Characteristics of the Chemical Composition of Groundwater on Tanegashima, Yakushima, and Nakanoshima Islands, Kagoshima Prefecture, Japan

地下水学会誌第 54 巻第 4 号 191 ～ 206（2012） Technical Report

Characteristics of the chemical composition of groundwater on Tanegashima, Yakushima, and Nakanoshima islands, Kagoshima Prefecture, Japan

Seizen AGATA＊, Maki ISHIKI＊＊, Hideaki SAKIHAMA＊＊, Akira TOKUYAMA＊＊＊, Hiroshi SATAKE＊＊＊＊† and Jing ZHANG＊＊＊＊

Abstract Twenty-six groundwater samples were collected From Tanegashima, Yakushima, and Nakanoshi- ma islands between July 27 and August 2, 2003. The interpretation of chemical analyses suggested that the groundwater on the islands have been affected by sea salt (ss). Chlorine (Cl－) concentrations on Yakushima, which were inversely related to elevation, were considerably lower than those on Na- kanoshima and Tanegashima, reﬂecting higher annual rainfall on Yakushima. However, groundwater samples from Yakushima had a higher ratio of ss components—more than 50% (except one sample). Non-sea salt (nss) components of the groundwater on Tanegashima, Yakushima, and Nakanoshima were found to be affected mostly by the chemical weathering of plagioclase feldspars. The groundwater on Tanegashima was also thought to be affected by ion exchange between clay minerals and Ca2＋. The effects of sulfuric acid and carbonic acid on chemical weathering were nearly equivalent on Na- 2－ kanoshima. Therefore, the average concentrations of nssSO4 and dissolved SiO2 were remarkably higher than those for Yakushima and Tanegashima. The arithmetic mean concentration of nssK＋ on Yakushima was high even though the total concentration of all nss components was low. The nssK＋ probably originates from chemical weathering of K-feldspar in granite. The total concentration of nss components on Yakushima was remarkably lower than that on Tanegashima or Nakanoshima. Low － NO3 -N concentrations in these mountainous and forested islands reﬂect a low load from anthropogenic activities.

Key Words: groundwater chemistry, chemical composition, chemical weathering, nitrate, Tanegashi- ma, Yakushima, Nakanoshima

1. Introduction islands, with population sizes of 35695, 13875, and 183 in 2000, respectively, are located between Ky- Tanegashima, Yakushima, and Nakanoshima ushu and Amamioshima islands in southwestern

＊ Yaeyama High School, 275 Tonoshiro, Ishigaki-shi, Okinawa 907-0004, Japan ＊＊ Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan ＊＊＊ Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishi- hara-cho, Okinawa 903-0213, Japan ＊＊＊＊ Earth and Environmental System, Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gohuku, Toyama-shi, Toyama 930-8555, Japan † the late doctor

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Japan, as shown in Figure 1 (Kagoshima Prefec- ter and the air mass on Yakushima, which is locat- ture, 2001; Toshima Village, 2001). These islands ed on the axis of the Kuroshio Current, are greater are in a subtropical climate region. Tanegashima than those for the other Nansei Islands. Therefore, Island has an area of 454 km2 and a maximum el- climate disturbance (i.e., rainfall) is strongly af- evation of 282 m. Mt. Miyanouradake (1935 m fected by low-pressure systems passing through a.s.l.) on Yakushima Island is the highest mountain the area south of Yakushima (Ishijima and Itokazu, in Kyushu, but many mountains on this small is- 1980; Eguchi, 1984). land (503 km2) are higher than 1500 m. Therefore, Nagafuchi et al. (2001) studied the compo- Yakushima Island has both temperate and subtrop- sition of rime ice on Yakushima. The geochemi- ical climate zones. Nakanoshima Island, a volcanic cal characteristics of acidic stream water and the island, has an area of 34.5 km2. Mt. Otake (979 m effects of acid deposition on the water quality of a.s.l.) is its highest mountain. The annual average mountainous streams were studied on Yakushima temperatures on these islands are 19.6 ℃, 19.2 ℃, by Nakano (2001) and Nagafuchi et al. (2003), re- and 18.9℃, and the annual amounts of rainfall are spectively. Yamanaka et al. (2005) described river 2322 mm, 4359 mm, and 3082 mm (JMA, 1971- and spring water chemistry on Yakushima. These 2000, 2003-2006), respectively. A warm current studies, with respect to the effect of acid deposi- (Kuroshio) in the East China Sea ﬂows northward tion on the stream water of Yakushima Island, re- west of Nakanoshima and passes eastward south of vealed that the acid species are transported from Yakushima and Tanegashima to the Paciﬁc Ocean. the Asian continent. The results show that the pol- Rainfall on Yakushima is much greater than that on lutants that acidify stream water on Yakushima Is- Tanegashima. Differences in temperature between land accelerate chemical weathering, mainly of pla- the surface seawater and the atmosphere and of gioclase. water vapor pressure between the surface seawa- Many chemical studies on meteoric water have been conducted on Yakushima Island, mainly because the island was registered as a UNESCO World Natural Heritage Site in 1993. However, the meteoric waters of Tanegashima and Nakanoshi- ma islands have yet to be studied. In this study, contributions from sea salt and chemical weathering of silicate rocks inferred from chemical data of groundwater will be presented and discussed to clarify the geochemical characteristics of groundwater on Tanegashima, Yakushima, and Na- kanoshima, each of which has different geographi- cal and geological environments—sedimentary rocks, granites, and volcanic rocks, respectively.

