Environmental Pollution 244 (2019) 958e965

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Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol

Assessment of the sources and inﬂow processes of microplastics in theriver environments of Japan*
Tomoya Kataoka a, *, Yasuo Nihei a, Kouki Kudou a, Hirofumi Hinata bab

Department of Civil Engineering, Faculty of Science and Technology, Tokyo University of Science, Chiba, 278-8510, Japan
Department of Civil and Environmental Engineering, Faculty of Engineering, Ehime University, Ehime, 790-8577, Japana r t i c l e i n f o

a b s t r a c t

Article history:
Received 18 August 2018
Received in revised form
25 October 2018
Accepted 26 October 2018
Available online 29 October 2018

The numerical and mass concentrations of microplastics collected at 36 sites on the surfaces of 29
Japanese rivers were mapped and compared with four basin characteristics (basin area, population
density, and urban and agricultural ratios) and six water quality parameters (pH, biochemical oxygen
demand (BOD), suspended solids (SS), dissolved oxygen (DO), total nitrogen (T-N), and total phosphorus(T-P)) in each river basin. Microplastics were found in 31 of the 36 sites, indicating that some plastics
fragment into small pieces before reaching the ocean. The microplastic concentrations are signiﬁcantly
correlated with urbanisation and population density, indicating that the microplastic concentrations in
the river depend on human activities in the river basin. Furthermore, we found a signiﬁcant relationshipbetween the numerical and mass concentrations and BOD, which is an environmental indicator of riverpollution. This result demonstrates that microplastic pollution in river environments has progressedmore in polluted rivers with poor water quality than in rivers with good water quality, leading to theconclusion that the sources and inﬂow processes of microplastics in river environments are similar to
those of other pollutants. Our ﬁndings can help identify potential sources (i.e., point and non-point
sources) of fragmented microplastics to improve waste management in Japan and model the transport
ﬂuxes of fragmented microplastics in Japanese rivers using water quality parameters and basincharacteristics.
© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:
Microplastics
Water quality
Basin characteristics
Japanese rivers
Statistical analysis

1. Introduction
Very small plastics, ranging in size from 0.3 mm to 5 mm inlength (Kershaw and Rochman, 2016) and commonly referred to as‘microplastics’, have become a major concern in aquatic environments (Andrady, 2011; Cole et al., 2011). Aquatic organisms caneasily ingest microplastics because their size is similar to that of thelarvae of several organisms, including plankton (Besseling et al.,2014; Boerger et al., 2010; Browne et al., 2008; Cole et al., 2013;
Kaposi et al., 2014; Tanaka and Takada, 2016; Thompson, 2004).
Since microplastics contain toxic chemicals from various processes,
such as production and absorption in marine environments(Koelmans et al., 2013; Mato et al., 2001; Nakashima et al., 2016),*

This paper has been recommended for acceptance by Eddy Y. Zeng.
* Corresponding author. Department of Civil Engineering, Faculty of Science andTechnology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510,
Japan.
E-mail address: tkata@rs.tus.ac.jp (T. Kataoka).

aquatic organisms and mammals are exposed to these chemicals
via the ingestion of microplastics. Recently, numerous studies havereported that toxic chemicals are transferred into natural organisms via microplastics (Besseling et al., 2017; Browne et al., 2013;
Koelmans et al., 2016; Koelmans et al., 2013; Tanaka et al., 2013,2015). Eventually, the transition of toxic chemicals originating frommicroplastics into natural organisms results in a chemical hazard;
ultimately, toxic chemicals derived from microplastics can reachhumans through the food web (Rochman et al., 2015; Thompson
et al., 2009; Van Cauwenberghe and Janssen, 2014; Wright and
Kelly, 2017).
Microplastic sources are categorised as primary and secondary
sources (Andrady, 2011; Cole et al., 2011). A primary source refers toparticles that were manufactured with small particle sizes (e.g.,cosmetics and skin scrubbers). Secondary sources are microplastics
produced by the breakdown or fragmentation of larger plastic
items due to exposure to solar ultraviolet radiation, weathering, or
gradual loss of weight due to physical damage. Beaches are themost likely source of secondary microplastics in marine environments (Andrady, 2011; Kataoka and Hinata, 2015; Kataoka et al.,https://doi.org/10.1016/j.envpol.2018.10.111
0269-7491/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

