地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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126 巻 , 1 号
小特集:富士山の地下水
選択された号の論文の8件中1~8を表示しています
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論説
  • 菅沼 悠介, 川又 基人, 白水 薫, 小山 拓志, 土井 浩一郎, 金田 平太郎, 青山 雄一, 早河 秀章, 小花和 宏之
    126 巻 (2017) 1 号 p. 1-24
    公開日: 2017/04/20
    ジャーナル フリー

     Unmanned aerial vehicle-based “Structure from Motion” (UAV-SfM) photogrammetry is becoming increasingly important for obtaining high-definition topographic data in a variety of earth science research. Antarctica is one of the best fields in which UAV-SfM surveys are applied, because detailed geomorphological data are essential for reconstructing past Antarctic ice sheet changes and understanding landform evolution processes in a hyper-arid and hypothermal environment. However, application of UAVs in Antarctica has been limited because of difficulties arising from low temperatures and the restricted availability of the Global Navigation Satellite System (GNSS)-assisted navigation system at high latitudes. In this article, we provide methodological solutions for these difficulties, and report several preliminary results of UAV surveys in central Dronning Maud Land and the Soya Coast in East Antarctica. A digital elevation model (DEM) obtained in central Dronning Maud Land clearly shows 3D structures of polygons developed on glacial tills. At the Soya Coast, a DEM analysis reveals detailed geomorphological characteristics, such as moraine ridges originating from a former ice stream and differential erosion of the basement due to weathering. Based on these results, we suggest that the UAV survey has certain merits for conducting an efficient field survey in the extremely large areas of Antarctica within a limited time, and has great potential for reconstructing past Antarctic ice sheet changes and obtaining a further understanding of landform evolution processes.

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小特集:富士山の地下水
総説
  • 土 隆一
    126 巻 (2017) 1 号 p. 33-42
    公開日: 2017/04/20
    ジャーナル フリー

     Mt. Fuji, at 3,776m, is a stratovolcano located on the Pacific coast of central Japan. It started to form at ca. 100,000 yr. B.P. and continued eruptions ejected huge quantities of basaltic lavas and volcanic ashes. Its activities are divided into two stages: before and after ca. 11,000 yr. B.P. The old stage, or the Ko-Fuji stage, is characterized by frequent volcanic mudflows and ashes, and the new stage, or the Shin-Fuji stage, is characterized by continuous effusions of enormous basaltic lava flows at its initial phase. Basaltic lava flows ejected at the initial phase of the Shin-Fuji stage flew extended to cover most of the foot of the volcano. There are many springs at the foot of Mt. Fuji. Among these, large springs such as the Kakidagawa and Wakuike are located at the termini of the lava flows of the Shin-Fuji stage. Groundwater recharged at the upper parts of Mt. Fuji flows down the flanks under confined conditions through clincker zones formed above and below the massive central cores of basaltic lava flows. At the termini of the lava flows, confined groundwater gushes out of the clinker zones under pressure, forming many large springs. Hydrologic studies indicate that the total recharge to these confined aquifers in Mt. Fuji is ca. 4,700,000 m3/day.

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論説
  • 神谷 貴文, 渡邊 雅之, 村中 康秀, 申 基澈, 中野 孝教
    126 巻 (2017) 1 号 p. 43-71
    公開日: 2017/04/20
    ジャーナル フリー

