2023 年 57 巻 1 号 p. 28-41
Observations in Oshino Village, Yamanashi Prefecture, Japan were conducted in January and August, 2017, and the water quality characteristics of the shallow and deep groundwater and spring water were elucidated. The water quality of the area’s shallow groundwater indicated the presence of relatively high levels of Ca-HCO3, Mg2+, and SO42– at some sites, and the deep groundwater contained Ca-HCO3, (Ca+Na)-HCO3, (Ca+Mg)-HCO3, and Na-HCO3. In August, the shallow groundwater at some sites was mixed with irrigation and paddy water affected by evaporation and fertilization. In deep groundwater, δ18O and δ2H levels were lower than those in shallow groundwater; therefore, the recharge area of deep groundwater probably increased as a result of elevation. The dissolved matter contents and isotope ratios of Deguchi-ike, which is one of the Oshino-Hakkai springs, were different from those of other Oshino-Hakkai springs, which may have been caused by differences in the recharge elevation. The vanadium and phosphorus concentrations of deep groundwater were relatively higher than those of the shallow groundwater and spring water, which may be ascribed to the influence of the basaltic rock of Mt. Fuji. Additionally, the observation of the groundwater level revealed two regional groundwater flow systems in Oshino Village: one flowed from south to north in the western part of the village, and the other flowed from east to west in the central to eastern part of the village. The former corresponded to a flow from Mt. Fuji to Oshino-Hakkai, and the latter to that from the Doshi Mountains to near Oshino-Hakkai.
In recent year, the global water issues, e.g., water pollution, overconsumption of water, unsafe drinking water, are becoming important. And sustainable water use is one of the important concepts for boosting a country’s economic growth and development. In order to consider the sustainable water use, it is necessary to grasp the water quality, groundwater flow, amount of groundwater storage, and so on. However, these information or data have not been sufficient yet in many regions in Japan.
Oshino-Hakkai, which comprises eight springs, is located at Oshino Village, Yamanashi Prefecture. These springs were registered as one of the constituent elements of the World Cultural Heritage Site in June 26, 2013, as “Fujisan, is a sacred place and source of artistic inspiration.” In Oshino Village, groundwater resources are abundant, in addition to those in Oshino-Hakkai, and groundwater is used as a source of tap water; therefore, groundwater resources play an important role in this region. Oshino Village has enacted the “Oshino Village Groundwater Resource Conservation Ordinance” (Oshino Village, 2011), and is working to conserve its water resources by limiting the use of groundwater and spring water.
Previous studies have reported the water quality and recharge area of the Oshino-Hakkai springs. For example, Yaguchi et al. (2016) discussed the formation process of water quality in one spring in Oshino-Hakkai and nine deep wells at the northern foot of Mt. Fuji through dissolved materials and stable isotopes of oxygen, hydrogen, and sulfur. The results of this study indicated that the water originated from meteoric water and that the weathering of olivine may play an important role in water quality. The origin of SO42– ions is attributed to the dissolution of anhydrite/gypsum formed by the volcanism of Mt. Fuji. Koshimizu and Tomura (2000) found that the vanadium concentration in the Oshino-Hakkai springs was higher than that at other sites because of the geological and geochemical characteristics of these areas. They also found that the vanadium concentration was present in a relatively narrow range, suggesting that vanadium leached out through the simple interaction between groundwater and Mt. Fuji rocks. Uchiyama et al. (2014) estimated the average recharge elevation of Oshino-Hakkai in Oshino Village using δ18O and δ2H, as it is less than 1250 m above sea level; they also indicated the contamination with domestic wasted river water using the concentration of water-soluble inorganic ions and trace elements. Nakamura et al. (2017) suggested that the main sources of springs in the Oshino area was precipitation at the northern foot of Mt. Fuji and water from Lake Yamanaka using δ18O and δ2H, and that the main source of nitrate in river water (R. Katsura) was from residential areas as based on δ15N. Asai and Koshimizu (2019) estimated the residence times of four springs located at the foot of Mt. Fuji using the 3H/3He method and found that the residence time of Sokonashi-ike, which is one of the Oshino-Hakkai springs, was approximately 39 years.
