IMPROVING STORMWATER QUALITYFOR MICROPLASTICS(25 - 106 MICRONS) USING ABIORETENTION CELL

K. Smyth (Civil and Mineral Engineering, University of Toronto), S. Tan (Chemical Engineering and Applied Chemistry University of Toronto), T. Van Seters (Toronto Region Conservation Authority), J. Gasperi (Laboratoire Eau et Environnement, Université Gustave Eiffel), B. Tassin (Laboratoire Eau Environnment Systèmes Urbaines, École des Ponts ParisTech), R. Dris (Laboratoire Eau Environnment Systèmes Urbaines, Université Paris-Est Créteil), C. Rochman (Ecology and Evolutionary Biology, University of Toronto), J. Drake (Civil and Environmental Engineering, Carleton University), E. Passeport (Civil and Mineral Engineering & Chemical Engineering and Applied Chemistry, University of Toronto)

ABSTRACT

CONTEXT

Traditionally, stormwater management has focused on flood prevention with less focus on water quality (Davis, Hunt and Traver, 2022). Common stormwater/wastewater contaminants may be targeted in designs and often untreated combined sewer overflows occur during large storm events. Bioretention cells, or rain gardens, are a type of low impact development (LID)/ green infrastructure used to manage stormwater as close to the source as possible. They consist of a ground depression filled with porous media and soil amendments where applicable, layered on top with mulch and plants tolerant of drought and flood conditions (Davis, Hunt and Traver, 2022). Bioretention cells are placed in highly impervious urban areas such as parking lots and along roadways to collect stormwater from these surfaces (Davis, Hunt and Traver, 2022). By mimicking the natural hydrologic cycle, they reduce flooding and improve water quality through mechanisms such as filtration (Davis, Hunt and Traver, 2022). They are effective at improving stormwater quality for various common stormwater contaminants including more recently microplastics (Smyth et al, 2021; Gilbreath et al, 2019).

Microplastics are plastic particles in the size range of 1 μm to 5 mm. They are considered primary if intentionally produced in this size range or secondary if formed through fragmentation (Alimi et al, 2022). Stresses such as mechanical, biological, thermal, hydrolysis and photolysis by light exposure cause microplastics to fragment into increasingly small pieces over time (Alimi et al, 2022). Relevant stresses in an urban stormwater context could include tire-pavement shear stress, road/parking lot maintenance, thermal stress from hot pavement and freeze-thaw cycles in cold climates and sunlight exposure. Biological stressors to microplastics additionally exist in a bioretention cell such as ingestion/excretion by organisms, aggregation with natural organic matter and biofilm growth. Under specific environmental conditions, biodegradation may also be a possible stress factor. Microplastic transport and environmental fate may be impacted by fragmentation as it can modify the particles’ physical and chemical characteristics (Pfohl et al, 2022).

Both aquatic and terrestrial organism health is impacted by microplastics and negative impacts are suspected for humans as well (Anbumani and Kakkar, 2018; Vazquez and Rahman, 2021). Health risks become amplified with increasingly small microplastic sizes with increased ease of intake such as through ingestion and inhalation and likewise internal transfer. (Leslie et al, 2022). Risks are also greater for environmental transport. In a bioretention cell, smaller microplastics may posed increased risks for transport within soil pores to connected water systems such as surface water, connected municipal sewer systems and groundwater. Previously, we found that microplastics from 106 μm to 5 mm in size were removed at a rate of 84% with a bioretention cell (Smyth et al, 2021). Microplastics smaller than approximately 100 μm may pose greater environmental transport risks in relation to bioretention systems as it is less known if they are equally well removed.

PURPOSE

The purpose of this work, building off our previous knowledge, is to determine the capacity of a bioretention cell to capture microplastics from 25 to 106 μm from stormwater runoff and characterize this size range of particles in this environmental matrix.

METHODS

The study was located about one hour North of Toronto, Ontario, Canada at a conservation area, Kortright Centre, operated by the Toronto Region Conservation Authority (TRCA). It included a bioretention cell with a 265-m² parking lot drainage area. The parking lot provides space for the public to access nearby hiking trails, a visitor centre and children’s camps and it is made of recycled tire-derived rubber pavers (Eco-Flex® Churchill). The bioretention cell from bottom to top includes 15 cm of clear stone wrapped in permeable geotextile, 10-cm perforated pipe underdrains, 40 cm of sandy porous media, overlaid with 7.5 cm of hardwood mulch and it was designed with a surface area of 30-m². The underdrains connect to a downstream monitoring hut. Added details on its design are listed separately (Spraakman et al, 2020).

