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Hydrobiogeochemical evolution along the regional groundwater flow systems in volcanic aquifers in Kumamoto, Japan

Authors:
Takahiro Hosono at Kumamoto University
Takahiro Hosono
Shahadat Hossain at Bangladesh Atomic Energy Commission
Shahadat Hossain
Jun Shimada at Kumamoto University
Jun Shimada
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Abstract and Figures

Kumamoto is known as the largest groundwater city in Japan. Geochemical modeling (saturation indices and mineral stability diagram) was applied in this area for better understanding a hydrogeochemical evolution in volcanic aquifers in regional scale with additional constraint from stable isotopic dataset. Geochemical evolutionary model was interpreted along the water flow dynamics. In total 136 water samples were collected from wells, springs, and rivers in and around the major groundwater flow lines for geochemical analyses. Our results indicated that plagioclase is the major weathering reactant in aquifers with secondary important weathering minerals of pyroxenes. These reactions facilitate current hydrochemical signatures and produce secondary minerals of kaolinite or halloysite and smectite in later stage. Observed hydrogeochemical processes can be distinguished into two distinct criteria along flow regime. The first processes are material loads from the surface (mixing of contaminants and river waters) and initial stage of silicate weathering, resulting in the formation of Ca-HCO3, Ca-NO3–HCO3, and Ca–Mg–Na-HCO3 type waters. These processes prevail in aquifers at the recharge to lateral flow zones with relatively shorter residence time of < ca. 40 years. The second processes are the precipitation of clay minerals, i.e., smectite, and cation exchange reaction of Ca²⁺ and Mg²⁺ to Na⁺ in downslope aquifers (Na-HCO3 type) with relatively longer groundwater residence time of > 55 years. Microbiological reduction reactions dominate over these areas and salinization occurs at the coast that changes aquifer waters to be Na-Cl type. Proposed models and approach shown in this study may be useful and applicable in interpreting systems in other volcanic aquifer systems at similar climate conditions and for sustainable water resources management.
Location of the study area showing major geographic features, groundwater region and sampling locations along major groundwater flow lines A–A′ and B–B′, which hydrogeologic sections are shown in Fig. 3. Division of flow zones are shown along each arrow. Solid contour lines represent groundwater potential for high water season during September and October (m, above sea level, after Kagabu et al. 2017). Green and yellow squares represent sampling location of spring waters from Aso caldera watershed and Kumamoto area, respectively. Yellow and white stars indicate sampling location of river waters from Shira and Midori Rivers, respectively
Location of the study area showing major geographic features, groundwater region and sampling locations along major groundwater flow lines A–A′ and B–B′, which hydrogeologic sections are shown in Fig. 3. Division of flow zones are shown along each arrow. Solid contour lines represent groundwater potential for high water season during September and October (m, above sea level, after Kagabu et al. 2017). Green and yellow squares represent sampling location of spring waters from Aso caldera watershed and Kumamoto area, respectively. Yellow and white stars indicate sampling location of river waters from Shira and Midori Rivers, respectively
… 
Conceptual cartoon showing hydrobiogeochemical evolution of groundwater in Kumamoto region along the two major flow lines. Microbiological reaction models are after Hosono et al. (2013, 2014) and Hossain et al. (2016a, b)
Conceptual cartoon showing hydrobiogeochemical evolution of groundwater in Kumamoto region along the two major flow lines. Microbiological reaction models are after Hosono et al. (2013, 2014) and Hossain et al. (2016a, b)
… 
Surface geology map of the study area after Kumamoto Prefecture Geological Map Compilation Committee Report (2008). Dotted line in the figure shows elevation line of ca. 5 m (above sea level). Ariake clay sediments are expected to be distribute at elevation lower than ca. 5 m (see text for details)
Surface geology map of the study area after Kumamoto Prefecture Geological Map Compilation Committee Report (2008). Dotted line in the figure shows elevation line of ca. 5 m (above sea level). Ariake clay sediments are expected to be distribute at elevation lower than ca. 5 m (see text for details)
… 
Simplified hydrogeological cross sections along A–A′ and B–B′ flow lines shown in Fig. 1. Arrows in the aquifers represent the groundwater flow direction and the size of arrows corresponds to flow velocity calculated by groundwater flow simulation (after Hosono et al. 2019)
Simplified hydrogeological cross sections along A–A′ and B–B′ flow lines shown in Fig. 1. Arrows in the aquifers represent the groundwater flow direction and the size of arrows corresponds to flow velocity calculated by groundwater flow simulation (after Hosono et al. 2019)
… 