2013a).
Rivers are major pathways by which waste plastics enter oceans(Carr et al., 2016; Jambeck et al., 2015; Lebreton et al., 2017) andtransport
primary
microplastics
releasedfromplastic
manufacturing and other human activities. In addition, rivers canbecome secondary sources of microplastics through two processes.
One process is the fragmentation of plastics on land. Plastic litter onland can be efﬁciently fragmented via processes similar to those onbeaches, such as photooxidative degradation and physical damage
due to human activity (Andrady, 2011). Plastic fragments generated
on land can be released into the marine environment via rainwaterdrainage to rivers. Alternatively, fragmentation of plastics can occur
on riverbanks or ﬂood plains. When the plastic debris released fromland are repeatedly washed ashore on riverbanks and ﬂood plains,depending on the magnitude of the ﬂooding, the degradation of theplastics proceeds based on the residence time out of water(Andrady, 2011; Kershaw and Rochman, 2016). In particular, macrophytes that ﬂourish on the ﬂood plain would signiﬁcantly
contribute to the degradative process because plastics can gettrapped by them. However, the movement of microplastics fromland to the ocean is poorly understood (Eerkes-Medrano et al.,2015).
The scientiﬁc concern for microplastics in rivers has rapidly
increased, resulting in the publication of many microplastic studies
on topics such as monitoring microplastic concentrations (Driset al., 2015a; Mani et al., 2014; Verster et al., 2017; Yonkos et al.,2014), estimating the outﬂow ﬂux of microplastics to the oceans(Lebreton et al., 2017; Nizzetto et al., 2016; Schmidt et al., 2017;
Siegfried et al., 2017), and understanding microplastic sources (Carret al., 2016; Leslie et al., 2013; McCormick et al., 2016; Murphy et al.,2016; Ziajahromi et al., 2016) and their impact on freshwater species (Besseling et al., 2017; Dris et al., 2015b; Hoellein et al., 2017;
Wagner et al., 2014). For example, the microplastic concentrations
along the Rhine River, one of Europe's major rivers that enters theNorth Sea, signiﬁcantly increase going downstream, except in the
tidal zone of the river (Mani et al., 2014). The microplastic concentrations in four estuarine rivers entering the Chesapeake Bay
were found to depend on population density and land use (Yonkos
et al., 2014). These studies have clearly demonstrated that microplastics are concentrated near densely populated areas (Mani et al.,2014; Yonkos et al., 2014). Recently, microplastic transport in rivercatchments has been mathematically modelled using an upgradedcatchment hydrology, soil erosion and sediment budget model(Nizzetto et al., 2016). In another approach, the global input of
microplastics from rivers to oceans was estimated based on waste
management, population density, and hydrological information(Lebreton et al., 2017). These recent studies imply a signiﬁcantrelationship between microplastic concentration and waste management, basin characteristics, and the hydrological regime.
The sources and inﬂow processes of microplastics in rivers need
to be better understood to efﬁciently prevent the release of
microplastics in aquatic environments. For instance, the inﬂow
processes from non-point sources of microplastics, as well as theirpoint sources (Carr et al., 2016; Leslie et al., 2013; McCormick et al.,2016; Murphy et al., 2016; Ziajahromi et al., 2016), should be discussed considering river pollutants. Since microplastics are generated from land according to human activities, similar to riverpollutants, a comparison between the microplastic concentration
and water quality could be used to identify microplastic sources
and inﬂow processes. In addition, the microplastic concentrationsin Japanese rivers have not been reported often. As demonstratedby Isobe et al. (2015) there are signiﬁcant concentrations of
microplastics in the East Asian seas surrounding Japan, suggestingthat this area is a microplastics hotspot with concentrations 27
times greater than those in other oceans (Isobe et al., 2015). Hence,959