Sampling campaigns for stormwater samples took place in the Summer and Fall of 2018 and 2019 using an autosampler at the inlet and another at the outlet of the bioretention cell. Of the twenty-two sampled events, eleven were paired inlet-outlet events. Twenty-four 1-L polypropylene sample containers were placed inside each autosampler. A 10-L bottle was used to make flow-weighted composite stormwater samples which ranged in volume depending on the runoff generated per storm from 0.5 to 2 L.

Initial processing of stormwater samples was done at the University of Toronto in Toronto, Canada, with final processing and analysis completed at the Université Gustave Eiffel in Nantes, France. Stormwater samples were dosed isopropyl alcohol at 10% by volume the day of sample collection to reduce microbial growth. Initial processing including wet sieving, digestion and density separation. Samplers were size-fractioned to less than 106 microns using a stainless-steel sieve and up to 1 L was stored per sample. A digestion step was employed to remove organic matter that built up over time between sample collection in 2018-2019 and processing in 2022. This removed organic material that visually obstructed microparticles in samples. Specifically, a 2-hour reaction with Fenton’s reagent and 30% hydrogen peroxide was used. Then, digested samples underwent density separation over a minimum of 24 hours with 1.8 g/cm³ zinc chloride solution. For transport to the Université Gustave Eiffel Campus Nantes, processed stormwater samples were vacuum filtered onto pre-cleaned 45-cm square pieces of stainless steel mesh and wrapped in aluminum foil. Upon arrival, these filtered samples were resuspended into ethanol with 10-20 minutes of sonication and immediately filtered onto aluminum oxide filters (Whatman Anodisc). Sample containing filters were stored in closed petri dishes.

Analysis of aluminum oxide filters was performed in transmission mode with a micro-Fourier Transform InfraRed spectrometer (μFTIR) (Thermo Scientific, Nicolet iN10 mx). The 25 x 25 μm imaging pixel resolution was used to estimate the lower detection limit of 25 μm. Mapping mode using one scan was used with a 4000-1200 cm^-1 spectra range. The instrument was cooled prior to use with liquid nitrogen and a spectra background was collected before every sample analysis of an empty Anodisc filter. Simple (version 1.1.β, Sept 18, 2020), an open source microplastics analysis software (Liu et al, 2019) was used to treat spectra maps with its provided single spectra reference databased (v1.02, July 21, 2019). Some limitations of this method include inability to record microplastic morphology data and inconsistent characterization of fibers as well as inability to analyze black rubber particles, a constraint for many spectroscopy methods (Wagner et al, 2018). Numerous quality assurance/quality control steps were employed to reduce microplastic contamination, including field and lab blanks, using 100% cotton lab coats, working in a laminar flow hood, washing all glassware with dish soap followed by triple rinses with distilled water, using a surfactant solution when rinsing glassware to reduce microparticle loss and spike & recovery tests.

FINDINGS

The microplastics analyzed varied extensively in size beyond the expected size range of 25 to 106 μm up to several millimeters in length despite size fractioning by sieving. Polymer types were largely the same in both the inlet and outlet, however proportions and quantities varied between the two. Polypropylene was the most commonly observed polymer type in the bioretention inlet followed by polyethylene. This predominance in these polymer types matches literature for road dust and stormwater (Monira et al, 2022). Similarly, polypropylene was the most common polymer type in the outlet followed by polyethylene but in this case with a higher outlet count than inlet. As black rubber could be analyzed due to spectroscopic limits, the impact of rubber pavement could not be evaluated. Other polymers observed included mainly polyester, polyester and polyamide as well as small proportions of polyvinyl chloride, polyurethane, acrylonitrile butadiene styrene, pan-acrylic, acrylic, alkyd and in the inlet only, phenoxy resin. Previous results on for larger microplastics greater than 106 microns on the other hand found polyester, polyurethane and polyethylene respectively to be the most prevalent.

Microplastic count data was censored using two non-parametric survival analysis techniques: Kaplan-Meier and regression-on-statistics. Detection limits for these techniques were obtained from field blank counts. Since spectroscopy allowed for simultaneous quantification and polymer type characterization, all results were report directly as microplastics. Between the bioretention inlet and outlet, a median removal rate of 79% was found for microplastic counts down to 25 microns in size. This is a similar removal rate to that previously found of 84% for larger microplastics in the range of 106 to 5000 μm suggesting that physical filtration effectively removes particles down to 25 μm in size.

SIGNIFICANCE

Since microplastics fragment into increasingly small sizes over time and smaller sizes pose increased health risks and likelihood of environmental transfer, methods to capture these increasingly small microplastics is vital. Bioretention cells, as an existing type of green infrastructure intended for stormwater volume management and other water quality parameters, are ideally suited as a filtration technique to prevent the spread of microplastics (>25 µm) into downstream connecting environments.

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