Although there are many springs and shallow/deep wells are present in Oshino Village besides the Oshino-Hakkai springs, they have rarely been studied thus far, and the details of groundwater flow in Oshino Village are unknown. Water quality and groundwater flow are important information for water quality conservation, sustainable use and management of groundwater, and flood control by the local government. The objectives of this study were to clarify the characteristics of water quality, estimate the main source of groundwater using stable isotopes, and understand the groundwater flow in Oshino Village derived from two surveys in 2017, for groundwater conservations in the future.
Oshino Village is located in the southeast Yamanashi Prefecture, with an area of 25.05 km2. The village is surrounded by mountains, including Mt. Fuji (3776 m asl) and others (about 950–1600 m asl) (Fig. 1). The Oshino Basin spreads throughout the central part of the village, whose ground slopes gently to the southwest. Mt. Shakushi and Mt. Ishiwari lie in the eastern part of the village, which corresponding to the western edge of the Doshi Mountains. There are two main rivers in Oshino Village: R. Shinnasho flows from east to west and R. Katsura flows from south to north. The waters of R. Katsura originated from the sluice gate situated northwest of Lake Yamanaka. Considering R. Shinnasho, as the water only flows during rainfall events in the headwater region of the river in Mt. Shakushi and Mt. Ishiwari, the water of R. Shinnasho is mainly derived from Lake Yamanaka and it flows through the channel in the mountains into the river channel.
Study area and sampling points (a. shallow groundwater, b. deep groundwater, artesian well water, and hot spring water, c. spring water and river water).
A geological map created using data from the Geological Survey of Japan, AIST ed. (2015) is shown in Fig. 2. In the mountainous area, the ejecta from a submarine volcano, known as the Misaka Group from the Neogene period, are deposited mainly at 1200–1600 m asl, and the volcanic ejecta of the Old Fuji volcano are deposited at 950–1200 m asl (Oshino Village, 1989). Non-alkaline mafic volcanic rocks in the Holocene were deposited in the southern part of the village, which corresponds to the Takamarubi Lava Flow that erupted during the Enryaku era (800 A.D.) (Takada et al., 2016). Non-marine sediments (alluvium), including gravel, silt, clay, and volcanic lava of Mt. Fuji in the Late Pleistocene to Holocene, are deposited in the central part of the village, and non-marine sediments (volcanic fan II deposits) in the Holocene are deposited in the southwest part of the village. The hydraulic conductivity of soil derived from the Shin-Fuji volcano is approximately 1 × 10–2 cm/s (Ogata et al., 2014).
Geological map of Oshino Village. The points of W-1, W-2, W-3, W-4, O-1, O-2, and O-3 on this figure indicate the geological column site.
In the study area, the average annual mean air temperature and average annual precipitation amount from 1981 to 2010 were 10.6°C and 1568 mm at Kawaguchiko (N35.50°, E138.76°; Japan Meteorological Agency, 2020), respectively, and those of the Oshino elementary school (N35.4609°, E138.8476°) were 10.6°C and 1411 mm, respectively (from April 2016 to March 2017). Monthly precipitation amounts were low in the winter (below 50 mm/month) and high in the summer (sometimes over 300 mm/month).
According to land use, forests are distributed in the mountainous area surrounding Oshino Village, which occupies a large area. The settlement is concentrated in the central area, whereas paddy fields are distributed near the R. Shinasho and R. Katsura, and farmland is sporadically distributed around the settlement and paddy fields (Fig. 3).
Land-use map.
Oshino-Hakkai comprises eight springs: Deguchi-ike (C10), Okama-ike (A01), Sokonashi-ike (C09), Choshi-ike (C04), Waku-ike (C06), Nigori-ike (C05), Kagami-ike (C07), and Shoubu-ike (C08), as shown in Fig. 1 and presented in Table S2. Seven springs of Oshino-Hakkai, except Deguchi-ike, are distributed approximately 450 m upstream from the confluence zone of R. Shinnasho and R. Katsura. Deguchi-ike is located approximately 800 m southeast of the distribution area of the other springs (Fig. 1c). The volcanic lava (basaltic rock) of Mt. Fuji was deposited around the eight springs. The amounts of discharge of Deguchi-ike, Okama-ike, Sokonashi-ike, Choshi-ike, Waku-ike, and Nigori-ike are 0.26, 0.18, 0.15, 0.02, 2.2, and 0.041 m3/sec, respectively; those of Kagami-ike and Shoubu-ike are low and show seasonal variation (Oshino Village, 2021). In Deguchi-ike, the water is discharged from a pore or crack in the lava, and is distributed at the edge of the northern foot of Mt. Fuji. In the other springs, water is discharged from the bottom of the pond or a crack in the lava is deposited around these springs.