estimating the emissions from Japan is important for clarifying thecontributions of Japan to the microplastic concentrations in the
East Asian seas surrounding Japan. Understanding the relationships
between microplastic concentration and basin characteristics andwater quality would allow for more accurate estimates of microplastic emissions from Japan to the surrounding seas via rivers.
Here, we describe the spatial distribution of microplastics at 36
sites on the surfaces of 29 rivers in Japan from August 2015 to May
2018 (Table S1). We discuss the sources and the inﬂow processes
through which microplastics reach the rivers from each source bycomparing the microplastic concentrations with four basin characteristics (basin area, population density, and two ratios of theurban and agriculture areas to the basin area) and six water quality
parameters (pH, biochemical oxygen demand (BOD), suspended
solids (SS), dissolved oxygen (DO), total nitrogen (T-N), and total
phosphorus (T-P)). Our research is the ﬁrst report of the microplastic concentrations on Japanese river surfaces, which may be asource of microplastics for the microplastic hotspot in the East
Asian seas. In addition, our research uses the relationship between
the microplastic concentrations and the basin characteristics in theriver basin by surveying more sites than any previous study. Our
results can be used as a basis in the future to estimate microplastictransport from inland areas and to clarify the contributions of Japanto the microplastic abundance in the East Asian seas.
2. Materials and methods
2.1. Field survey for collecting microplastics
Microplastics were collected at 36 sites on 29 rivers in Japan(Table S1). The target rivers were selected considering river basinsthat include various land uses (i.e., urban and agriculture). The
collection sites were selected to avoid the tidal reach. In the tidal
reach of a river, microplastics can be transported from both the sea
and the upstream area due to tidal movements. Sampling within
the tidal reach should thus be avoided because of the difﬁculty indiscussing the inﬂow processes by which microplastics reach eachriver.
Microplastics were collected on bridges over the rivers according to a survey of water quality (Fig. S1). Although the collection of
microplastics has previously been conducted using ships in rivers
and oceans (Isobe et al., 2015; Mani et al., 2014; Yonkos et al., 2014),
bridge surveys are advantageous because the microplastics can besafely collected regardless of the ﬂow conditions (e.g., ﬂooding). In
rivers, the concentrations of microplastics depend strongly on the
ﬂow conditions. In fact, Kataoka et al. (2013b) reported that theﬂuxes of macro-debris with sizes >20 mm in rivers rapidly increase
under ﬂood conditions and greatly contribute to the annual ﬂuxesof microplastics in rivers. Nonetheless, we investigated the amountof microplastics under normal ﬂow conditions. It is difﬁcult tocollect microplastics at multiple sites simultaneously when collecting samples under ﬂood conditions, and the amount of microplastics depends strongly on the magnitude of the ﬂood.
Consequently, it is difﬁcult to compare the amounts of microplastics among multiple sites or to assess the potential for generating microplastics from a river basin (e.g., the major sources of
microplastics in the basin). In addition, to compare the amounts of
microplastics with the water quality parameters that are regularly
measured under normal ﬂow conditions, we also sampled microplastics under normal ﬂow condition.
A small plankton net (No. 5512-C; RIGO Co. Ltd., Japan), originally designed for sampling plankton, ﬁsh larvae, and ﬁsh eggs near
the sea surface, was used for microplastic sampling (Fig. S1). The
diameter and length of the net were 30 cm and 75 cm, respectively.
The net mesh size was originally selected as 100 mm but was

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T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

changed to 335 mm, which has often been used in microplastic
surveys in the oceans (Isobe et al., 2015), after the survey on 27 July
2016 because the 100-mm net was easily clogged with suspended
materials. The net change did not signiﬁcantly affect the numericalconcentrations evaluated at Noda Bridge on 5 September 2016
(Supplementary Notes). The net was hung from the bridge at thecentre of the stream for 5e30 min, according to how clogged thenet became. When the water volume that passed through the net(hereinafter, ‘ﬁltered water volume’) was low, the microplastic
sampling was repeated three times at the same location to collect aﬁltered water volume. Since the net is not ﬁxed in the river water,
the upper edge of the net was kept on the water surface by carefully
adjusting the length of the rope (Supplementary Video). The
ﬁltered water volume was estimated by multiplying the projectedarea of the mouth of the net (0.25  0.30  0.30  p ¼ 0.0707 m2)
by the ﬂow velocity inside the net, measured using a ﬂow metre(No. 5571-A, RIGO Co. Ltd., Japan) installed at the mouth of the net
or an electromagnetic current metre (AEM1-D; JFE Advantech Co.,
Ltd., Japan) (Supplementary Notes). Notably, the ﬁltered water
volume was potentially overestimated because the net frame was
not fully submerged during sampling. When the height of the upper edge of the net frame that emerged above the water surface
during the sampling was checked using the video (Supplementary
Video), it was found to range from 3 cm to 5 cm. Hence, the ﬁlteredwater volume in the present study may have varied by 5%e10%.
Supplementary video related to this article can be found athttps://doi.org/10.1016/j.envpol.2018.10.111.
2.2. Evaluation of the microplastic concentrations in the laboratory
All of the samples were placed into stainless-steel bottles byrinsing the net during the ﬁeld surveys. Next, the samples werebrought back to the laboratory to separate the microplastics fromother suspended materials. River water was collected in situ to rinse
the net and was ﬁltered using the 100-mm net to prevent
contamination from unexpected plastics in the rinse water. In thelaboratory, all the samples in the stainless-steel bottles wereﬁltered using the 100-mm net. If large suspended materials werecollected, the samples underwent salinity-based density separation
using a saturated sodium chloride solution made with tap water(speciﬁc gravity (SG) z 1.20) to enhance the efﬁciency of the
microplastic identiﬁcation analysis (Supplementary Notes). Then,the settled particles were checked visually. This process can be used
to extract only microplastics with a lower SG than the solution (i.e.,polyethylene(PE;
SG ¼ 0.91e0.94),
polypropylene(PP;
SG ¼ 0.83e0.85), and polystyrene (PS; SG ¼ 1.05); Andrady (2011)).
Microplastics with a higher SG (e.g., polyethylene terephthalate(PET; SG ¼ 1.37) and polyvinyl chloride (PVC; SG ¼ 1.38); Andrady(2011)) are not typically recovered using this process.
The ﬁltered samples were transferred to a glass Petri dish with a
lid and then were dried at 60  C for 24 h. Thereafter, particles thatappeared to be plastics were picked up with stainless-steel tweezers from the ﬁltered samples by visual observation based on theircolour and shape (Hidalgo-Ruz et al., 2012). Each particle was thenprocessed following three steps: First, the size of each particle wasmeasured by analysing a picture of the particle taken by a stereoscopic microscope (SZX7; Olympus Co. Ltd., Japan) installed with a
USB camera (HDCE-20C; AS ONE Co. Ltd., Japan) using imageprocessing software (ImageJ, downloaded from http://imagej.nih.
gov). Second, the mass of each particle was measured using an
XPR Ultra-Microbalance (XPR2UV, Mettler Toledo Co. Ltd., Japan)
with maximum and minimum weights of 2.1 g and 0.1 mg, respectively. Third, the compositions of the particles were identiﬁed using
Fourier transform infrared spectrophotometry (FTIR; FT-IR alpha,
Bruker Optics K.K., Japan or IRAfﬁnity-1S, Shimadzu Co. Ltd., Japan).