     To elucidate geochemical characteristics of groundwater and sources of dissolved ions in groundwater at the southern slope and the base of Mt. Fuji, central Japan, in 2011 and 2012, dissolved ions in 199 samples of both well water or spring water from the study region and 12 monthly precipitation samples at each of four elevations were collected and analyzed. Most of the groundwater was of the Ca-HCO3 type. The study region was divided from the near summit to the base of the mountain into four areas based on water quality, topography, geology, and land use at the watershed. The concentration of chlorine ion (Cl) in the groundwater and precipitation tended to decrease with elevation, and the Na+/Cl ratio in the precipitation (0.56 on average) was close to that of seawater (0.56) at all sites. The atmosphere-derived fractions of major cations in the groundwater, which were estimated by assuming that all Cl was derived from sea salt in the atmosphere, were in ascending order: Ca (average 0.05), Mg (0.08), K (0.28), and Na (0.29). The concentrations of PO43− and F and the cation: Cl ratios of all cations were high in the groundwater in the eastern area, where the surface geology consists of Gotemba mud flow sediments, which are composed of volcanic materials of the Old Fuji period. These results indicate that groundwater quality is strongly affected by the basaltic materials composing Mt. Fuji, which are enriched with Ca and Mg, and are susceptible to chemical weathering, but differs between aquifers in volcanic rocks of the Old Fuji period (about 100 ka) and those in rocks of the New Fuji period (after about 10 ka). In the southwestern and southeastern areas, the groundwater was characterized by a high concentration of NO3. Because tea plantations are distributed at upper elevations in these areas, we inferred the major source of nitrogen in the water to be fertilizer. Moreover, spring water was enriched more with NO3 than well water collected at similar elevations, suggesting that the recharge area of well water is at a higher elevation than the spring water recharge area.

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  • 中村 高志, 長谷川 達也, 山本 真也, 内山 高, 瀬子 義幸, 清水 源治, 米山 由紀, 風間 ふたば
    126 巻 (2017) 1 号 p. 73-88
    公開日: 2017/04/20
    ジャーナル フリー

     Lake Sagami, which is fed by the Katsuragawa River, supplies about 60% of tap water in Kanagawa Prefecture. However, excess growth of phytoplankton due to high loading of nutrients such as nitrogen in summer has been an issue since the 1980s. Therefore, identifying nitrate loading in the Katsuragawa River watershed can be considered to be critical. The headstream of the river, at the northern foot of Mt. Fuji, is selected for a study of nitrate loading, in which oxygen, hydrogen, and nitrogen isotopes in water and nitrate are used to determine the sources of springs and nitrate contamination. Hydrogen and oxygen isotopes in water samples indicate that precipitation at the northern foot of Mt. Fuji and water from Lake Yamanakako are the main sources of springs in the Oshino area. Low nitrate concentrations with high nitrogen isotope values in spring water indicate low-level wastewater contamination in the springs. Furthermore, a watershed investigation identifies the characteristics of nitrate loading in the Katsuragawa River. The distribution of nitrate concentration and nitrogen isotope values indicates that springs and wastewater drainage from residential areas are significant sources contributing to the upstream nitrate load. However, nitrate concentrations and nitrogen isotope values downstream are reduced by the diluting effect of tributary water originating from mountain areas.

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総説
  • 戸崎 裕貴, 浅井 和由
    126 巻 (2017) 1 号 p. 89-104
    公開日: 2017/04/20
    ジャーナル フリー

     Current studies on the groundwater ages in Mt. Fuji are reviewed. Mt. Fuji is one of the largest Quaternary stratovolcanoes in Japan (volume of 1,200-1,500 km3). The large amount of precipitation on mountain slopes (annual volume of approximately 2 × 109 m3) suggests that Mt. Fuji contains substantial reservoirs of groundwater in its main body. In fact, numerous springs located around the foot of the mountain originate mainly from confined groundwater in Holocene lava flows. Early groundwater studies in the Mt. Fuji area focused on the development of groundwater resources, followed by studies on measures to address groundwater problems including depletion, salinization, and nitrate contamination. Application of isotope hydrological tools since the 1990s has provided valuable information on groundwater flow processes in Mt. Fuji. Groundwater age in Mt. Fuji has been a key issue since the 1960s, and relatively extensive data on tritium (3H) are available. Besides, new age-dating techniques including tritiogenic 3He (3H/3He method), chlorofluorocarbons (CFCs), and bomb-produced 36Cl have been applied in the Mt. Fuji area in recent years. These groundwater age data are compiled and discussed in terms of the hydrogeological structure of Mt. Fuji (lava flows of the Younger Fuji volcano, and mudflow deposits of the Older Fuji volcano). Compiled multi-tracer groundwater age data show distinct differences between Younger Fuji (< 30-40 years) and Older Fuji (> 60 years) aquifers, although data on Older Fuji groundwater are still limited. Possible explanations relate to differences in permeability or volume between Younger Fuji and Older Fuji deposits.

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