2. Geological Features

Tanegashima is elevated from the main sub-

Fig. 1 Locations of Tanegashima, Yakushima, and marine bedrock, which consists of Kumage Group Nakanoshima islands in Kagoshima Prefec- formations including the Sumiyoshi, Fukago, ture. Hamatsuwaki, and Nijyuuban formations (Figure

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2). The formations consist of sandstone, shale, and Tiba (1990) reported that andesitic to dacitic vol- tuff-silt rocks that represent neritic marine depos- canic rocks on Nakanoshima contain plagioclase, its. These rocks are widely distributed around hills orthopyroxene, cliopyroxene, and amphibole. The at about 200 m (a.s.l.) in the northern, central, and island has hot springs in Satomura (Nishiku and southwestern parts of this island. Eroded surfaces Amadomari hot springs) and Funagura (Higashiku of Kumage Group formations are partially covered Hot Spring) (Daishi, 1985). unconformably by Kukinaga Group formations. Unconformity surfaces of Kumage and Kukinaga 3. Experimental Methods Group formations are covered by Masuda Forma- tion sandstone, which is distributed below 120 m Thirt een, eight, and five groundwater samples (a.s.l.) in the southern part of the island. Addition- from Tanegashima, Yakushima, and Nakanoshi- ally, unconformity surfaces of Kumage and Kuki- ma islands, respectively, were collected into 0.5-L naga Group formations and the Masuda Formation polyethylene bottles between July 27 and August are overlaid by the Hase Formation, which consists 2, 2003. Water temperature, pH, and EC were mea- of gravels distributed in the southern part of the is- sured at each sampling point using a glass elec- land (Hayasaka, 1985). A camptonite dike about 20 trode pH meter (HM-10P; Toa Corp.) and an elec- km long exists in the northern part of this island, trical conductivity meter (CM-10P; Toa Corp.). as presented in Figure 2. Minerals in the intrusive After filtration with a 0.45-µm membrane filter, Na＋, ＋ 2＋ 2＋－ 2－－ rock include biotite, alkali amphibole, and plagio- K , Ca , Mg , Cl , SO4 , and NO3 concentra- clase (albite to oligoclase), which is associated with tions were determined in the laboratory using an calcite (Hayasaka, 1985). Yagi et al. (1975) report- ion chromatograph (L-7000; Hitachi Ltd.). The － ed that this intrusive rock is older than the Kukina- HCO3 concentrations were determined by alkalin- ga Group, which includes pebbles and boulders of ity titration (JISC, 1992) and dissolved silica by the the camptonite dike. Tanegashima is cov ered with molybdate yellow method using a spectrophotom- andesitic two-pyroxene volcanic ash (Hamazaki, eter (AE-300; Erma Inc.; JISC, 1992). 1972). The volcanic ash soils consist mainly of hypersthene, augite, basic plagioclase, and an uniden- 4. Results and Discussion tified iron mineral. Clay minerals in the soils include mostly allophane, chlorite, vermiculite, illite, Table 1 presents the chemical composition of and kaolinite, among others. groundwater from Tanegashima, Yakushima, and Figure 3 shows granite in the central part of Nakanoshima islands. Ion charge balances (Σcation Yakushima Island that is surrounded by sedimen- - Σanion)/(Σcation + Σanion) for 23 of 26 samples tary rocks, including siltstones, sandstones, and were in the ± 5% range. The other three samples gravels (Hayasaka, 1985). Major minerals in the (Nos. 2, 3, and 4) on Tanegashima had balances granite are biotite, quartz, K-feldspar, and plagio- ranging from － 5.1% to － 6.3%. The non-sea salt clase feldspar. Argillaceous rock contains biotite, (nss) component concentrations shown in Table 1 muscovite, chlorite, and others (Hayasaka, 1985). were calculated using the following equation (Ishiki On Nakanoshima Island, Otake and Shizaki et al., 2009): volcanic rocks, consisting of either lavas or tuff nssX=X -(X /Cl ) × Cl （1） breccias, are distributed in the northern and the groundwater sea sea groundwater southern halves of the island, respectively (Fig- where X is the concentration of the dissolved spe- ure 4). Both volcanic rocks consist mostly of two- cie and Cl is the chloride concentration. Arithme- pyroxene andesites (Daishi, 1985). Yokoyama and tic mean concentrations for the respective islands

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Fig. 2 Locations of groundwater sampling points on a geological map of Tanegashima Island and hexadiagrams for groundwater samples. Hexadiagrams for non-sea salt (nss) components are also shown by dotted lines. Numbers showing sampling points are the same as in Table 1.

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Fig. 3 Locations of groundwater sampling points on a geological map of Yakushima Island and hexadiagrams for groundwater samples. Hexadiagrams for non-sea salt (nss) components are also shown by dotted lines. Numbers showing sampling points are the same as in Table 1.