For each collection site, the number of particles analysed using FTIR
(i.e., candidates) is listed in Table S1. The ratio of microplastics tototal candidates was 35%. In the FTIR identiﬁcation, we did not
deﬁne a threshold for matching the IR spectra of candidates withthe reference spectra of pure polymers to avoid incorrectly identifying polymers. This method was used because the spectra of thesampled plastics were usually contaminated by unknown materials, such as vegetation and other particles, that adhered to theirsurface. Instead, we identiﬁed the plastics by visually checking thewavenumber (or wavelength) of the IR spectrum peaks of thesampled plastics. The numerical and mass concentrations werethen calculated by dividing the number and mass of the microplastics identiﬁed with FTIR by the ﬁltered water volume.
Contamination with unexpected plastics was prevented by thefollowing measures: Plastic utensils, such as beakers, Petri dishes,and sieves, were avoided when possible, and a white robe made of
100% cotton was worn throughout the sorting and processing
procedure of identifying plastic particles from the suspended materials. In addition, it is important to recognise blanks during thesorting and processing. To estimate the contamination from the air
and tap water, two experiments were conducted. In one experiment, airborne particles were collected by placing glass Petri dishes(f 91 mm) on the tables for sorting and processing for 1 day. In theother experiment, waterborne particles were collected by sampling
tap water in a 10-L glass beaker. In both experiments, no plastics
were found. Nevertheless, two measures were taken during thesorting and processing to prevent contamination as follows: lids
were generally placed on all utensils to prevent air blanks in thelaboratory, and the tap water was used only after it was passed
through a 75 mm ﬁlter.
2.3. Basin characteristics data
We obtained river basin characteristics from the National Land
Numerical Information (NLNI) download service (http://nlftp.mlit.
go.jp/ksj-e/index.html). The NLNI has various data classiﬁed by
the designated regional area, coastal zone, nature, land use, national average, facility, census, and hydrology. The vector data forthe river basin and land use for every square of a 100-m mesh was
downloaded from the NLNI. The basin, urban, and agricultural areasin the upstream region of each microplastic collection site were
calculated using these vector data, and then the proportions of theurban and agricultural areas to the basin area (i.e., urban andagricultural ratios, respectively) were computed. In addition, thepopulations in the upstream region of each microplastic collection
site were calculated using 250-m mesh population data downloaded from a portal site for Japanese Government Statistics (eStat: http://www.e-stat.go.jp/SG1/estat/eStatTopPortalE.do). The
population density was calculated by the ratio of the population in
the upstream region to the basin area. QGIS (version 2.14) was used
to establish the basin characteristics data.
2.4. Water quality data
The water quality is regularly monitored at a frequency of one tothree months in public waters, such as rivers, lakes, groundwater,
and coastal seas, by national and local governments based on the
Basic Environment Law. In the present study, the four indices ofwater quality, pH, BOD, SS, and DO, were compared with the numerical and mass concentrations of the microplastics collected ateach site. In addition to the four indices, T-N and T-P werecompared with the microplastic concentrations because theyreﬂect human activities in a river basin. Here, the annual mean of
the water quality data was used because the water quality parameters were not simultaneously measured in situ during the