Water samples, including groundwater, spring water, and artesian well water, were collected from Oshino Village on January 18–19 and August 8–10, 2017. In this study, groundwater was divided into shallow and deep groundwater based on well depth. At a depth of 10–30 m, there is an aquitard (low-permeability layer) comprising lake sediment (e.g., humus, clay), loam, basaltic lava, basaltic andesite, and other materials; an aquifer is separated by this layer (Fig. S1). Therefore, a well below a depth of 30 m contains unconfined groundwater (shallow groundwater) and that above a depth of 30 m contains confined groundwater (deep groundwater).
Water was sampled at 72 and 96 sites in January and August, respectively (Table S1). Shallow groundwater (unconfined groundwater), which mainly occurred in open wells, was sampled from near the bottom of the wells using a water sampler. Deep groundwater (confined groundwater) and hot spring water were sampled from a faucet attached to a well after continuous water flow for a few minutes. Artesian well water was sampled the effluent water from the well. These wells were used daily. Spring water was sampled from a gushing point, for example, a crack in lava or rocks, as close as possible. Water was sampled directly from the river channel. Lake water was sampled northwest of L. Yamanaka (N35.4269°, E138.8504°) from the water surface. The sampled water was filtered through a syringe filter with a pore size of 0.22 μm (RJP3222NH). Additionally, the electrical conductivity (EC), pH (D-54, HORIBA), water temperature (TL-1, ThermoProbe), and oxidation-reduction potential (PH72, YOKOGAWA) were measured at each site. The groundwater level was measured whenever possible (WL50M, YAMAYO).
Dissolved inorganic materials (F–, Cl–, NO2–, Br–, NO3–, SO42–, PO43–, Li+, Na+, NH4+, K+, Mg2+, and Ca2+) were analyzed using an ion chromatography system (ICS; ICS-3000, Thermo Fisher Scientific, USA); five standards and a blank were used for calibration. The analysis ranges were set from 0.1 to 40 mg/L for Cl–, NO3–, SO42–, PO43–, Na+, K+, Mg2+, and Ca2+; from 0.05 to 5 mg/L for F–, NO2–, Br–, and NH4+; and from 0.05 to 1 mg/L for Li+. The accuracy of the ICS was 2% or better and the precision was 1%. HCO3– concentration was determined through an alkalinity analysis at a pH of 4.8 involving titration. The calculation method is described by Rounds and Wilde (2012). The trace elements (Li, B, Al, Si, P, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, W, Pb, and U) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS; 7500cx, Agilent, USA) after appropriate dilution; six standards from 0.5 ng/L to 500 μg/L made from XSTC-1 and XSTC-622 (SPEX), and a blank were used for the calibrations. 1% (v/v) concentrated HNO3 were added to all samples. The relative standard deviation (RSD) approximately ranged from 1 to 5%, and the recovery ranged from 97 to 104%. The stable isotopes of hydrogen and oxygen were analyzed using cavity ring-down spectroscopy (L2130-i, Picarro, USA). The analytical precision values (1σ) of the L2130-i with liquid specifications were 0.025 and 0.1‰ for δ18O and δ2H, respectively.
The EC was lowest in deep groundwater and highest at the artesian well in both January and August, except for river water and hot spring water, whose values were observed only in August. The pH values of the deep groundwater and artesian well water were higher than those of the shallow groundwater and spring water. When the residence time was long, the pH generally increased because of the elution of sodium and calcium from the soil and the change of carbon dioxide in water to hydrogen carbonate ions. The average water temperature in August was higher than that in January because of the influence of air temperature (Table S2).