－ are also shown in Table 1. Here, nssHCO3 , nss 4. 1 Chemical Features of Groundwater on － NO3 -N, and dissolved nssSiO2 concentrations are Tanegashima excluded from Table 1 because those concentra- Hexadiagrams using data from 13 groundwa- tions derived from seawater are generally extreme- ter samples taken from Tanegashima Island are ly low (Berner and Berner, 1987). Hexadiagrams shown in Figure 2. According to a grouping meth- for groundwaters on these islands are shown in od (Nagai, 1968; Horiuchi, 1992), the groundwater

Figures 2, 3, and 4, respectively, and those for nss was found to be the Na·Ca-HCO3·Cl type at Nos. components are shown in the same ﬁgures by dot- 1 and 3; the Na-HCO3 type at Nos. 2, 4, and 5; the ted lines. Ca-HCO3 type at Nos. 6, 7, and 9; the Na-Cl type at

Nos. 8, 10, 12, and 13; and the Ca·Na-HCO3·SO4·Cl type at No. 11. However, compositions of nss com-

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Fig. 4 Locations of groundwater sampling points on a geological map of Nakanoshima Island and hexadiagrams for groundwater samples. Hexadiagrams for non-sea salt (nss) components are also shown by dotted lines. Numbers showing sampling points are the same as in Table 1. ponents on Tanegashima were found to be the (Kobayashi, 1971). Judging from hexadiagram re-

Ca-HCO3 type, except for Nos. 5, 8, 10, and 11, sults, sea salt is thought to be included in the wa- which belong to either the Na-HCO3, Mg-SO4, Na- ter.

HCO3·SO4, or Ca·Na-HCO3·SO4 types (Figure 2). Figure 5 presents a key diagram (Nagai, 1991) Chloride (Cl－) concentrations were in a range based on relative percentages in milliequivalents from 17.3 to 50.7 mg L－1 with an average of 25.8 for groundwater on Tanegashima, Yakushima, and mg L－1, as shown in Table 1. These values are re- Nakanoshima islands. The mixing line of seawater markably higher than the average Cl－ concentra- with groundwater at No. 7 was shown as a straight tion of river waters throughout Japan, 5.8 mg L－1 line ( ■ － 7) because No. 7 on Tanegashima had

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bored well used as a source of tap water) shows

that it is the Na·Ca-HCO3·Cl type (Figure 2). The －＋ 2＋ 2－ Cl , sea salt (ss) Na , ssMg , and ssSO4 concentrations at this location were the highest on Tanegashima at 50.7, 28.2, 3.4, and 7.1 mg L－1, respectively. The well has a depth of 100 m below ground level and is only 100 m from the coastline. Judging from the high concentrations of seawater components, the groundwater is probably affected by seawater intrusion. The hexadiagram for No. 13 (Arasaki Spring) shows that the groundwater groups into the Na-Cl type (Figure 2). The Cl－, ＋ 2＋ 2－ ssNa , ssMg , and ssSO4 concentrations were second highest on Tanegashima at 39.5, 22.0, 2.6, and 5.5 mg L－1, respectively. This spring is only 20 m from the coastline. Correlations of the concentration of dissolved ＋ 2＋＋ SiO2 with those of nssNa , nssCa , nssK , and ns- sMg2＋ were not positive for the groundwater samples from Tanegashima. Figure 6 shows correla- － 2＋ 2＋＋ tions of HCO3 with nssCa (A), nss(Ca ＋ Na ) (B), and nss(Ca2＋＋ Na＋＋ K＋＋ Mg2＋) (C). － 2＋＋ Correlations of HCO3 with nss(Ca ＋ Na )

Fig. 5 Key diagram using total concentration (mil- concentrations appeared to be positive in the liequivalents) of each component in ground- groundwater (Figure 6B). This indicates that ns- water samples on Tanegashima, Yakushima, sCa2＋ and nssNa＋ are probably enriched by the re- and Nakanoshima islands. The mixing line action of plagioclase feldspars in the volcanic ash of seawater with groundwater at No. 7 is soils and sedimentary rocks with groundwater con- presented as a straight line ( ■ –7). The ion taining CO , as shown by the following equation exchange direction is shown by an arrow. 2 ○ : Tanegashima □ : Yakushima △ : Na- (Stumm and Morgan, 1981; Tamari et al., 1988). kanoshima ■ : Seawater 4 Na0.5Ca0.5Al1.5Si2.5O8 ＋ 6 CO2 ＋ 17 H2O Non-carbonate hardness: Ca•Mg–SO •Cl type 4 andesine Non-carbonate alkali: Na•K–SO4•Cl type ＋ 2＋ → 2 Na ＋ 2 Ca ＋ 4 H SiO Carbonate hardness: Ca•Mg–HCO3 type 4 4 － Carbonate alkali: Na•K–HCO3 type ＋ 6 HCO3 ＋ 3 Al2Si2O5(OH)4 （2）

kaolinite

－ 2＋－ 2＋ the highest HCO3 and Ca concentrations. The The HCO 3 and nssCa concentrations groundwaters in the carbonate hardness area were in a range from 0.16 to 3.03 meq L－1 and were shown to be the farthest from seawater. The from 0.25 to 1.75 meq L－1, respectively, with groundwaters on Tanegashima were plotted along averages of 1.37 and 0.73 meq L－1 (Table 1). － 2＋ this line, which suggests that sea salt affects more Concentrations of HCO3 and nssCa at No. 7 or less the groundwater components. were the second highest overall and the high- The hexadiagram for No. 3 (Tanowaki 1, a est on Tanegashima, 2.85 and 1.75 meq L－1,

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－ HCO3 is shown in Figure 6A. As this figure shows, equivalent concentrations of nssCa2＋－ were deficient compared to those of HCO3 . However, equivalent concentrations of nss(Ca2＋＋－＋ Na ) were only slightly deﬁcient against HCO3 concentrations (Figure 6B). These results suggest that the chemical composition of the groundwater has been affected by ion exchange. Ion exchange between clay minerals and Ca2＋ can be illustrated by the following reaction (Stumm and Morgan, 1970):