Bivariate plot of δD and δ¹⁸O for river, spring and aquifer waters collected from the study area. Local meteoric lines and compositions of soil pore waters are after Okumura et al. (2018). The global meteoric water line is after Gat and Gonfiantini (1981). Average compositions of 100 core soil waters (10 m deep, 10 cm interval) are shown for four soil cores (Okumura et al. 2018), which sampling locations are shown in Fig. 1
+5
Bivariate plot of δD and δ¹⁸O for river, spring and aquifer waters collected from the study area. Local meteoric lines and compositions of soil pore waters are after Okumura et al. (2018). The global meteoric water line is after Gat and Gonfiantini (1981). Average compositions of 100 core soil waters (10 m deep, 10 cm interval) are shown for four soil cores (Okumura et al. 2018), which sampling locations are shown in Fig. 1
… 
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Environmental Earth Sciences (2020) 79:410
https://doi.org/10.1007/s12665-020-09155-4
ORIGINAL ARTICLE
Hydrobiogeochemical evolution alongtheregional groundwater ow
systems involcanic aquifers inKumamoto, Japan
TakahiroHosono1,2 · ShahadatHossain3· JunShimada1
Received: 18 September 2019 / Accepted: 27 August 2020 / Published online: 7 September 2020
© Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract
Kumamoto is known as the largest groundwater city in Japan. Geochemical modeling (saturation indices and mineral stabil-
ity diagram) was applied in this area for better understanding a hydrogeochemical evolution in volcanic aquifers in regional
scale with additional constraint from stable isotopic dataset. Geochemical evolutionary model was interpreted along the
water flow dynamics. In total 136 water samples were collected from wells, springs, and rivers in and around the major
groundwater flow lines for geochemical analyses. Our results indicated that plagioclase is the major weathering reactant
in aquifers with secondary important weathering minerals of pyroxenes. These reactions facilitate current hydrochemical
signatures and produce secondary minerals of kaolinite or halloysite and smectite in later stage. Observed hydrogeochemical
processes can be distinguished into two distinct criteria along flow regime. The first processes are material loads from the
surface (mixing of contaminants and river waters) and initial stage of silicate weathering, resulting in the formation of Ca-
HCO3, Ca-NO3–HCO3, and Ca–Mg–Na-HCO3 type waters. These processes prevail in aquifers at the recharge to lateral flow
zones with relatively shorter residence time of < ca. 40years. The second processes are the precipitation of clay minerals,
i.e., smectite, and cation exchange reaction of Ca2+ and Mg2+ to Na+ in downslope aquifers (Na-HCO3 type) with relatively
longer groundwater residence time of > 55years. Microbiological reduction reactions dominate over these areas and salini-
zation occurs at the coast that changes aquifer waters to be Na-Cl type. Proposed models and approach shown in this study
may be useful and applicable in interpreting systems in other volcanic aquifer systems at similar climate conditions and for
sustainable water resources management.
Keywords Groundwater· Volcanic aquifer· Saturation indices· Silicate weathering· Cation exchange· Flow dynamics
Introduction
Kumamoto is the largest groundwater user area in Japan
(Fig.1). About one million people in and around the area
completely depend on groundwater for their all purposes
(e.g., Taniguchi etal. 2019). The area comprises Kumamoto
City and its surrounding districts and towns, extended to the
western flank of Aso caldera mountains in central Kyushu
(Fig.1), which is one of the largest volcanic calderas in
Japan. The abundant groundwater resources in this area
are a blessing of combination of both high local precipi-
tations and highly permeable surface geological structures
that comprise of Quaternary pyroclastic flow deposits of
the Aso volcanic eruptions. Since the people in Kumamoto
relies on groundwater for drinking purpose, its quality is one
of major concerns to the local citizens and policy makers
in a management point of view. However, the groundwater
quality is becoming a serious issue in this area due to some
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s1266 5-020-09155 -4) contains
supplementary material, which is available to authorized users.
* Takahiro Hosono
hosono@kumamoto-u.ac.jp
1 Faculty ofAdvanced Science andTechnology, Kumamoto
University, 2-39-1 Kurokami, Kumamoto860-8555, Japan
2 International Research Organization forAdvanced Science
andTechnology, Kumamoto University, 2-39-1 Kurokami,
Kumamoto860-8555, Japan
3 Atomic Energy Centre, Chittagong, Bangladesh Atomic
Energy Commission, 1018/A, Bayezid Bostami Road, East
Nasirabad, Chittagong4209, Bangladesh
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Groundwater flow systems and basic water quality in Kumamoto are well acknowledged through traditional borehole measurements (water table, temperature, and water quality measurements) using many wells installed over the entire area (Fig. 1a and b;Nagai et al., 1983, Kumamoto Prefecture andKumamoto City, 1995;Taniguchi et al., 2003). More detailed aquifer environmental conditions were revealed in recent years from multidisciplinary approaches including groundwater age determination (Kagabu et al., 2017, geochemical modeling (Hossain et al. 2016a;Hossain et al., 2016b;Hosono et al., 2020a), multiple stable isotope (Hosono et al., , 2020b. (b) The locations of the wells for groundwater sampling by this study. ...