The distribution maps of the stiff diagrams for January and August are shown in Figs. 4 and 5, and the trilinear diagrams for January and August are shown in Fig. 6. The relationships between δ18O and δ2H are shown in Fig. 7, along with the global meteoric water line (GMWL; δ2H = 8 × δ18O + 10; Craig, 1961) and local meteoric water line (LMWL) on Mt. Fuji (δ2H = 8 × δ18O + 15.1; Yasuhara et al., 2007).
Distribution map of Stiff diagram in January, 2017 (a. shallow groundwater, b. deep groundwater and artesian well water, c. spring water).
Distribution map of Stiff diagram in August, 2017 (a. shallow groundwater, b. deep groundwater, artesian well water, and hot spring water, c. spring water and river water).
Trilinear diagram of water samples (a. in January 2017, b. in August 2017). Type indicate that Gw-s is shallow groundwater, Gw-d is deep groundwater, Aw is artesian well, Sp is spring water, R is river water, and HSp is hot spring.
Relationship between δ18O and δ2H of water samples. The global meteoric water line (δ2H = 8 × δ18O + 10; Craig, 1961) and local meteoric water line in Mt. Fuji (δ2H = 8 × δ18O + 15.1; Yasuhara et al., 2007) are shown in each figure.
For shallow groundwater, almost all the sites showed relatively high levels of Ca-HCO3, Mg2+, and SO42– (Figs. 4a and 5a), which can be ascribed to the influence of the basalt and tuff of Mt. Fuji. At the sites located near the upper stream of R. Shinnasho, isotope ratios and NO3– concentrations increased in August. For example, the NO3– concentration was approximately 20 mg/L (B01, B02, and B23) and 11–17 mg/L (B06–B08 and B15) in January; however, it increased to over 30 mg/L (B01, B02, and B23) and 20–28 mg/L (B06–B08 and B15) in August. At C02, located in the central part of the Oshino Basin, NO3– increased remarkably from 4.2 mg/L in January to 124.9 mg/L in August. In most of these sites, the SO42– concentration also increased in August. The fertilizer (e.g., ammonium sulfate; (NH4)2SO4) includes nitrogen (N) and sulfur (S); therefore, we suggested that shallow groundwater is affected by the fertilizer. The well depths of these sites were below 10 m, and permeable sediments were deposited here. According to the land-use map, farmland and paddy fields can be found near the upper stream of R. Shinnasho. Agricultural activity occurs from spring to autumn in Oshino Village. The components of fertilizer sprayed on farmland infiltrate the soil with irrigation water or rainfall, thus likely increasing the NO3– concentration of shallow groundwater. Nakamura et al. (2017) studied nitrogen isotope values (δ15N) and indicated that the source of NO3– in Oshino-Hakkai, except in Deguchi-ike, and some sites of R. Katsura were wastewater drainage from residential areas; however, according to our observations, the source of NO3– in shallow groundwater, at least in summer, is largely due to agricultural activity. We suggested that the source of NO3– in this shallow groundwater can be clarified by measuring the nitrogen isotope and interpreting the nitrogen and oxygen isotope ratios of nitrate in shallow groundwater samples in the future. Additionally, the oxygen and hydrogen stable isotope ratios of such shallow groundwater were higher, probably because of the mixing with irrigation water and paddy water, which experienced extensive evaporation on the surface. The isotope ratios of these shallow groundwater samples deviated from the meteoric water line with a slope of 8 (Fig. 7), and the deuterium-excess (d-excess; d-excess = δ2H – 8 × δ18O) was relatively low, suggesting that these shallow groundwater samples were influenced by evaporation. Vanadium and phosphorus concentrations in shallow groundwater show large variations (Fig. 8). They are relatively low near the confluence zone of R. Shinnasho and R. Katsura and the upper stream of R. Shinnasho, and are high in the southwestern part of the Oshino Basin near the foot of Mt. Fuji and the boundary of the Takamarubi Lava Flow (Figs. S2a and S3a). As the concentrations of dissolved components and NO3–, and isotope ratios of oxygen and hydrogen are relatively high in the water at the former location, the vanadium and phosphorus concentrations of shallow groundwater in this area likely decreased during dilution by irrigation water and paddy water. However, at the latter location, the vanadium and phosphorus concentrations of shallow groundwater were relatively high than other areas. Sakai et al. (1997) indicated that a relatively large amount of vanadium was observed in groundwater samples from locations with basaltic soils or rocks, suggesting that the geochemical interactions of groundwater with such soils or rocks could enhance the vanadium concentrations. Kobayashi and Koshimizu (1999) showed that the source of phosphorus in groundwater and spring water around Mt. Fuji was mainly related to geology because basaltic rock has a high phosphorus content. Therefore, the vanadium and phosphorus concentrations in the southwestern part of the Oshino Basin are likely caused by the geology of Mt. Fuji.