T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

microplastics sampling. It is difﬁcult to relate the microplastic
concentration to the water quality because the microplastics andpollutants originate from different human activities, and both themicroplastic concentration and water quality parameters havelarge temporal variabilities. Hence, to remove the temporal variability and characterise the anthropogenic effects in the river basin
for comparison with the microplastic concentration, we used theannual means of the water quality parameter in the same year thatthe microplastics were sampled. Notably, the microplastic concentrations at three sites (Site Nos. 12, 13, and 18; Table S1) werecompared with the averages of the water quality parameters during
the several years when microplastics were sampled. At Kaihei
Bridge (Site No. 17), the annual mean in 2017 was used for thecomparison because the annual mean in 2018 has not yet beenobtained at present. The microplastic concentrations were alsoclassiﬁed based on the class of the nearest water quality station, as
shown in the Supplementary Notes. The numbers of Class AA, A, B,
C, and D sites were 1, 19, 8, 3, and 3, respectively (Table S1). There
was no class E site.
2.5. Statistical analysis
Two types of statistical analyses were applied to investigate therelation of the microplastic concentrations with the basin characteristics (basin area, population density, urban ratio, and agricultural ratio) and water quality parameters (pH, BOD, SS, DO, T-N, and
T-P). First, the microplastic concentrations were regressed with thebasin characteristics and water quality parameters, and the significance of the regression was tested by a t-test at a 95% conﬁdence
level. Second, the microplastic concentrations and basin characteristics at all collection sites were classiﬁed under the water
quality classes (Table S1) and then averaged for the same class. To
test whether the averages of the microplastic concentrations andbasin characteristics are the same in all water quality classes, theKruskal-Wallis test, which is a non-parametric method, wasapplied because the variance of the data under the water quality
class was heterogenous at a 95% conﬁdence level, as shown by the
Bartlett test (Table S2). The null hypothesis in the Kruskal-Wallis
test is that there is no difference in the means. A comparison by
the Kruskal-Wallis test can investigate the signiﬁcant differences
between the water quality classes. R (version 3.3.2) was used tocarry out the statistical analysis.
3. Results

961

Fig. 1. Number (black) and mass (grey) proportions of all polymers collected in all
survey rivers.

concentrations at each site on all collection dates are listed inTable S1. To investigate the seasonal variation of the microplastic
concentration under normal ﬂow conditions, microplastics were
sampled repeatedly at ﬁve collection sites (i.e., Site Nos. 11, 12, 13,
15, and 18; Table S1). For these sites, the standard deviations arepresented in Table S1. As seen from Fig. 2, both the numerical andmass concentrations of the three major polymer materials varyconsiderably among the sites. The averages of these concentrations
are 1.6 pieces m3 and 0.44 mg m3, respectively; these values are
smaller than their standard deviations (2.3 pieces m3 and0.77 mgm3, respectively; Table 1), which indicates that a large
spatiotemporal variability exists in the microplastic concentrations.
The average and median numerical concentrations in the Japaneserivers are on the same order of magnitude as those in the East Asianseas, while the variance of the former was smaller than that of thelatter by one order of magnitude (Table 1). The maximum numerical (mass) concentration of 12 pieces m3 (3.2 mgm3) wasrecorded at Kachi Bridge (Site No. 11) (Kisaki Bridge (Site No. 12)).
Note that the collection site that recorded the maximum numericalconcentration is different from the one with the maximum massconcentration because the mass concentration is dependent on thesize and polymer types of the collected plastic particles. Overall, thenumerical concentration tends to be high in the Kanto region,
which is close to metropolitan areas (Fig. 2c), and high concentrations of microplastics can also be found in the Kyushu region(Fig. 2d). This spatial distribution of microplastics can be seen on
the map of mass concentration (Fig. S2).

3.1. Concentrations of microplastics in Japanese rivers
Three polymer materials with lower SG (i.e., PE, PP, and PS) were
found in the suspended materials from Japanese rivers collectedwith a net. As with the East Asian seas (Isobe et al., 2015), thesematerials were predominant among the polymer materials found at
many sites, with the exception of ﬁve sites (i.e., Site Nos. 2, 16, 26,
27, and 34; Table S1) (80% by number and 72% by mass; Fig. 1).
Various other polymer materials (e.g., acryl, nylon, ethylene-vinyl
acetate, and polyacetylene) were listed as “others”. Thus, we
focused on the three major polymers, and microplastics refer to thethree major polymers throughout this paper, unless otherwise
noted. In addition, we focused on secondary microplastics (i.e.,fragments) because primary microplastics (e.g., resin pellets) were
only collected at the Yoshikoshi Bridge (i.e., Site No. 15; Table S1) on
4 December 2015. The concentrations of plastic pieces smaller than
1.0 mm were higher than those of other sizes of plastic pieces inJapanese rivers, corresponding to those collected from the East
Asian seas (Supplementary Notes).
The numbers of identiﬁed microplastics and mean microplastic

