Multi-temporal surveys for microplastic particles enabled by a novel and fast application of SWIR imaging spectroscopy – Study of an urban watercourse traversing the city of Berlin, Germany☆
Graphical abstract
Introduction
Plastic debris and especially microplastic particles (MPPs) defined as smaller than 5 mm have been found to be ubiquitous in marine habitats from pole to pole (Do Sul and Costa, 2014), in the oceanic gyres (Eriksen et al., 2014), in deep-sea sediments (Van Cauwenberghe et al., 2015) and even at shorelines of remote islands (Imhof et al., 2017; Lavers and Bond, 2017). Recent studies show that MPPs are not only accumulated in the marine system but also in freshwater habitats (for reviews see Dris et al., 2015b; Eerkes-Medrano et al., 2015; Duis and Coors, 2016; Reifferscheid et al., 2017) with concentrations in some places even reaching those in the oceanic gyres (Driedger et al., 2015). MPPs were, for example, found in the North American great lakes (Zbyszewski and Corcoran, 2011; Eriksen et al., 2013), the European Lake Garda (Imhof et al., 2013, 2016), Lake Chiusi, Lake Bolsena and in several Swiss lakes (Faure et al., 2012; Faure and Alencastro, 2014; Fischer et al., 2016). Even remote lakes in Mongolia or on the Tibet plateau were shown to be contaminated with MPPs (Free et al., 2014; Zhang et al., 2016). Next to lakes, rivers can be a significant component of the global microplastic life cycle (e.g. McCormick et al., 2014), which can be severely polluted by microplastics they receive from terrestrial sources and themselves create a substantial input to the oceans (e.g. Lebreton et al., 2017; Schmidt et al., 2017). MPPs were, for example, reported in the water surface and/or the sediments of the Danube (Lechner et al., 2014), Rhone (Faure and Alencastro, 2014), Seine (Dris et al., 2015a) and Rhine (Wagner et al., 2014b; Klein et al., 2015; Mani et al., 2015) as well as rivers in the Los Angeles region (Moore et al., 2011), the Chicago region (McCormick et al., 2014) and the St. Lawrence river in Canada (Castañeda et al., 2014).
As it takes several decades to hundreds of years for plastic products to decompose under environmental conditions, MPPs accumulate in aquatic habitats and are fragmented further into microscopic particles (Lambert and Wagner, 2016). Possible detrimental physical effects of plastic debris and MPPs on organisms, such as impairment through entanglement or ingestion are manifold and have been demonstrated for marine organisms of a wide size range (Wright et al., 2013; GESAMP, 2015). Concerning the consequences of microplastics in lakes and streams it is assumed that “all harmful consequences of plastic contamination described in marine systems may operate in rivers and lakes and deserve closer attention“ (Lechner et al., 2014). The possible ingestion was already shown for limnetic organisms at the base of the food web (Imhof et al., 2013). In addition, a high proportion of fish caught in the Seine as well as in Lake Victoria, Tanzania, had ingested MPPs (Sanchez et al., 2014; Biginagwa et al., 2016). Next to physical effects, the chemical toxicity of plastic monomers and leaching additives have raised ecotoxicological concerns (Oehlmann et al., 2009; Wright et al., 2013; Koelmans et al., 2014; Eerkes-Medrano et al., 2015). Due to their hydrophobic nature and their high surface-to-volume-ratio MPPs have also been shown to serve as transport vectors for hydrophobic contaminants (Teuten et al., 2009), but the relative importance of this transport is currently under discussion (Bakir et al., 2016; Koelmans et al., 2016). Similarly, MPPs may transport pathogenic bacteria, toxic algae or invasive species (Zettler et al., 2013; McCormick et al., 2014; Kirstein et al., 2016).
A major issue in order to prioritize starting points of mitigation measures is the classification of possible sources and pathways from/along which MPPs enter the (aquatic) environment. Recent insights on the factors governing the quantity and distribution of MPPs suggest that urban environments may be hotspots of MPP contamination, as MPP concentrations in surface waters appear to be influenced by population densities and industrial activities close to the water body as well as the proximity to urban centers (Wagner et al., 2014a; Driedger et al., 2015; Dris et al., 2015a; Mani et al., 2015). Especially, wastewater treatment plants (WWTPs) have been implicated as potentially important pathways of MPPs to freshwater resources (McCormick et al., 2014; Mani et al., 2015; Mason et al., 2016; Mintenig et al., 2017) although the retention rate of MPPs, depending on the technical properties, may be quite high (Carr et al., 2016; Murphy et al., 2016; Talvitie et al., 2017). Furthermore, MPP counts have been suggested to increase with decreasing water body size, increasing water residence time (Free et al., 2014) and after rainstorms (Lattin et al., 2004; Moore et al., 2011; Faure and Alencastro, 2014).