Relation between phosphorus and vanadium concentration (a. in January 2017, b. in August 2017).
In deep groundwater, the water quality almost showed the same tendencies in January and August (Fig. 6); however, it differed near the foot of Mt. Fuji and other areas (Figs. 4b and 5b). The site near the foot of Mt. Fuji contained Ca-HCO3 and the concentration of dissolved components was relatively low, whereas those in other areas contained (Ca+Na)-HCO3, (Ca+Mg)-HCO3, and Na-HCO3, and the concentration of dissolved components was relatively high. The water quality including relatively large amount of Na+, Mg2+, and HCO3– of deep groundwater in the latter area may have been caused by differences in the groundwater flow or well depth, which reflected the geology and land use of the recharge area. Yaguchi et al. (2016) showed the water quality of deep groundwater whose depth is approximately 1500 m is enriched in Na+, Cl–, Ca2+, and SO42– due to mixing of meteoric water with seawater. And they suggested that the formation mechanism of water quality in deep groundwater is caused by the dissolution of anhydrite and/or gypsum, plagioclase weathering, calcite precipitation, and the cation exchange reaction of smectite. In our observation site, the plagioclase weathering, and the cation exchange reaction of smectite may have related to the formation mechanism of deep groundwater quality. The vanadium and phosphorus concentrations of deep groundwater were relatively higher than those of shallow groundwater and spring water, and were especially high in municipal water wells (A03–A05), with V and P concentrations of 85–140 and 160–300 μg/L, respectively (Fig. 8). These values are higher than those of groundwater and spring water around Mt. Fuji (vanadium concentration of approximately 50–90 μg/L; Koshimizu et al., 1998; Kobayashi and Koshimizu, 1999). The municipal water wells (at a depth of approximately 100 m and well screens were installed at depths of 80–95 m) are located near the foot part of the northern slope on Mt. Fuji (Figs. S2 and S3). As basaltic rocks have a high vanadium content (Ando et al., 1989; Togashi et al., 1997; Yamamoto et al., 2004), the vanadium concentration in municipal water wells may have increased because of the influence of the basaltic rock of Mt. Fuji.
The isotope ratios of deep groundwater were almost the same in January and August, as was the water quality; however, the averages of δ18O and δ2H values were relatively lower than those of shallow groundwater (Table 1), indicating that the recharge area of deep groundwater was likely higher because of elevation (Yasuhara et al., 2007; Yabusaki, 2020). The δ18O and δ2H values of the deep groundwater were relatively low in the central to eastern part of the Oshino Basin (Figs. S4b and S5b). In these areas, the concentrations of vanadium, phosphorus, and dissolved components were different from those at other sites, which may have been caused by the difference in the recharge area or groundwater flow. One possibility is that if the source of the deep groundwater from the central to eastern part of the Oshino Basin is an eastern mountainous area, the vanadium and phosphorus concentrations in this area are relatively low because the geological conditions are different from those in the western part. The infiltration rate and groundwater recharge processes are also different due to geological conditions; therefore, the ion concentration, δ18O, and δ2H are also different.