3.2. Relationship with basin characteristic data
To discuss the sources and inﬂow processes through whichmicroplastics reach rivers, both the numerical and mass concentrations were compared with the basin area, population density,urban ratio, and agricultural ratio in the upstream region from thecollection sites (Fig. 3aed). Here, the urban (agricultural) ratio isdeﬁned as the proportion of urban (agriculture) area in the basin
area. Both the numerical and mass concentrations were signiﬁcantly related to the population density and urban ratio at the 95%
conﬁdence level (Fig. 3bec; Table S3). The positive slopes of theregression lines for the population density and urban ratio suggestthat microplastics are abundant both in count and mass in river
basins where the population density and urban ratio are high. A
signiﬁcant relationship between the microplastic concentrations
and the population density within the range of 101 to 102 persons
km2 has also been determined for four estuarine rivers that ﬂow
into the Chesapeake Bay (Yonkos et al., 2014). On the other hand,

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T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

Fig. 2. Maps of the mean numerical concentrations of the three major polymers on all sampling days at each site. Panels (c)e(d) are enlarged maps of the areas shown in squares inpanel (a). The number around each circle indicates the survey site number shown in Table S1. The bold and thin lines denote the river streams and prefectural boundaries,
respectively. The colours of the circles show the concentration according to the scale in the bottom-right of panel (a). (For interpretation of the references to colour in this ﬁgurelegend, the reader is referred to the Web version of this article.)

Table 1
Statistics of microplastic concentrations in all sampled rivers and the East Asian seas.
Statistics

Numerical conc. [pcs m3]

Mass conc. [mg m3]

Numerical conc. in the East Asian seas [pcs m3]aMaximum
Minimum
MedianAverageStandard deviation

1.2  101
0.0
7.9  101
1.6  100
2.3  100

3.2  1000.0
4.9  102
4.4  101
7.7  101

4.91  102
3.00  102
7.40  101
3.74  100
1.04  101aIsobe et al., 2015.

we sampled microplastics in rivers where the population in thebasin is more dense than that studied by Yonkos et al. (2014) (i.e.,
the range of the population density was between 101 and 103
persons km2; Table S1). Hence, our results offer more evidence
that microplastic concentrations strongly depend on urbanisationand the population density.

3.3. Relationship with water quality data
Both microplastic concentrations were compared with the pH,
BOD, SS, DO, T-N, and T-P (as water quality parameters) at 34
collection sites (Fig. 3eej). Note that the water quality parameters
were not obtained at two sites (i.e., Site Nos. 1 and 2; Table S1), and
T-N and T-P were not monitored at two sites (i.e., Site Nos. 25 and36; Table S1). Both the numerical and mass concentrations weresigniﬁcantly related to the BOD, as well as the DO, T-N, and T-P(Fig. 3f, h-j; Table S3). The microplastic concentrations have apositive relationship with BOD, which is interesting because BOD isan environmental indicator of river pollution, while they have anegative relationship with DO.

However, the linear relationship with the water quality parameters may be questionable because of the temporal variabilityof the microplastic concentrations. Thus, to justify the signiﬁcantrelationship of the microplastic concentrations with the water
quality parameters, we analyse the averaged microplastic concentration for each water class by applying the Kruskal-Wallis test.
Microplastic concentrations tended to be relatively high in morepolluted water (Fig. 4). The Kruskal-Wallis test indicates a statistically signiﬁcant difference in the mean numerical and mass concentrations among the water quality classes (Table S2). On the other
hand, the higher the mean population density and mean urban
ratio are, the more polluted the riverine environment tends to be(Fig. S3). These signiﬁcant differences demonstrate that microplastics are abundant on the surface of highly polluted rivers thatﬂow through river basins with high population densities.

4. Discussion and conclusions
Our ﬁndings clearly demonstrate that microplastics alreadyexist in river environments in Japan, indicating that a certain

T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

963

Fig. 3. Correlation of the numerical and mass concentrations with four basin characteristics and six water quality parameters: (a) basin area, (b) population density, (c) urban ratio,(d) agricultural ratio, (e) pH, (f) biochemical oxygen demand (BOD), (g) suspended solids (SS), (h) dissolved oxygen (DO), (i) total nitrogen (T-N) and (j) total phosphorus (T-P). Only
signiﬁcant regression lines are included in this ﬁgure, and the statistical signiﬁcance of the regression lines is shown in Table S3. The legend of the symbols and lines is shown on theright side of panel (j).