... The thickness of the aquifers is about ~ 250 m, which tends to be thicker toward the coastline, and groundwater flows along the topographical gradient from north or east to west (Fig. 1a). The aquifers are divided into an upper unconfined shallow aquifer (layer thickness ~ 90 m, which we call the 1st aquifer) and a lower confined to semiconfined deep aquifer (layer thickness 20 ~ 250 m, which we call the 2nd aquifer) across the impermeable layer of the lacustrine sediments (Hosono et al., 2020a;Hosono et al., 2020b). Groundwater is recharged from the surrounding mountains, the plateaus (50 to 200 m above sea level), and the river water in the midsection of the Shira River, and flows laterally to the plains (Fig. 1a). ...
... Most of the groundwater discharge near Ezu Lake, at the locations where the topographical gradient becomes gentle, but a part of it still stays under the ground and exists as a relatively slow-flowing groundwater toward the coast (Fig. 1a). See previous studies (Hosono et al., 2013, Hosono et al. 2014, Hosono et al., 2020aHosono et al., 2020b, Rahman et al., 2021 for a more detailed description of regional aquifer conditions. ...
Stable N and O isotopic indicators coupled with social data analysis revealed long-term shift in the cause of groundwater nitrate pollution: Insights into future water resource management
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... In many regions of the world, volcanic aquifers have become valuable resources in providing water for human consumption and economic activities (Bertrand et al. 2010;El Maghraby 2015;Hosono et al. 2020;Koh et al. 2018;Morán-Ramírez et al. 2016). Approximately 65% of the population in the Central Valley (Valle Central), Costa Rica, equivalent to 1.7 million people, depends on groundwater resources from the Barva-Colima volcanic aquifer system (Reynolds- Vargas et al. 2006). ...
... DA has been successfully applied in volcanic systems, combined with other multivariate methods (Barzegar et al. 2019;Panagopoulos et al. 2016) and machine learning procedures (Ha et al. 2021;Sajedi-Hosseini et al. 2018). Moreover, principal component analyses (PCA; Gan et al. 2018;Panagopoulos et al. 2016;Pazand and Javanshir 2016), correlation analyses (Morán- Ramírez et al. 2016;Kim et al. 2020), factor analysis (Gan et al. 2018); hierarchical cluster analysis Egbueri 2020;Gan et al. 2018); hydrochemical diagrams (Ebrahimi et al. 2022;Fenta et al. 2020;Hosono et al. 2020;Shishaye et al. 2020;Wisitthammasri et al. 2020), base exchange indices (Haji et al. 2021;Mechal et al. 2017), multiple linear regression , and the assessment of saturation indexes of aqueous mineral phases (Esteller et al. 2017;Hosono et al. 2020;Morán-Ramírez et al. 2016) are widely employed to facilitate the interpretation of geochemical processes with large hydrochemical data series in volcanic aquifers. ...
... DA has been successfully applied in volcanic systems, combined with other multivariate methods (Barzegar et al. 2019;Panagopoulos et al. 2016) and machine learning procedures (Ha et al. 2021;Sajedi-Hosseini et al. 2018). Moreover, principal component analyses (PCA; Gan et al. 2018;Panagopoulos et al. 2016;Pazand and Javanshir 2016), correlation analyses (Morán- Ramírez et al. 2016;Kim et al. 2020), factor analysis (Gan et al. 2018); hierarchical cluster analysis Egbueri 2020;Gan et al. 2018); hydrochemical diagrams (Ebrahimi et al. 2022;Fenta et al. 2020;Hosono et al. 2020;Shishaye et al. 2020;Wisitthammasri et al. 2020), base exchange indices (Haji et al. 2021;Mechal et al. 2017), multiple linear regression , and the assessment of saturation indexes of aqueous mineral phases (Esteller et al. 2017;Hosono et al. 2020;Morán-Ramírez et al. 2016) are widely employed to facilitate the interpretation of geochemical processes with large hydrochemical data series in volcanic aquifers. ...