| Min. | Max. | Ave. | S.D. | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| All | Gw shallow | Gw deep | Aw | Sp | Rw | HSp | |||||
| EC (mS/m) | Jan | 7.3 | 39.0 | 18.7 | 20.1 | 14.4 | 36.0 | 18.8 | — | — | 6.8 |
| Aug | 6.0 | 44.0 | 14.0 | 16.6 | 9.8 | 28.0 | 12.1 | 8.4 | 14.9 | 6.7 | |
| pH | Jan | 6.7 | 8.7 | 7.6 | 7.5 | 8.0 | 8.5 | 7.7 | — | — | 0.5 |
| Aug | 6.6 | 10.0 | 7.6 | 7.3 | 8.1 | 8.1 | 7.5 | 7.8 | 10.0 | 0.6 | |
| WT (°C) | Jan | 5.6 | 19.0 | 11.1 | 10.8 | 11.6 | 19.0 | 11.9 | — | — | 2.3 |
| Aug | 10.2 | 25.7 | 15.6 | 16.7 | 13.4 | 20.5 | 14.3 | 20.8 | 17.5 | 3.6 | |
| ORP (mV) | Jan | –177.0 | 242.0 | 117.6 | 146.6 | 46.1 | — | 242.0 | — | — | 86.2 |
| Aug | 128.0 | 666.0 | 248.8 | 229.5 | 302.9 | 233.0 | 219.4 | 213.5 | 167.0 | 94.4 | |
| δ18O (‰) | Jan | –11.3 | –7.7 | –9.5 | –9.3 | –10.3 | –11.3 | –9.1 | — | — | 0.7 |
| Aug | –11.4 | –6.5 | –9.6 | –9.4 | –10.0 | –11.4 | –9.1 | –8.0 | –10.3 | 0.7 | |
| δ2H (‰) | Jan | –77.2 | –53.5 | –63.6 | –62.5 | –68.7 | –77.2 | –61.4 | — | — | 4.2 |
| Aug | –77.6 | –47.5 | –63.7 | –63.2 | –65.8 | –77.6 | –60.9 | –54.2 | –68.7 | 4.3 | |
The water quality of the hot spring (A38) was categorized as Na-HCO3 and its pH was as high as 10, which is a common characteristic of a long water residence time. In general, the pH increases with the water-rock interaction due to the dissolution of minerals or the ion-exchange reaction (Shikazono et al., 2010). The water quality of the artesian well (C27) was categorized as (Na+Ca)-SO4, showing high dissolved components without NO3–, and the isotope ratios of oxygen and hydrogen were the lowest in this study area. Additionally, the water temperature was relatively high (19–20.5°C), and the vanadium concentration was low (about 5 μg/L); therefore, the source of this artesian well water is unlikely to be Mt. Fuji. The isotope ratio of the artesian well is positioned on the extension of the meteoric water line on the δ-diagram (Fig. 7); therefore, isotopic fractionation does not occur during the water-rock interactions in hydrothermal systems, and the origin of the artesian well is probably meteoric water. A hot spring well (N35.45945°, E138.80606°) is located approximately 2.5 km west from the artesian well in Fujiyoshida City. The well depth was approximately 1500 m, and the hot spring water was collected from depth of approximately 600 m. Hot spring water was sampled on October 28, 2022, and the ion concentration, trace elements, δ18O, and δ2H were analyzed. The water quality of the hot spring water was categorized as Ca-SO4, showing high dissolved components, low vanadium concentration (about 0.4 μg/L), and the absence of NO3– (Table S2). The isotope ratios were relatively low (δ18O and δ2H were –12.5 and –87.8‰, respectively), and these values are also positioned as extensions of the meteoric water line. The isotope ratios of artesian well water are positioned on the mixing line between those of deep groundwater in Oshino and hot spring water in Fujiyoshida (Fig. 7). Therefore, we considered that the water quality of the artesian well water is formed by mixing with the groundwater and hot spring water. The strontium isotope ratio (87Sr/86Sr) might be useful for the consideration about source of the artesian well water because 87Sr/86Sr reflects the difference of geology; therefore, we will attempt the analyze of 87Sr/86Sr of water samples in future.
The water quality of the spring water indicated the presence of Ca-HCO3 in both January and August (Fig. 6). The vanadium and phosphorus concentrations were almost the same as those in shallow groundwater (Fig. 8). In C14 and C15, located int the central part of the Oshino Basin, the NO3– concentration increased in August, the isotope ratios were higher and d-excess values were lower than those in the Oshino-Hakkai springs (Fig. 7 and Table S2). These facts suggest that the two sites have some relationship with shallow groundwater, which was mixed with recharged water affected by evaporation.