Fig. 4. Mean numerical and mass concentrations of each water quality class. The error
bars are the standard deviation of each class. The statistical signiﬁcance of the differences among water quality classes is shown in Table S2.

amount of plastics fragment into small pieces before entering theocean. In particular, the pollution of river environments by microplastics has progressed the most in polluted rivers with poor water
quality (Fig. 4) due to the high pollution density and urbanisation(Fig. S3). It is interesting that the microplastic concentrations haveincreased signiﬁcantly and have a signiﬁcant relationship with the
BOD, DO, T-N, and T-P (Fig. 3) as the water quality decreases. The
large spatiotemporal variation in the microplastic concentrations isdue to the seasonal variation that is dependent on the hysteresis of
ﬂooding and land use in a river basin (Borah and Bera, 2004). The

mean numerical and mass concentrations at Noda Bridge over the
Edo River from April to September (3.8 pieces m3 and0.62 mg m3, respectively) were 3.5 and 2.7 times higher, respectively, than those from October to March (1.1 pieces m3 and0.23 mg m3, respectively). This result indicates that there may beseasonal variations in the microplastic concentrations.
Based on these results, the question that arises naturally is why
the microplastic concentrations are relatively high in polluted river
basins (Fig. 4). One possible reason is that the sources and inﬂow
processes of microplastics are similar to those of pollutants. In
general, pollutants are classiﬁed as having ‘point’ or ‘non-point’
sources. Point sources include domestic outputs, livestock andmanufacturing industries, commerce, and wastewater treatment
plants (WWTPs), and non-point sources include forest, urban, andagricultural areas. For instance, pollutant loads from domestic
wastewater (or urban areas) corresponding to a point source (or anon-point source) are evaluated by their generation rates per unit
person (or per unit urban area) (Borah and Bera, 2004). Hence, thepollutant loads from these point and non-point sources increase
according to the population and urbanisation. The signiﬁcant relationships between the microplastic concentrations and the population density and urban ratio (Fig. 3bec) suggest that the
microplastics ﬂoating on river surfaces could be generated on land
and reach public water through pathways similar to those ofpollutants.
Another question based on the results of this research considers
where the small plastic fragments that ﬂoat on river surfaces comefrom; in other words, what are the sources of secondary microplastics? Domestic wastewater (a point source) drains directly into

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T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

