Distribution, Sources and Mass Balance of Polycyclic Aromatic Hydrocarbons in the Seto Inland Sea and Surrounding Area, Japan

概要

Doctoral Thesis

Distribution, Sources and Mass Balance of
Polycyclic Aromatic Hydrocarbons in the Seto
Inland Sea and Surrounding Area, Japan
㸦℩ᡞෆᾏ࿘㎶ᇦ࡟࠾ࡅࡿከ⎔ⰾ㤶᪘Ⅳ໬Ỉ⣲ࡢ
ศᕸࠊⓎ⏕※ࠊ≀㉁཰ᨭ㸧

(Summary)

HIROAKI TSUJI
Department of Environmental Dynamics and Management
Graduate School of Biosphere Science
Hiroshima University
March 2021

In this study, the concentration of polycyclic aromatic hydrocarbons (PAHs) in
marine sediments as well as in the river water and atmosphere of the Seto Inland Sea and
its surrounding area was determined in order to evaluate environmental pollution and
dynamics, elucidate distribution, possible sources and pathways and fluxes of PAHs, and
to calculate their mass balance within the study area.
Chapter 1 describes the background of environmental dynamics analysis of trace
chemical substances, which is the focus of this study, the concept of trace chemical
substances specifically PAHs in the environment, the geography and natural environment
of the Seto Inland Sea, and the purpose of this research. PAHs are trace harmful chemical
substances that are distributed in the environment such as air, water and soil, and show
carcinogenicity, mutagenicity and teratogenicity. The chemical structure of PAHs is
composed of two or more benzene rings containing carbons and hydrogens. PAHs are
produced in the environment mainly by fossil fuel combustion and biomass burning. In
particular, when PAHs flow into the ocean, they tend to migrate and accumulate in
seafloor sediments due to their hydrophobicity, high persistence, and high affinity with
organic matters. Therefore, in the marine environment, PAHs are more abundant in
seafloor sediments than in overlying seawater. The Seto Inland Sea, which is the target
area of this study, is a semi-enclosed sea that is in contact with the open ocean only
through the narrow Kanmon Straits, Bungo Channel and Kii Channel. Thus, the
exchange rate of seawater in the Seto Inland Sea and the open ocean is slower than that
of Tokyo Bay and Ise Bay, and pollutants are not rapidly diluted and diffused into the

open ocean. Therefore, pollutants can easily accumulate in the Seto Inland Sea.
Chapter 2 describes a study on the distribution and source estimation of PAHs
in the coastal sediments in the Seto Inland Sea. The concentrations of 17 PAHs in marine
surface sediments and sediment cores collected in Osaka Bay in June 2015 and in AkiNada, Harima-Nada, Osaka Bay and Kii Channel within the Seto Inland Sea in July 2016
were measured. Sediment core samples from the estuary in Osaka Bay were dated by
measuring 210Pb concentration to determine the historical changes of PAHs concentration
discharged into the marine environment. The total concentration of 17 PAHs (Σ17PAH)
in the surface sediments collected in 2015 and 2016 was 108 ~ 1,590 ng g -1dry weight
(dw) (average = 615 ng g-1dw) and 65.8 ~ 604 ng g-1dw (average = 245 ng g-1dw),
respectively. In Aki-Nada and Osaka Bay, a decreasing PAHs concentration trend
towards the offshore was observed. From the historical changes of PAHs concentration
in the estuary of Osaka Bay from 1965 to 2010, the highest PAHs concentration (2,810
ng g-1dw) was detected around 1978, and then their concentration decreased and remain
unchanged until around 2010. PAHs diagnostic ratios and principal component analysis
of PAHs sources revealed the possible sources of PAHs in surface sediments and
sediment cores collected in 2015 and 2016 to be industrial and traffic activities, and
biomass burning.
Chapter 3 describes a study on PAHs concentration in sediments, river water
and atmosphere in the Seto Inland Sea and its surrounding area, the relationship between
PAHs and organic carbon contents and grain size of sediments, and the risk assessment

of PAHs in sediments. Also, based on the measured values obtained in this study and the
literature, the transport fluxes in the Seto Inland Sea and its surrounding area were
estimated, and the mass balance of PAHs was calculated. 17 PAHs in surface sediments
collected from Suo-Nada, Beppu Bay, Bungo Channel, Hiroshima Bay, Iyo-Nada,
Hiuchi-Nada, Osaka Bay and Kii Channel in the Seto Inland Sea in December 2018,
February–March 2019 and July 2019 were analyzed. Likewise, 17 PAHs in river water
samples collected from the Kurose River and Yodo River in November 2019 were also
analyzed. In addition, 17 PAHs in rainwater samples collected in Higashi-Hiroshima in
December 2020. Furthermore, 12 particulate PAHs in atmospheric samples were
collected once a month for 24 h at 4 or 5 air pollution monitoring stations in Osaka city
for a total of 4 years from April 2011 to March 2014 and from April 2017 to March 2018.
Σ17PAH in the surface sediment was 14.6 ~ 1,160 ng g-1dw (average = 198 ng g-1dw),
which was the highest in Beppu Bay and the lowest in Bungo Channel. Σ17PAH in
suspended river water averaged 176 ng g-1dw in the Kurose River and 42.1 ng g-1dw in
the Yodo River. The particulate Σ12PAH in the atmosphere of Osaka city was 0.38 ~ 7.4
ng m-3 (average = 2.3 ng m-3). Atmospheric particulate PAHs showed seasonal variations
wherein concentration is high in winter and low in summer. From the measurement of
organic carbon in sediments and grain size distribution of sediments, the PAHs showed
high concentrations in sediments when the organic carbon ranged between 2 ~ 5 % wet
weight (ww), silt content 85 ~ 90 %, clay content 10 ~ 15 %, and gravel + sand content
is < 5 %. From the PAHs risk assessment in sediments, only fluorene concentration in

Beppu Bay exceeded the ERL (concentration at which a biological adverse effect occurs
with 10 % probability) set by the sediment quality guidelines, but was still below the
ERM value (biological adverse effect is 50 % or less); all PAHs concentrations were
below the ERL at all sites except Beppu Bay. From the mass balance calculation in the
Seto Inland Sea regions, the flux of PAHs flowing into the Seto Inland Sea through rivers
was 2.3 ± 0.3 ton yr-1, and the flux of PAHs deposited from the atmosphere into the Seto
Inland Sea was 13 ± 1 ton yr-1, and the flux of PAHs deposited on the bottom sediments
of the Seto Inland Sea was 5.5 ± 3.3 ton yr-1. The difference between the total flux of
PAHs input into the Seto Inland Sea and the flux of PAHs deposited on the seafloor (2.3
+ 13 – 5.5 = 9.8 ton yr-1) is considered as the flux that disappears due to discharge to the
open ocean, uptake by marine organisms, photodegradation and/or biodegradation, etc.
It was estimated that the outflow flux through the uptake by bivalves and microbial
degradation were 0.17 and 4.4 ton yr-1, respectively, but other outflow fluxes are
unclarified.
Chapter 4 describes the general discussion, summary, conclusions and
suggestions for future work.