For river water, B24 is in the headwaters and B25 is in the upper stream of R. Shinnasho which was derived from L. Yamanaka; the water quality in this zone indicated the presence of Ca-HCO3, and the dissolved components and vanadium concentration were low (Fig. S3c). The water quality and isotope ratios of B25 were almost identical to those of L. Yamanaka (Fig. 5c and Table S2); hence, these values changed little from the sluice gate of L. Yamanaka to that of B25.
Interaction between groundwater and Oshino-Hakkai springsCompared to those in other Ohino-Hakkai springs, the EC was 4–6 mS/m lower, pH was 0.4 higher, water temperature was approximately 2°C lower, and NO3– was 3–7 mg/L lower in Deguchi-ike. As the Deguchi-ike is located at the end of the northern foot of Mt. Fuji and is discharged from the pores or cracks in volcanic rock, there is no mixing with shallow groundwater, irrigation water, or paddy water; therefore, the NO3– concentration may not increase. Nakamura et al. (2017) indicated that the source of NO3– in Deguchi-ike is the forest, which is consistent with our estimation results. In contrast, the Oshino-Hakkai springs other than Deguchi-ike are located in the western part of the Oshino Basin; therefore, the NO3– concentration and EC values may slightly increase because of the mixing of shallow groundwater or irrigation water of farmland where fertilizer is used.
The vanadium concentration of Deguchi-ike was 20 μg/L higher than that of the other Oshino-Hakkai springs (Figs. S2c, S3c, and Table S2). If these other springs are diluted with shallow groundwater or irrigation water, the phosphorus concentration will also decrease; however, the phosphorus concentration is almost the same at all sites of Oshino-Hakkai; thus, the cause of the difference in vanadium concentration is probably not limited to dilution. Another possible reason is the difference in the geology. The vanadium content may vary in the same basaltic rock depending on the location (Togashi et al., 1997; Yamamoto et al., 2004); therefore, Deguchi-ike is likely recharged in the area where the vanadium content is high.
The δ18O and δ2H values of Deguchi-ike were 0.4 and 3‰ lower than those of other Oshino-Hakkai springs, respectively. In general, the cause of the difference in isotope ratios in groundwater or spring water is, for example, mixing with other waterbodies or differences in recharge elevation. The altitude effect of δ2H near Mt. Fuji has been estimated to range from –1.4 to –0.8‰/100 m (Yasuhara et al., 1997). The elevation of the recharge area in Deguchi-ike is 250–370 m higher than that of the other Oshino-Hakkai springs, as estimated by the isotopic altitude effect. Similar results were reported by Uchiyama et al. (2014). In Fig. 7, δ18O and δ2H of the Oshino-Hakkai springs, except Deguchi-ike, are positioned nearly parallel between the GMWL and LMWL, whose slopes are approximately 8; therefore, this suggests that the evaporation of the Oshino-Hakkai springs is negligible. This result slightly differs from that of Nakamura et al. (2017), which may be caused by the sampling period, sampling frequency, and other factors. However, the Oshino-Hakkai springs, except Deguchi-ike, likely mixed with irrigation water or shallow groundwater from the result of NO3– and EC values (see above); thus, the mixing of water undeniably contributed to the difference in isotope ratios of the Oshino-Hakkai springs. Therefore, future research is required to study the mechanism causing the differences in isotope ratios of the Oshino-Hakkai springs using multiple factors.
Dynamics of the groundwater flow systemThe groundwater flow was determined using the water levels measured from the shallow and deep wells (Fig. 9). The water levels of the shallow wells rose in August; this water level increase was less than 1 m in the western part of the Oshino Basin and 2–3 m in the eastern part. The infiltration amount increased in summer because the precipitation was higher. According to the monthly water balance data shown in Fig. S6, the recharge rate (precipitation amount–potential evapotranspiration amount) was relatively large during the summer. Additionally, irrigation water from farmland and paddy water infiltrated the soil and recharged the groundwater. The increase in water levels in the shallow wells was especially high in the eastern part of the Oshino Basin because of the presence of numerous paddy fields and farmlands in the area. In contrast, the water level in deep wells decreased by 0.2–0.7 m in August; however, the difference in water level between January and August was smaller than that between shallow wells (Figs. 9c, d), which may have been caused by low infiltration from the soil surface.