2＋＋ 2 NaY ＋ Ca → CaY2 ＋ 2 Na （4）

where Y is the monovalent cation exchanger. The direction of ion exchange is shown in Figure 5. The key diagram illustrates that the chemical composition of No. 5 formed as a result of ion exchange (Agata, 1993). Figure 6C shows that concentra- － tions of HCO3 are mostly equivalent to those of nss(Ca2＋＋ Na＋＋ K＋＋ Mg2＋). Judging from the nss components, the groundwater at No. 11 (a well

in Noma) was found to be the Ca・Na-HCO3・SO4

－ 2＋ type, which plots far from the equivalent line in Fig- Fig. 6 Correlation of HCO3 with nssCa (A), nss(Ca2＋＋ Na＋) (B), and nss(Ca2＋＋ Na＋＋ ure 6C. Hamazaki (1972) reported that the volca- K＋＋ Mg2＋) (C) concentrations in groundwa- nic ash soils of the island contained an unidentiﬁed ter samples from Tanegashima Island. iron mineral, which could be a kind of pyrite. Stud- 2－ ies have suggested that nssSO4 in the groundwa- respectively. This sampling point is located ter might originate from the oxidation of sulfur in at the southernmost extent of the camptonite the volcanic ash soils (Stumm and Morgan, 1970; － 2＋ dike. Therefore, the high HCO3 and nssCa Agata et al., 2001). concentrations in the groundwater are thought to The nssK＋ concentrations positively correlate be related to the reaction between calcium carbon- with nssMg2＋ (r=0.88), except at Nos. 12 and 13. ate in the camptonite dike and CO2 in groundwater, These non-sea salts are probably attributable to as shown by the following equation (Stumm and the chemical weathering of hypersthene, augite, Morgan, 1981): and illite in the volcanic ash soils and sedimentary rocks. CaCO ＋ CO ＋ H O → Ca2＋＋ 2 HCO －（3） 3 2 2 3 Figure 7 shows a histogram of total nss con- 2＋－ T h e n s s C a a n d H C O 3 c o n c e n t r a - centrations. The amounts of nss components at tions and the pH of groundwater in Kumage Nos. 2-7 and 13 (northern part of island) taken Group formations in the northern part of Tane- from Kumage Group formations were larger than gashima were higher than those in the Masu- those at Nos. 8-12 (southern part of island) in da Formation at the southern part of the is- the Masuda Formation area, which suggests that land. The relationship between nssCa2＋ and chemical weathering reactions differ considerably

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(38.8% for a well in Miyanoura). These results suggested that the amounts of nss components were relatively small and that dissolved chemical species were mainly from sea salt. The total concentrations of dissolved chemical species at No. 16 (Oko Spring), including the Cl－ concentration (24.0 mg L－1), were the highest of any on Yakushima Island. This result suggests that the groundwater at No. 16 is strongly affected by sea salt as a result of it being only 100 m from the coastline. The Cl－ concentrations on Yakushima were in the range of 4.2-14.4 mg L－1 except for one sampling point with its highest 24.0 mg L－1 and the average 12.7 mg L－1 (Table 1). Yamanaka et al. (2005) reported that Cl－ concentrations in river waters of Yakushima were mostly lower than 10 mg L－1 (1.50-10.1 mg L－1, and 23.9 mg L－1) and were about one-half of those on Tanegashima Island (avg. 25.8 mg L－1). The annual rainfall on Tane- Fig. 7 Histogram of total concentrations of non- gashima is 2322 mm (JMA, 1971-2000). The annu- sea salt components on Tanegashima, al weighted mean Cl－ concentrations in rainwater Yakushima, and Nakanoshima islands. (A) at Yaku Town (3685 mm of annual rainfall) and Mt. Tanegashima: Southern part, Northern Dichu-dake (1497 m high and 5721 mm of annual part. (B) Yakushima. (C) Nakanoshi- rainfall) on Yakushima were 5.7 and 3.5 mg L－1, re- ma. spectively (Hara, 1992). These Cl－ concentrations between the Kumage Group and the Masuda For- are remarkably lower than those on Ishigaki Island mation in Tanegashima; i.e., the nss components (avg. 24.9 mg L－1 in ground and river waters [Aga- in the northern region might originate from mud- ta, 1994]; annual weighted mean of 10.4 mg L－1 in stone and tuff-silt of the Kumage Group (Tamari et rainwater; 2429 mm of annual rainfall [Agata et al., al., 1988). 2006]). The low Cl－ concentrations reﬂect the cli- It is therefore concluded that the chemical matic characteristics of Yakushima Island, which characteristics of groundwater on Tanegashima has high rainfall-Cl－ concentrations in meteoric wa- are affected mostly by sea salt, chemical weather- ter decrease concomitantly with increasing rainfall ing of plagioclase feldspars, and ion exchange. (Agata et al., 2006). The lowest Cl－ concentrations of 4.2 and 7.2 4. 2 Chemical Features of Groundwater on mg L－1 were from No. 14 (Kigenmeisui) and No. Yakushima 20 (Yakuunsui) at elevations of 1250 m and 600 m, Figure 3 shows data from eight samples (Nos. respectively, which suggests that the Cl－ concen- 14-21) taken from Yakushima. All of the samples trations decrease concomitantly with increasing were found to be the Na-Cl type, as shown by the elevation. The Cl－ concentrations are probably de- hexadiagrams, and the proportion of sea salt com- termined both by elevation and distance from the ponents was greater than 50%, except in No. 18 coast (Tokuyama and Shimoji, 1978).