Although recently a number of studies concerning pathways of MPPs have been published, still several knowledge gaps exist. This is partly due to missing extensive, large-area, multi-temporal studies that are needed to understand spatio-temporal processes such as transport dynamics, but require methods for the rapid processing and analysis of a large number of samples. Though established methods for microplastic identification, such as Fourier-transform infrared (FTIR) and Raman spectroscopy provide highly accurate identification of potential microplastic particles (Hidalgo-Ruz et al., 2012; Löder and Gerdts, 2015), they involve great efforts for the manual or semi-automatic identification process. Larger microplastic particles (>500 μm) can be accurately identified by attenuated total reflection ATR-FTIR spectroscopy (Löder and Gerdts, 2015), but particles need to be operated manually one by one. Smaller particles can be identified using FTIR (<500–20 μm) and Raman microspectroscopy (50–1 μm) (see Fig. S1 in the supplementary material and Ivleva et al., 2017). These methods provide detailed and accurate results with semi-automated or automated workflows (e.g. Primpke et al., 2017), but are unfortunately intensely time-consuming. Even measurement times with the “high-throughput” focal plane array (FPA) based FTIR microspectroscopy (Löder and Gerdts, 2015) range between 9 h for a single analytical filter of 47 mm diameter (Tagg et al., 2015) and 10 h for a filter of 10–11 mm diameter containing MPP (Löder et al., 2015; Mintenig et al., 2017) depending on instrument settings.
In contrast to the above mentioned methods, short-wave infrared (SWIR) (1000–2500 nm) imaging spectroscopy is well established in the recycling industry where it is applied for the quick and automated identification and classification of post-consumer plastic waste (Eisenreich and Rohe, 2006). Spectral analysis algorithms for such data have been developed enabling real-time identification and sorting of plastic types on conveyor belts (Feldhoff et al., 1997; Kulcke et al., 2003; Serranti et al., 2011). Recently, different imaging spectrometers have also been tested with individual MPPs picked from marine water samples but have not yet been used for the analysis of complete samples including all of the particular matter which resists the laboratory purification processes (Karlsson et al., 2016).
The study presented here tested and applied the combination of a novel fast spectroscopic method for the identification of MPPs extracted from environmental samples with a multi-temporal survey concept performed along the Teltow Canal (TC), a major manmade watercourse traversing the city of Berlin. The time-efficient identification process allowed the analysis of 57 samples which were taken during six surveys of MPPs along the TC within 4 months. The retrieved MPP concentrations, size distribution and plastic types allowed for a discussion of MPPs in a spatial and temporal context with respect to (i) the impact of urban areas, (ii) the impact of effluents from three major wastewater treatment plants draining into the watercourse and (iii) the effects of strong precipitation events.
Section snippets
Study area
The Teltow Canal (TC) is a 38.84 km long manmade canal running through the south of Berlin, Germany. It is fed by the river Dahme in the East and flows into the river Havel in the West. The tributary Britz Canal flows into the TC at kilometer 28.3, connecting the TC to the river Spree (Fig. 1). The TC receives rainwater from a drainage area of about 94.3 km2 while its discharge is controlled by locks located at kilometer 8.34 (Fig. 1). Average discharge is 8.63 m³/s with flow velocities around
Correction and final values of all classified filters
As described above, the automatic filter image analysis by the PlaMAPP algorithm was able to detect the majority of the MPP present (95%), and the majority of the detects (75%) were confirmed to be MPPs, even though the test subsample contained the more challenging filters with largest number of particles preferentially. The detailed visual inspection enabled the identification of 5 different error types which had led to misidentifications by PlaMAPP in its developmental stage at hand.
Conclusion
In the present study, a novel imaging spectroscopic method for microplastic particle (MPP) identification was applied to surface water samples filtrated onto glass fiber filters, by scanning these filters with a short-wave infrared imaging spectrometer. In about 20 minutes 10 whole filters of 47 mm diameter could be scanned (measurement speed: 52048 mm2 per hour), currently with a lower limit for a reliable detection of two pixels, i.e. particles with a size of about 560 μm by 280 μm or 450 μm
Acknowledgements
The environmental sampling and sample purification has been performed by Lena Katharina Schmidt with support of the Water and Matter Transport in Landscapes group of the University of Potsdam. The data correction methods for the SWIR HySpex measurements have been developed by Mathias Bochow, together with Christian Rogass and Christian Mielke at GFZ Potsdam, Remote Sensing section. The algorithm for the identification of MPPs, named PlaMAPP, has been mainly developed by Mathias Bochow, together
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This paper has been recommended for acceptance by Maria Cristina Fossi.