Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 117–123.
© by TERRAPUB, 2011.

Spatial Variation of Submarine Groundwater Discharge (SGD)
in the Central Part of Seto Inland Sea, Japan
Mitsuyo S AITO1, Xinyu G UO1, Shin-ichi O NODERA2, Yuta SHIMIZU2,Yoshiaki KATO2, Minoru TOKUMASU 3 and Hidetaka TAKEOKA1
1Center for Marine Environmental Studies (CMES), Ehime University,Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
2
Graduate School of Integrated Arts and Sciences, Hiroshima University,Kagamiyama 1-7-1, Higashi-Hiroshima 739-8521, Japan
3
Saijyo City Office, Akeyashiki 164, Saijyo 793-8601, Japan(Received 1 November 2010; accepted 12 January 2011)

Abstract—We measured the spatial distribution of salinity and radon-222(222Rn) concentration within seawater in the Hiuchi-Nada, a small bay locatedin central part of the Seto Inland Sea, to clarify the groundwater discharge
distribution in weak stratification period of 2009. The results suggest that theshallow (unconfined) groundwater discharged near the coastline, while deep(confined) groundwater discharged from offshore seabed in southwestern areaof the study area. The average contribution of groundwater to seawater in thestudy area is estimated to be about 2%.
Keywords: submarine groundwater discharge (SGD), spatial variation, Seto
Inland Sea, Radon-222 (222Rn)

INTRODUCTION

Submarine groundwater discharge (SGD) represents all direct discharge ofsubsurface fluids across the land-ocean interface (Taniguchi et al., 2002). Recentstudies have revealed that SGD is one of the important pathways for nutrients andthe other dissolved materials such as carbon and trace metals from terrestrial area
to the marine environment (Moore, 2010). On the global scale, Zektser andLoaiciga (1993) estimated that the amount of dissolved material load by direct
groundwater discharge is more than 50% of that from rivers. Accordingly, theeffect of groundwater cannot be negligible in considering nutrient environmentof coastal area.
Based on the conceptual figure shown by Taniguchi et al. (2002), submarine
fresh groundwater discharge (SFGD) includes shallow (unconfined) groundwaterand deep (confined) groundwater. In additj"b, shallow groundwater generally
discharges near the coast line, while deep groundwater would discharges from theseabed of offshore area because it has higher water pressure than shallow
groundwater. However, in the previous studies, discharge area and its flux of the117

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Fig. 1. Study area (a: location of the study area and b: sampling stations).

deep groundwater in the coastal areas were not well evaluated.
Seto Inland Sea is the largest semi-enclosed coastal sea in Japan. Recently,some researchers tried to evaluate SGD at the specific area of the Seto Inland Sea.
For example, Taniguchi and Iwakawa (2004) evaluated shallow groundwater
discharge in the Osaka Bay using seepage meters. Onodera et al. (2007) examinedshallow groundwater discharge and derived nutrient transport in a small tidal flat
located on an island. Shimizu et al. (2009) estimated the shallow groundwater
discharge along the coast line of Bisan-Seto using topographic model. Nevertheless,deep groundwater discharge was not evaluated in these studies.
The objective of the study is to confirm the presence of SGD including bothof shallow and deep groundwater and its spatial distribution in the central area of
Seto Inland Sea.
STUDY AREAThe Hiuchi-Nada is a semi-enclosed bay in the central part of the Seto Inland

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Fig. 2. Vertical profiles of (a) salinity, (b) temperature and (c) sigma-t measured at all stations.

Sea. The bay has a size of about 50 km × 30 km and an average depth ofapproximately 20 m. Kurushima Strait, a narrow and deep strait (>50 m), islocated at its western side, and Bisan Strait, a wide and shallow strait, at its easternside.
The study area (Fig. 1) is southwestern part of the Hiuchi-Nada and has a sizeof approximately 30 km × 13 km. Based on the horizontal distribution of residual
current in the Hiuchi-Nada (Guo et al., 2004), we know that dominant dub-tidal
current flows from east to west along coastal line. Only some small rivers (KamoRiver, Nakayama River etc.) flow into the study area, whereas the terrestrial area
is characterized by abundant volume of groundwater recharged in the catchmentof mount Ishizuchi, which is the highest mountain (max. 1982 m amsl.) in westernJapan.
METHOD

We conducted the measurement of vertical profiles in salinity, watertemperature and chlorophyll and sampling of sea water at intervals of 5 m fromthe surface to the bottom at 15 stations with depth of from 10 m to 25 m in
September 2009 (Fig. 1). River water samples and terrestrial groundwater
samples were collected around the study area. Radon-222 (222Rn) concentration
was measured at surface and bottom layers of the sampling station using
electronic radon detector (RAD7, Durridge Co.). 222Rn is one of the radioactive
isotopes of radon in the uranium-238 (238U) series. It is unreactive in nature withshort half-life (t1/2 = 3.83 d). Burnett et al. (1996) pointed out that the Radon-222(222Rn) is a useful tracer of SGD because groundwater has extremely high120