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As shown in Figure 5, the chemical composi- percentages (meq L－1) in the total nss cation con- tion of groundwater on Yakushima grouped into centrations on Yakushima and Tanegashima were the non-carbonate alkali type and distributed along 28.3% and 7.0%, respectively. －－＋ a line at about 25% HCO3 . The nss components Correlations of HCO3 with both nssK and ＋ 2＋ are the Na-HCO3 type, except for No. 18 which be- nss(K +Mg ) concentrations were positive in the 2＋ longs to the Ca・Na-HCO3・SO４ type, as shown groundwater samples (Figure 9). The nssMg in Figure 3. The results suggest that the chemical concentrations were very low and the average was composition of groundwater on Yakushima is af- 0.043 meq L－1. Those in four samples were lower fected mostly by chemical weathering reactions. than 1.0 mg L－1 (Table 1). These indicate that the The percentages (meq L－1) of nssNa＋ in the nssK＋ and the nssMg2＋ components are probably total nss cation concentrations on Yakushima and attributable to chemical weathering of K-feldspar Tanegashima were 36.8% and 27.9%, respectively. and biotite in the granites (Tamari et al., 1988). The －＋ 2＋＋ Correlations of HCO3 with nssNa and nss(Ca nssK is probably liberated by a reaction between ＋＋ Na ) in the groundwater were positive (Figure K-feldspar and CO2 in groundwater, as shown by 8), which indicates that the nssNa＋ and nssCa2＋ the following equation (Drever, 1982). components are probably attributable to the chemi- 2 KAlSi O ＋ 2 CO ＋ 11 H O cal weathering of sodium-enriched plagiocla se feld- 3 8 2 2 K-feldspar spars in the granites (Tamari et al., 1988). Table 1 → 2 K＋＋ 4 H SiO shows the nssK＋ concentrations in groundwater 4 4 ＋ 2 HCO －＋ Al Si O (OH) （5） samples from Yakushima, Tanegashima, and Na- 3 2 2 5 4 kaolinite kanoshima. The arithmetic mean of nssK＋ con- －＋＋ 2＋ centrations was high despite low concentrations of Correlations of HCO3 with nss(Na +K +Ca total nss components on Yakushima. The nssK＋ +Mg2＋) concentrations were also positive (Figure

－＋－＋ Fig. 8 Correlation of HCO3 with nssNa (A) Fig. 9 Correlation of HCO3 with nssK (A) and and nss(Ca2＋+Na＋) (B) concentrations in nss(K＋+Mg2＋) (B) concentrations in ground- groundwater samples from Yakushima Is- water samples from Yakushima Island. land.

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10). This chemical characteristic of the groundwa- L－1 (Figure 7B). Thus, total nss components on ter on Yakushima reﬂects the chemical weathering Yakushima were remarkably lower than those on of granite. Tanegashima (avg. 3.47 meq L－1). Total concentrations of nss components on Yakushima were 0.15-1.28 meq L－1 with an aver- 4. 3 Chemical Features of Groundwater on Na- age of 0.60 meq L－1. Those in seven samples of kanoshima eight were distributed between 1.0 and 2.0 meq Three groundwater samples (Nos. 22, 23, and 26) among ﬁve from Nakanoshima Island were collected from the southern foot of Mt. Otake in the boundary area between Otake and Shizaki volcanic rocks. Hot springs are also located along the coast- al site (Figure 4). The groundwater of Nos. 22, 23, and 26 were

found to be the Ca·Na·Mg-Cl·HCO3·SO4 type and those of Nos. 24 and 25 were the Ca·Mg·Na- － Cl·HCO3 type. The Cl concentrations were in a range from 19.9 to 31.7 mg L－1 and the average 26.4 mg L－1 (Table 1). These values were higher Fig. 10 Correlation of HCO －with nss(Na＋＋K＋＋ 3 than those on Tanegashima (avg. 25.8 mg L－1) or Ca2＋＋ Mg2＋) concentrations in groundwater －1 samples from Yakushima Island. Yakushima (avg. 12.7 mg L ). The hexadiagrams show that sea salt contributed to the groundwater

Table 1 Chemical compositions of groundwaters on Tanegashima, Yakushima, and Nakanoshima Islands.

― 201 ― 地下水学会誌第 54 巻第 4 号 191 ～ 206（2012） components on Nakanoshima (Figure 4). tions. However, nssK＋ and nssMg2＋ do not. Thus, Judging from the nss components, groundwa- nssK＋ is probably attributable to the reaction be- ter samples at Nos. 24 and 25 grouped into the Ca- tween K-feldspar in the volcanic rocks and CO2 in

HCO3 type (Figure 4). However, No. 22 was found groundwater (eq. 5). 2－ to be the Ca·Mg-HCO3·SO4 type and Nos. 23 and Figure 12 shows that correlations of nssSO4 ＋＋ 2＋＋ 2＋ 26 were the Ca-HCO3·SO4 type. In Figure 5, these with nssK , nssNa , nss(Ca ＋Na ), and nss(Ca groundwaters were plotted in the boundary area ＋ Na＋＋ K＋＋ Mg2＋) were positive (r=0.87, 0.90, between the non-carbonate hardness and the non- 0.95, and 0.90, respectively). Chemical composi- 2－ carbonate alkali areas because of high SO4 con- tions of hot spring waters on Nakanoshima are centrations that are in a range from 0.050 to 1.29 shown in Table 2 (Toshima Village, 1999). The wa- meq L－1 and average 0.56 meq L－1 (Table 1). The ter chemistry of the hot springs is the sulfur type— 2－－1 SO4 concentration at No. 22 (1.29 meq L ) was the sample area smells of sulfur. It is thought that the highest on Nakanoshima. the nssK＋, nssNa＋, nssCa2＋, and nssMg2＋ are orig- Total nss on Nakanoshima (avg. 2.32 meq L－1) inated from the reaction between sulfuric acid and was higher than that on Yakushima (avg. 0.60 meq minerals in silicate rocks (e.g., K-feldspar, pyrox- L－1) but lower than that on Tanegashima (avg. 3.47 meq L－1). 2＋ Correlations of dissolved SiO2 with nss(Ca ＋ Mg2＋) concentrations were positive in the groundwater (Figure 11A). Correlations between concentrations of nssCa2＋ and nssMg2＋ also were positive (r=0.87), which suggests that the nssCa2 ＋ and ns- sMg2＋ components are derived from the reaction between pyroxenes in the two-pyroxene andesites and CO2 in groundwater, as shown in the following equation (Berner and Berner, 1987):