Map of groundwater level (m asl), and groundwater flow in 2017 (shallow groundwater in January (a) and August (b), deep groundwater in January (c) and August (d)).
The groundwater flow in shallow wells showed the same tendency in January and August, and two groundwater flows were observed. One flowed from south to north in the western part of the Oshino Basin, which corresponded to the flow from Mt. Fuji to the Oshino-Hakkai springs. The other flowed from east to west in the central to eastern part of the basin, which corresponds to the flow from the Doshi Mountains to near Oshino-Hakkai. Furthermore, this flow moved north in the direction of R. Shinnasho in the eastern part of the basin (Figs. 9a, b). As a result of river discharge observations in Oshino Village in August 2019 and January 2020 (Yabusaki et al. in preparation), the discharge of R. Shinnasho increased and the water quality changed near the area where the groundwater flowed into the river. This result indicates that the river water was recharged by shallow groundwater in this area. In the case of deep wells, the groundwater flow could not be determined in detail because of the few observation sites; nevertheless, the groundwater flow showed almost the same tendency as that of shallow groundwater (Figs. 9c, d).
In the shallow groundwater flowing from south to north, a decrease in the NO3– concentration and isotope ratios was associated with the groundwater flow. As a large amount of groundwater or spring water, which flowed from Mt. Fuji, discharge occurred near the confluence zone of R. Shinnasho and R. Katsura, the NO3– and isotope ratios may have changed through the mixing of the discharged water. In the deep groundwater flowing from south to north, the decreases in the isotope ratio and vanadium concentration are associated with groundwater flow. This may have been caused by the deep groundwater flowing from the east with low isotope ratios and vanadium concentrations. In the deep groundwater that flowed from east to west, the water quality and isotope ratios were almost constant with the groundwater flow because the amount of groundwater recharge was small. The above results show that the groundwater flow system was consistent with the change in the water quality and isotope ratios of shallow and deep groundwater in the Oshino Basin.
Observations at Oshino Village revealed, the following hydrological aspects: the water quality of the area’s shallow groundwater indicated the presence of relatively high levels of Ca-HCO3, Mg2+, and SO42– at some sites, and the deep groundwater contained Ca-HCO3, (Ca+Na)-HCO3, (Ca+Mg)-HCO3, and Na-HCO3. In August, for some sites with shallow groundwater, the mixing of irrigation water and paddy water was affected by evaporation and fertilization. The δ18O and δ2H values in deep groundwater were lower than those in shallow groundwater; hence, the recharge elevation of the deep groundwater is likely higher because of the altitude effect. The Oshino-Hakkai springs indicated the presence of Ca-HCO3; however, the EC, pH, water temperature, vanadium concentration, and isotope ratios of Deguchi-ike were different from those of the other springs, which may have been caused by differences in the recharge elevation, namely, the average recharge elevation of Deguchi-ike was higher than that of other springs. The vanadium and phosphorus concentrations of the deep groundwater were relatively higher than those of the shallow groundwater and spring water, which may be ascribed to the influence of the basaltic rock of Mt. Fuji. Observations of the groundwater level revealed two regional groundwater flow systems in Oshino Village. In the case of shallow wells in the western part of the village, groundwater flowed from south to north, which corresponds to a flow from Mt. Fuji to Oshino-Hakkai. In the central to eastern part of the village, groundwater flowed from east to west, which corresponds to a flow from the Doshi Mountains to near Oshino-Hakkai. In the case of deep wells, the groundwater flow almost showed the same tendency as that of shallow groundwater. The comparison between water quality and groundwater flow showed that the groundwater flow system was almost consistent with the changes in the water quality and isotope ratios.
The authors want to express our gratitude the owner of the wells and the inhabitants of Oshino Village, Mr. Amano, a mayor of Oshino Village, and the village office staffs for their cooperation. And we would like to thank the staffs of property management contract division in Fujiyoshida City for providing detailed information about hot spring well. We are also grateful to the anonymous reviewers for their valuable comments and suggestions. This study was funded by the R&D commission of Oshino Village and partly supported by the project (14200076 and 14301001), Research Institute for Humanity and Nature.