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Fig. 3. Salinity distribution in surface (a) and bottom (b) seawater.

concentration in 222 Rn compared with river water and seawater. Therefore,relatively high 222 Rn concentration in seawater suggests groundwater dischargearea.
RESULTS AND DISCUSSION

Spatial distribution of groundwater discharge area
Figure 2 summarized vertical profiles of (a) salinity, (b) water temperatureand (c) sigma-t at all sampling stations. Salinity and sigma-t were relatively low
while temperature was high in the upper layer. These results indicate the presenceof stratification with a pycnocline at depth of about 5 m.
Salinity in the surface layer was relatively low near the coastline, indicating
the influence of river water inflow. Figure 3 shows horizontal distribution ofsalinity in (a) surface and (b) bottom seawater. In the surface layer, low salinity
less than 26 was detected at some stations near the coastline; in the bottom layer,salinity was about 32–33 with a smaller spatial variation than in surface layer.
Figure 4 is the horizontal distribution of 222Rn concentration in (a) surfaceand (b) bottom seawater. Here, 222Rn concentration is expressed as the ratio of theSGD in the Seto Inland Sea

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Fig. 4. 222Rn distribution in surface (a) and bottom (b) seawater.

concentration in each station to the lowest concentration (HN-11). 222Rn
concentration in the surface layer was relatively high along the coast line. This
result suggests the effect of river inflow and shallow (unconfined) groundwater
discharge near the coast line. Meanwhile, in the bottom layer, high 222Rn
concentration was detected in the offshore area at southern part of the study area.
As mentioned above, the observation was conducted in stratification period (Fig.
2). Therefore, the result indicates that deep (confined) groundwater discharged
more offshore area than the shallow groundwater.
The spatial variation of salinity and 222Rn concentration presented almost thesame trend, which was more significant in 222Rn than in salinity. This can beeasily understood from the fact that salinity is almost same between river waterand groundwater, whereas 222Rn concentration is generally higher in groundwater
than in river water (Burnett et al., 1996). For this reason, 222Rn is a good tracer
for separating the effect of river water and groundwater within seawater, as
shown by our measurement results. It must be noted that our measurements
confirmed presence of deep groundwater discharge in the coastal area (Fig. 4b),which correspond well to the concept of SGD given by Taniguchi et al. (2002).

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Fig. 5. Relation between salinity and 222Rn in the seawater, groundwater and river water.

Estimation of groundwater contribution in the seawater
Figure 5 shows the relation between salinity shown as electric conductivity(EC) and 222Rn concentration in the surface and bottom seawater. Surface
seawater is characterized by lower salinity than bottom seawater. Here, wehypothesized that the seawater in the study area is composed of simple binary
mixing between offshore seawater and terrestrial water (river water orgroundwater). The offshore seawater is assumed to be the bottom seawater atstation HN-11 located in the northeastern area (Fig. 1c). According to the
distribution of residual currents in the Hiuchi-Nada given by Guo et al. (2004),
offshore seawater would flow in the study area from eastern part, and flow out
from western part (Fig. 1c). Since the bottom seawater at station HN-11 ischaracterized by the lowest 222Rn concentration and relatively high salinityamong all stations, it should be little influenced by fresh terrestrial water inflow.
The gray color zone in Fig. 5 shows the mixing between offshore seawaterand river water or that between offshore seawater and groundwater. Here, themixing is not given as a single “line”, because the 222Rn concentration interrestrial water has spatial variations among sampling sites (Fig. 1c). The result
suggests that the bottom seawater in the study area would be originated by themixing between offshore seawater and the groundwater, while the surface
seawater would be influenced by river water. The average contribution ofgroundwater is estimated to be about 2% among all sampling stations.

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CONCLUDING REMARKS

To confirm the groundwater discharge area in the Hiuchi-Nada, we havemeasured the spatial distribution of salinity and radon-222 (222Rn) concentrationin the surface and bottom seawater during the stratification period of 2009.
Significantly low salinity in the surface seawater indicates the effect of river
inflow. Spatial distribution in 222Rn concentration suggests that shallow(unconfined) groundwater discharges near the coastline, while deep (confined)
groundwater discharges from the offshore seabed in southwestern area of thestudy area. The average contribution of groundwater to the seawater in the study
area is estimated to be about 2%.
Acknowledgments—This research was supported by the research grant of ResearchInstitute for the Seto Inland Sea. The authors are grateful to Mr. Onishi and students fromCenter for Marine Environmental Studies, Ehime University and Mr. Ikawa from Saijyo
City Office for their help to our field work. Xinyu Guo acknowledges the support from theGlobal COE (Centers of Excellence) Program and the Grants-in-Aid for Scientific
Research (21310012) from JSPS.
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M. Saito (e-mail: misaito@sci.ehime-u.ac.jp)