CaMgSi2O6 ＋ 4 CO2 ＋ 6 H2O pyroxene 2＋ 2＋－ → Ca ＋ Mg ＋ 2 H4SiO4 ＋ 4 HCO3 （6）

The percentage (meq L－1) of nssNa＋ in the total nss cation concentration is 14.3% on Nakanoshi- ma, which was lower than that on Tanegashima (27.9%) and Yakushima (36.8%). The nssNa＋ and nssCa2＋ components are thought to come mainly from the reaction between plagioclase in the andesites and CO2 in groundwater, as shown in equation 2, because the nssCa2＋ concentration was much greater than the nssMg2＋ concentration and the 2＋＋ correlation of dissolved SiO2 with nss(Ca ＋ Na ) 2＋ Fig. 11 Correlation of dissolved SiO2 with nss(Ca was positive (Figure 11B). +Mg2＋) (A), nss(Ca2＋+Na＋) (B), and nssK＋＋ Figure 11C shows that nssK concentrations (C) concentrations in groundwater samples correlate positively with dissolved SiO2 concentra- from Nakanoshima Island.

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enes, plagioclase). The sulfuric acid is probably attributable to oxidation of sulfur in the hot springs, as the following equation shows (Iwasaki, 1976):

3S ＋ 3H2O → H2SO4 ＋ 2H2S （7）

2－ The nssSO4 concentrations, with an average of 0.560 meq L－1, were remarkably higher than those of Yakushima (avg. 0.082 meq L－1) and Tanegashima (avg. 0.257 meq L－1) (Table 1). It is thought that both sulfuric acid and carbonic acid in the groundwater contribute to the chemical weathering of silicate rocks. Therefore, the dissolved

SiO2 concentration in the groundwater on Na- kanoshima (0.980 m mol L－1) was also remarkably higher than that on Yakushima (0.206 m mol L－1) and Tanegashima (0.348 m mol L－1). The effects on chemical weathering caused by sulfuric acid and carbonic acid on Nakanoshima were estimated 2－ to be nearly equal because the nssSO4 concentrations (avg. 0.560 meq L－1) were nearly equal to the －－1 HCO3 concentrations (avg. 0.595 meq L ).

4. 4 Nitrate–nitrogen Concentration － The NO3 -N concentrations in groundwater on Tanegashima, Yakushima, and Nakanoshima were in ranges from 0.0 to 6.5 mg L－1 (avg. 2.5 mg L－1), 0.0 to 3.0 mg L－1 (avg. 0.9 mg L－1), and 0.0 to 0.4 －1 －1 － mg L (avg. 0.1 mg L ), respectively. The NO3 -N concentratio ns of Nos. 11 and 12 (located in a residential and farm area, respectively) were high. 2－＋ Fig. 12 Correlation of nssSO4 with nssK (A), － The concentration of NO3 -N is a severe prob- nssNa＋ (B), nss(Ca2＋+Na＋) (C), and lem for Okinoerabu and Miyako islands of the Nan- nss(Ca2＋+Na＋+K＋+Mg2＋) (D) concentrations in groundwater samples from Na- sei Islands (Tashiro and Taniyama, 1995; Agata et kanoshima Island. al., 2003). Tashiro and Taniyama (1995) reported －－1 NO3 -N concentrations of 4.5-13.2 mg L (avg.

Table 2 Chemical compositions of hot spring waters on Nakanoshima Island.

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8.7 mg L－1) in Kunigami area of Wadomari Town tion, decreasing with increasing elevation of the on Okinoerabu Island. The proportion of cultivat- sampling location. All of the Yakushima groundwa- ed land in the area is very high (81%) mainly be- ter samples were the Na-Cl type, which suggests cause of intensive ﬂoriculture. According to Agata that the chemical compositions have been strong- － et al. (2003), differences in NO3 -N concentrations ly affected by sea salt. The proportion of sea salt among residential areas, farm villages, farms, and components here was higher than 50%, except for forest areas are distinct in groundwater on Miyako sample No. 18 (a well in Miyanoura). － Island. The NO3 -N concentrations in tap water The nss components of the groundwater on sources of farm and forest areas in Shirakawada Tanegashima and Nakanoshima, that are the Ca- －1 area are 9.28 and 6.16 mg L , respectively, and HCO3 type, are probably formed by chemical those in residential areas are also high. The differ- weathering of plagioclase feldspars in the volcanic ence in concentrations must be attributable to an- ash soils, sedimentary rocks, and volcanic rocks. thropogenic activities, such as overuse of fertiliz- However, for No. 7, Ca-HCO3 groundwater compo- ers. nents are probably formed by reactions between