public water without passing through WWTPs when there is nosewer system. When a sewer system is developed, domestic
wastewater drains into public water after most of the debris isremoved at a WWTP (Dris et al., 2015a). Livestock and industrial
wastewater also drain into rivers after most of the debris isremoved at private WWTPs. In this case, the generation of microplastics in public water would depend on the debris ﬁlter size in the
WWTPs. On the other hand, some microplastics ﬂoating on theriver surface may originate from non-point sources. For example, inurban areas, this may include physical damage from human activities such as plastic that is broken into fragments from being
crushed by vehicles. Microplastics were found to be abundant in
the soil under roadside trees in Kawasaki, one of the largest cities inJapan that contributes to the high population density in the basin of
the Tama River (Fig. S4). Microplastics from urban areas are mostlytransported via the rainwater in roadside gutters. Open landﬁll sites
near rivers could also be a candidate source of plastic fragments; forexample, the plastics littered on the landﬁll site may be fragmented
and then released into the river due to rainfall and/or strong wind.
Riverbanks and ﬂoodplains could be other sources; for example,
large quantities of plastic debris including microplastics haveaccumulated on river walls surrounding a tidal ﬂat, such as the riverwall near the mouth of the Ara River (Fig. S4). Plastic debris
deposited onto riverbanks would not be ﬂushed downstream unless the water level reaches the debris. Similar to the residence timeof debris on beaches (Kataoka et al., 2013a, 2015), the residence
time of debris at non-point sources is an important parameter thatinﬂuences the generation of plastic fragments in a river basin.
Finally, the inﬂow processes of microplastics into riversdor how
they reach rivers from their sourcesdalso need to be considered.
These inﬂow processes depend on the development of sewer systems in a given river basin. At the end of March 2017, 78.3% of the
population of Japan had access to sewers (http://www.jswa.jp/rate/); therefore, 21.7% of the population did not use sewer systems. In
areas with undeveloped sewer systems, domestic wastewater isreleased into public water via a septic tank, which is a point source.
Other areas use either combined or separate sewer systems. In a
combined sewer system, a single sewer transports both rainwaterand sewage (i.e., combined wastewater) to a WWTP (Dris et al.,2015a). Relatively large pieces of debris would be ﬁltered out, but
a portion of the small debris, such as microplastics, would not beﬁltered out. Even if 83e95% of the microplastics are removed by
WWTPs (Dris et al., 2015a), 5e17% would still be released.
Furthermore, when the rainwater discharge passing through a
combined sewer exceeds a level three times that of the wastewater
discharge on a day with good weather conditions, the combined
wastewater with microplastics is released directly into rivers as‘combined sewer overﬂow’. On the other hand, a separate sewer
system transports rainwater and sewage to WWTPs in individual
sewers. Only sewage is treated at the WWTPs, regardless of theweather conditions, and rainwater is released directly into therivers or ocean after the large debris is ﬁltered out. Small plastic
fragments generated in urban areas (e.g., microplastics from sediment on the roadside in Kawasaki; Fig. S4) could therefore be
released into the public water with rainwater.
In summary, microplastics are mainly released into rivers fromthe following sources: homes that are not connected to sewer
systems (i.e., point sources), combined sewer overﬂow (i.e., point
and non-point sources) during heavy rainfall events, and rainwaterfrom separate sewer systems (i.e., non-point sources). The signiﬁcant relationships of the microplastic concentration with BOD, DO,
T-N, and T-P (Fig. 3f, and h-j) indicate that areas in which there are
no sewer systems are important sources of microplastics collectedunder normal river conditions. Furthermore, microplastic concentrations tend to increase when there is a large difference in waterlevels, implying that massive quantities of microplastics are
released from non-point sources during ﬂood events due to heavy
rainfall in the river basin (Fig. S5). In the future, we will investigate
the relationship between the microplastic concentration and riverdischarge by sampling microplastics under ﬂood conditions.
Our ﬁnal goal is to evaluate the outﬂow of microplastics frominland Japan into the surrounding seas. Evaluating the outﬂow of
microplastics is essential to reducing the generation of microplastics and mitigating the adverse impacts of microplastics onriver and marine environments. Considering the above assumptions for the sources and inﬂow processes of microplastics in riverenvironments, several issues arise. First, we need to regularlycollect data on the microplastic concentrations, as well as water
quality, in various Japanese rivers. The additional collection of
microplastic concentration data would enable us to more accurately evaluate the outﬂow than that estimated on a global scale(Lebreton et al., 2017). To efﬁciently collect microplastic concentration data, we need to select effective monitoring sites in river
basins. Our results suggest that the microplastic concentrations
should be preferentially monitored at stations with poor water
quality. The strategic monitoring of the microplastics would enable
us to more efﬁciently address this serious environmental problem
in river basins, as well as coastal regions and to detect areas withhigh microplastic concentrations in river basins by sampling
microplastics from the upper reaches to lower reaches along a river.
Second, we need to understand the spatiotemporal behavioursof microplastics in a river. The signiﬁcant difference in microplastic
concentrations according to the water quality classes (Fig. 4) provides a guideline for surveys to understand the microplastic
behaviour. In particular, the responses of microplastics at non-point
sources are similar to those of pollutants under ﬂood conditions.
Flood conditions signiﬁcantly increase the pollutant ﬂux in rivers
because pollutants instantaneously ﬂow out from non-point sources (Borah and Bera, 2004; Kataoka et al., 2013b; Nizzetto et al.,2016; Schmidt et al., 2017). Hence, understanding the ﬂuxes of
microplastics in rivers under ﬂood conditions is essential. Additionally, to evaluate the outﬂow of microplastics under ﬂood conditions, hysteresis must be considered, i.e., when and at what
concentration have microplastics been ﬂushed out due to ﬂoods inthe past? In particular, massive quantities of microplastics will betransported in the ﬁrst ﬂush in ﬂood events. In addition, we need tounderstand the spatial distribution of microplastics in the crosssection of rivers. In the present study, we measured the microplastic concentration on the surfaces of rivers at one or a few points
in a transverse direction. In the transport process by whichmicroplastics ﬂow down rivers, they may be subjected to turbulence due to horizontal and vertical mixing. In the future, thehorizontal and vertical distributions of microplastics in rivers
should be investigated.
Acknowledgements
The authors wish to thank Professor Atsuhiko Isobe, who provided data on the microplastics sampled in the East Asian seas,
Kochi Prefecture, which collected microplastics in Mononobe Riverand Niyodo River, and Chiba Prefecture, which provided private
water quality data. We are grateful to many students from the hydraulics laboratory of the Tokyo University of Science for theirassistance in sampling the microplastics. In particular, we would
like to thank Mr. Fumiya Kitaura, Mr. Hayata Miyamoto, Mr.
Shunsuke Kobayashi, and Ms. Sako Hasegawa for their help incomputing the basin characteristics and/or evaluating the microplastic concentrations. We would also like to thank the anonymous
native speakers working at American Journal Experts for providing
English language editing. This work was supported by JSPS

T. Kataoka et al. / Environmental Pollution 244 (2019) 958e965

KAKENHI Grant Number 17H04937 and the River Fund of the River
Foundation (Grant Number: 2017-5211-016), Japan.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.envpol.2018.10.111.
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