sieved in the laboratory using a vibratory sieve shaker (Retsch AS200) and a stainless sieve (mesh size of 2 mm). Soil lumps remaining on the sieve were crushed using pestle and mixed with the < 2 mm fraction (ISO 11464, 2006). The fraction > 2 mm, which generally included grass roots and stones, was excluded from analyses. Measurements of EC were done according to the standard ISO 11 265 (1994) using the CDM210 conductivity meter. Measurements of pH (in a 1:5 suspension of soil in water) were done according to the standard ISO 10390 (2005), using the WTW pH 330 instrument. Chloride and loss on ignition (LOI) analyses were done by an accredited commercial laboratory (ALS Scandinavia AB, Luleå).

Chloride analysis was done according to standard DIN EN ISO 12457–4 (2003). The LOI analysis was determined at 1000°C and reported in % of sample dry weight (DW).

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19 3.5.2. Analysis of total metal concentrations

Total concentrations of 13 metals have been examined in this thesis. The group consists of common urban-related metals Zn, Cu, Pb, Cd, Cr, Cu, Ni and Co, all of which, except Co, are considered stormwater priority pollutants (Eriksson et al., 2007). This group was expanded to include W, Mn, Ti, V and Ba, which were all reported as traffic- related elements, originating from such sources as e.g., asphalt, tire and brake wear, and tire studs (Apeagyei et al., 2011; Mummullage et al., 2016; Huber et al., 2016). Zr was also selected since it was validated as a tracer exhibiting a concentration deficit in sediment accumulations in Sustainable Urban Drainage Systems (Tedoldi et al., 2018). This deficit is caused by dilution of sediments containing Zr from anthropogenic sources by sediments of mineral origin. Analyses of total metal concentrations were done by ALS Scandinavia AB in Luleå. Metals Cd, Cu, Co, Ni, Pb and Zn were determined by digestion in a heating block with nitric acid, while for the remaining metals (Cr, V, Ba, Mn, Ti, W and Zr), 0.1 g of dried sample was fused with 0.4 g LiBO2 (lithium metaborate) and subsequently dissolved in dilute nitric acid. The total metal concentrations were analysed using Inductively Coupled Plasma Sector Field Mass Spectrometry (ICP-SFMS) following SS EN ISO 17294-1, 2 and EPA-method 200.8. All metal concentrations were reported in mg/kg DW except for Mn and Ti, which were reported as MnO and TiO2, respectively, and converted to mg/kg DW. The laboratory performing the analysis reported analytical uncertainties in concentrations of Cd, Co, Cu, Pb and Zn as 19-33%

of the reported values.

3.5.3. Sequential extraction analysis

Results of analyses of total metal concentrations were complemented by results from a five-step sequential extraction analysis and the residue analysis. In this procedure, extractants of increasing reactivity are sequentially applied, so that the successive fractions exhibit lesser mobilities and lower risks of metal release due to changes in the ambient environmental chemistry. Such changes may include the changes in pH and other factors (Stone and Marsalek, 1996). A set of 11 soil samples from swale L2, which was noted for the highest metal concentrations among the three swales studied, were selected for this analysis. The selected samples included three top layer samples (0-5 cm) - two on the road shoulder and one on the no-runoff side; and, eight samples from two soil cores from the swale bottom section. Each core comprised samples from four layers of successively increasing depths. The selected samples were analysed by ALS Scandinavia AB in Luleå using a five-step sequential extraction analysis, and the residual analysis, following the method adopted from Hall et al. (1996a, 1996b). The total metal concentrations were analysed using ICP-SFMS following SS EN ISO 17294-1, 2 and EPA-method 200.8.

Analytical uncertainties in the reported concentrations of all metals in all steps were in the range of 17-37% of the reported values, except for Zn in step 2, which had higher uncertainty (range of the uncertainty for the 11 samples analysed was 48-62%). Samples were ground prior to the first extraction step. Concentrations were reported in µg/L and recalculated to mg/kg DW. The five extraction steps are listed below:

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Step 1 (Fraction 1): Extraction of 1 g sample with 10 ml 1.0 M acetate buffer (pH 5) by shaking for 6 h at room temperature to remove and measure adsorbed and exchangeable metals and carbonates.

Step 2 (Fraction 2): Extraction of the solid residue from Step 1 with 50 ml 0.1 M pyrophosphate solution (pH 9) by shaking for 1 h at room temperature to remove and measure labile organic forms, which are the forms associated with reaction sites such as those present in humic and fulvic substances (Hall et al. 1996b).

Step 3 (Fraction 3): Extraction of the solid residue from Step 2 with 10 ml 0.25 M hydroxylamine hydrochloride for 4 h at 50°C to remove and measure amorphous Fe/Mn oxides.

Step 4 (Fraction 4): Extraction of the solid residue from Step 3 with 15 ml 1 M hydroxylamine hydrochloride in 25% acetic acid for 3 h at 90°C to remove and measure crystalline Fe oxides.

Step 5 (Fraction 5): Removal and measurement of stable organic forms and sulphides by adding 0.75 g potassium chlorate to the solid residue from Step 4 followed by adding 15 ml 12 M hydrochloric acid for 30 min at room temperature and then 10 ml 4 M nitric acid for 20 min at 90°C.

Additionally, metal residuals were also determined. The residual content of Ba, V and Cr, was determined according to ASTM D3682: 2013 and ASTM D4503: 2008 (fusion with LiBO2). For obtaining the residual content of Cd, Ni, Pb, Zn, Cu, and Co, the samples were digested with HNO3/HCl/HF according to SS EN 13656: 2003. The ICP- SFMS analyses were carried out according to SS EN ISO 17294-2: 2016 and EPA- method 200.8: 1994. The residual concentrations were reported in mg/kg DW.

Analytical uncertainties in the reported residual concentrations of metals were 14-34% of the reported values.

3.6. Grit material applied during the winter road maintenance

Three samples of stocked anti-skid grit materials were collected from the municipal storage in April 2018:

- Material A (aggregate sizes 2-6 mm) - Material B (aggregate sizes 4-8 mm) - Material C (aggregate sizes 0-6mm) + salt

This material is applied on the roads, parking lots and bicycle paths throughout the winter and, once applied, may be ground by vehicle tires.

A single sample of each material was analysed for total metal concentrations by the ALS Scandinavia AB laboratory in Luleå. Prior to the analysis, the material was crushed and the total metal content was analysed as described in section 3.5.2. Analytical uncertainties

Enhancement of stormwater quality in grass swales: Removal and immobilisation of metals

LICENTIATE T H E S I S

Snežana Ga vr ić Enhancement of stor mw ater quality in g rass s w ales

Enhancement of stormwater quality in grass swales:

Removal and immobilisation of metals

Snežana Gavri ć

Urban Water Engineering

Enhancement of stormwater quality in grass swales:

Removal and immobilisation of metals

Preface

Abstract

Sammanfattning

Table of Contents

List of papers

1. Introduction

2. Background

3. Methods