The amounts of cultivated land on Tanegashi- calcite in the camptonite dike and CO2 in ground- ma and Yakushima islands are 19.5% and 1.9%, re- water. The Na-HCO3 type of groundwater is prob- spectively (KAFS, 2006-2007). Almost all the land ably caused by a contribution from ion exchange on Nakanoshima is covered with mountains and between clay minerals and Ca2＋. forests, so only a small area is available for farm- The nss components of groundwater on Na- － ing. Therefore, the low NO3 -N concentrations are kanoshima of the Ca-HCO3·SO4 type might be thought to be a result of low anthropogenic nitro- formed by reactions between plagioclase feldspars gen load. and sulfuric acid and carbonic acid in the ground- 2－ water. The nssSO4 concentrations (avg. 0.560 5. Conclusions meq L－1) were remarkably higher than those on Yakushima (avg. 0.082 meq L－1) and Tanegashima Chemical characteristics of groundwater on (avg. 0.257 meq L－1). Both sulfuric and carbonic ac- Tanegashima, Yakushima, and Nakanoshima is- ids in the groundwater were thought to contribute lands, which are located between the East China to the chemical weathering of silicate rocks. There-

Sea and Paciﬁc Ocean, were investigated. Ground- fore, the dissolved SiO2 concentrations in groundwater components reflect the geological and geo- water on Nakanoshima (avg. 0.980 m mol L－1) graphical environments and also anthropogenic ac- were also higher than those in groundwater on tivities, as summarized below. Yakushima (avg. 0.206 m mol L－1) and Tanegashi- All of the groundwater on Tanegashima, ma (avg. 0.348 m mol L－1). The effects of sulfuric Yakushima, and Nakanoshima might be affected acid and carbonic acid on chemical weathering on by sea salt. The averages of chloride (Cl－) concen- Nakanoshima were nearly equivalent because the －1 2－－1 trations were 25.8, 11.7, and 26.4 mg L , respec- concentration of nssSO4 (avg. 0.560 meq L ) －－ tively. The Cl concentrations for Tanegashima and was nearly equal to the concentration of HCO3 Nakanoshima were remarkably higher than the (avg. 0.595 meq L－1). The nss components of average for river waters throughout Japan, 5.8 mg the groundwater on Nakanoshima of the Ca·Mg- －1 － L . Concentrations of Cl were especially low on HCO3·SO4 type might be formed by reactions of Yakushima, which reﬂects the high annual rainfall plagioclase and pyroxene with sulfuric and carbon- (3685 mm) of the island＇s climate. The Cl－ concen- ic acids in the groundwater. trations are also thought to be affected by eleva- The nss components of groundwater on

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Yakushima were the Na-HCO3 type, except for one ters of some Miyako Islands in Okinawa. Journal of sampling point (No. 18). The nssNa＋ and nssCa2＋ Groundwater Hydrology, 45(2), 115-132 (in Japanese might come from the chemical weathering of sodi- with English abstract). um-rich plagioclase in the granite. The nssK＋ and Agata S., M. Kumada and H. Satake (2006): Characteris- nssMg2＋ components probably also are originated tics of hydrogen and oxygen isotopic compositions and from the chemical weathering of the granite. Al- chemistry of precipitation on Ishigaki Island in Okina- though the total concentrations of all nss compo- wa, Japan. Chikyukagaku, 40(3), 111-123 (in Japanese nents were low, the arithmetic mean of nssK＋ was with English abstract). high. The total concentration of nss components Berner E.K. and R.A. Berner (1987): The Global Water on Yakushima (avg. 0.60 meq L－1) was remarkably Cycle. Prentice Hall, 155p, 212p. lower than that on Nakanoshima (avg. 2.32 meq L－1) Daishi M., Y. Kato and M. Matsumoto (1985): Geology of and Tanegashima (avg. 3.47 meq L－1). Tokara volcanic archipelago. In Kizaki K. (ed.) Geolog- － The low NO3 -N concentrations on these is- ical Records of Ryukyu Archipelago. Okinawa Times, lands reflect a low load of anthropogenic nitrogen. 35-49 (in Japanese). Drever J.I. (1982): The Geochemistry of Natural Waters. Acknowledgments Prentice Hall, 98p. Eguchi T. (1984): Climate of Yakushima Island, espe- We thank Mr. K. Sameshima, Mr. K. Ogata, cially regionality of precipitation distribution. In Con- Mr. K. Takesako, and Mr. E. Iwashita for collect- servation Reports of the Yakushima Wilderness Area, ing samples in Nishinoomote City; Mr. S. Nag ata Kyushu, Japan. Nature Conservation Bureau, Environ- and Mr. T. Hamawaki for collecting samples in Na- ment Agency, Japan, 3-26 (in Japanese with English katane Town on Tanegashima Island; and Mr. H. abstract). Terada and Mr. K. Hamasaki for collecting samples Hamazaki T. (1972): Some problems on volcanic ash soils in Kamiyaku Town on Yakushima Island. Finally, in Tanegashima Island, Kagoshima Prefecture. Pedolo- we are grateful to three anonymous reviewers for gist, 16(2), 78-91 (in Japanese). valuable and critical comments to improve this pa- Hara H. (1992): Precipitation. Chemistry of Terrestrial per. water. Kikan Kagaku Sosetsu, Japan Scientific Societ- ies Press, 69-78 (in Japanese). References Hayasaka S. (1985a): Geology of Tanegashima and Um- age Island. In Kizaki K. (ed.) Geological Records of Agata S. (1993): A geochemical study of the ground wa- Ryukyu Archipelago. Okinawa Times, 19-27 (in Japa- ters on Hateruma Island in Okinawa Prefecture. Kogyo nese). Yosui (Industrial Water), 418(7), 15-34 (in Japanese). Hayasaka S. (1985b): Geology of Yakushima. In Kizaki K. Agata S. (1994): A geochemical study of the river and (ed.) Geological Records of Ryukyu Archipelago. Oki- spring waters on Ishigaki Island in Okinawa Prefec- nawa Times, 29-34 (in Japanese). ture. Kogyo Yosui (Industrial Water), 434(11), 35-46 Horiuchi S. (1992): Ground water. In Chemical Society of (in Japanese). Japan (ed.) Chemistry of Terrestrial Water. Japan Sci- Agata S., H. Satake and A. Tokuyama (2001): Chemical entific Societies Press, 79-89 (in Japanese). characteristic and isotopic compositions of spring and Ishijima S. and M. Itokazu (1980): Change of the four sea- river waters of Okinawa Island. Chikyukagaku, 45(1), sons and change of climate. In Kizaki K (ed.) Ryukyu 27-41 (in Japanese with English abstract). no shizenshi. Tsukiji Shokan, 60-83 (in Japanese). Agata S, T. Miyazato, M. Ishiki and A. Tokuyama (2003): Ishiki M., H. Sakihama, S. Agata and A. Tokuyama (2009): Geochemical study of the ground and surface wa- Chemistry of the Urauchi River Water in Iriomote Is-

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land National Park, Okinawa, Japan. Water, Air, and (2001): Geochemical characteristics of acidic stream Soil Pollution, 198, 111-124. water on Yakushima Island, a world natural heritage Iwasaki I. (1976): Chemistry of hot springs -Introduction site. Water, Air, and Soil Pollution, 130, 869-874. of ingredients in hot springs-. Journal of Hot Spring Stumm W. and J.J. Morgan (1970): Aquatic Chemistry. Sciences, 27(2,3), 42-69 (in Japanese). Wiley and Sons, 488-490, 540-542. Japan Meteorological Agency (JMA): Climate statistics Stumm W. and J.J. Morgan (1981): Aquatic Chemistry. from 1971-2000 and 2003-2006. http://www.data.jma. Wiley and Sons, 530p, 539p. go.jp/odb/stats/etrn/index.php. (Accessed 15 July Tamari Y., H. Tsuji and Y. Kusaka (1988): Relation be- 2008). tween land-water quality and its surrounding geology: Japanese Industrial Standards Committee (JISC, 1992): analysis from leaching experiments of rocks. Chikyu- Testing method for industrial matter, 35p, 191p (in Jap- kagaku, 22(2), 139-147 (in Japanese with English ab- anese). stract). Kagoshima Agriculture and Forestry Society (KAFS, Tashiro Y. and T. Taniyama (1995): Intensive farming 2006-2007): Statistics of Agriculture, Forestry and and changes in nitrate concentrations of groundwater Fisheries (in Japanese). on Okinoerabu Island, a typical subtropical island cov- Kagoshima Prefecture (2001): Statistics of the Population ered with coral limestone. Japanese Journal of Tropi- Census in 2000 (in Japanese). cal Agriculture, 39(2), 82-88 (in Japanese with English Kobayashi J. (1971): Chemical composition of Japanese abstract). river water. Healthiness check of waters. Iwanami Sho- Tokuyama A. and H. Shimoji (1978): Chemical study of ten, 1-11 (in appendix, in Japanese). river water and ﬂux in northern Okinawa Hontoo. Bul- Nagafuchi O., H. Mukai and M. Koga (2001): Black acid- letin of Science and Engineering Division University of ic rime ice on the remote island of Yakushima, a World the Ryukyus (Mathematics and Nature Sciences), 26, Natural Heritage area. Water, Air, and Soil Pollution, 31-52 (in Japanese). 130, 1565- 1570. Toshima Village (1999): Chemical Composition of the Nagafuchi O., S. Akune, K. Yoshimura, A. Kume, S. Abise Hot Springs in Nakanoshima (in Japanese). and K. Tetsuka (2003): Effect of acid deposition on the Toshima Village (2001): Toshima Data File of 2000. Ka- water qu ality formation of mountainous streams in the goshima Prefecture, (in Japanese). western part of Yakushima Island, a World Natural Yagi K., H. Ishikawa and M. Kojima (1975): Petrology of Heritage site. Journal of Japan Society on Water Envi- a lamprophyre sheet in Tanegashima Island, Kagoshi- ronment, 26(3), 159-166 (in Japanese with English ab- ma Prefecture, Japan. Journal of Japanese Association stract). of Mineralogists, Petrologists and Economic Geolo- Nagai S. (1968): Interpretation of the ground-water sys- gists, 70, 213-224. tem by water quality. Kogyo Yosui (Industrial Water), Yamanaka M., M. Okumura, T. Nakano and Y. Shimano 114(3), 66-75 (in Japanese). (2005): Valuable water springs in Yakushima Island. Nagai S. (1991): Hydrochemical approach of groundwa- Visit to valuable water springs (69). Journal of Ground- ter contamination - ﬁelds and the problems of inorgan- water Hydrology, 47(2), 253-262 (in Japanese). ic contamination -. Journal of Groundwater Hydrology, Yokoyama K. and T. Tiba (1990): Petrological study of 33(3), 145-154 (in Japanese). the volcanic rocks from the Tokara Island. Memoirs of Nakano T., M. Okumura, M. Yamanaka and K. Satake the National Science Museum, 23, 7-14.

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