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The volcanic Barva and Colima multiaquifer system is crucial for the drinking water supply of about 1.7 million people, about 30% of the population in Costa Rica. The complexity of this system and the simultaneous occurrence of anthropogenic and natural processes complicate the understanding of hydrochemical and hydrogeological dynamics, both essential for groundwater protection. This research aimed to develop a model for the discrimination of groundwaters according to the main withdrawn aquifer and flow path, assess interaquifer connections, and evaluate the main hydrochemical processes governing water quality. Samples (571) from 38 sampling sites, collected quarterly from 2016 to 2020, were analyzed for nitrate, major ions, and silica. Principal component analysis and discriminant analysis exhibited and validated sample grouping according to the primary aquifer system captured, i.e., Upper Barva, Lower Barva, Upper Colima, and Lower Colima, and the occurrence of two flow paths within the Lower Barva and Upper Colima aquifers. Hydrochemical and statistical analyses showed resilience to seasonal chemical variation in deeper aquifers and also three groundwater mixing processes. Lower Barva groundwater enriches in bicarbonate, magnesium, calcium, sodium, and silica, mainly due to weathering of mafic andesitic-basaltic lavas. The Upper and Lower Colima showed higher silica and major ion content, except for calcium, indicating longer residence times, dissolution from felsic andesitic minerals, and calcium adsorption by normal ion exchange. Weathering of aluminosilicates is the primary process governing groundwater quality in the four studied aquifers, whereas cation exchange, interaquifer leakage, and anthropogenic processes might modify the groundwater chemistry.
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... The unconfined aquifer is composed of recent pyroclastic-flow deposits (uppermost Aso-4) and partial marine sediments (<50 m deep). The underlying confined/semi-confined aquifer consists of older pyroclastic deposits and volcanic flow lavas (60-250 m) [45,54]. ...
... The Kumamoto area lies in central Kyushu Island, southwestern Japan ( Figure 1). Hossain et al. [43,44] and Hosono et al. [45] reported its geological and hydrogeological settings and the geochemical processes controlling its groundwater. The eastern part of the study area is a volcanic area of the Quaternary age covered by pyroclastic-flow deposits erupted from the Aso volcano and alluvial deposits of the Pleistocene age. ...
... The eastern part of the study area is a volcanic area of the Quaternary age covered by pyroclastic-flow deposits erupted from the Aso volcano and alluvial deposits of the Pleistocene age. The western portion (especially coastal area) comprises the Kumamoto alluvial plain, consisting of Holocene Ariake clay [45,46]. The pyroclastic-flow deposits erupted from Mt. Aso over four periods between 90 and 30 ka and are classified into four types: Aso-4, Aso-3, Aso-2, and Aso-1. ...
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... The main challenge for developing such a model is the collection of long-term tracer measurement data over a regional scale. The model domain of this study addresses regional groundwater flow systems in Kumamoto in the central part of Kyushu Island in southern Japan (Fig. 1); several studies (Taniguchi et al. 2003;Hosono et al. 2013Hosono et al. , 2014Hosono et al. , 2020Hossain et al. 2016a, b;Zeng et al. 2016;Kagabu et al. 2017;Okumura et al. 2018) have characterized the groundwater age, flow dynamics, and biogeochemical processes using several tracers such as groundwater age tracers including sulfur hexafluoride (SF 6 ), tritium ( 3 H), chlorofluorocarbons (CFCs) and krypton ( 85 Kr), stable isotope ratios of water molecular (δD and δ 18 O), and aquifer temperature profiles. These studies demonstrated that the groundwater age tracers such as 3 H and 85 Kr concentrations are the most useful variables, while SF 6 and CFCs cannot be used for proper age determination due to contamination effects (Kagabu et al. 2017;Ide et al. 2020). ...
... The aquifers in the study area are separated by the Futa and Hanafusa impermeable sediment layers into unconfined (named as the first aquifer) and semiconfined to confined aquifers (named as the second aquifer; Fig. S1 of the ESM; Taniguchi et al. 2003;Hosono et al. 2013Hosono et al. , 2019Hosono et al. , 2020Rahman et al. 2020b). The first aquifer consists of unwelded Aso-4 pyroclastic deposits and alluvium sedimentary deposits, and the depth of this aquifer varies between a few meters to 90 m. ...
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View
... Understanding the material cycle in the ecosystem and biogeochemical processes is essential for comprehending changes in the physical and chemical properties of water within a basin. These processes also apply to groundwater and surface water Hosono et al. 2020). Water quality, watershed management, and systemic hydrological environmental factors are all crucial (Wu and Lu 2021;Xu et al. 2022a;Zhao et al. 2022). ...
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... G2, G3, G4, and most of G1 are saturated with quartz and kaolinite (Table 5). Following Hosono et al. [72], incongruent dissolution reactions of silicate weathering in the study area can be hypothesized by following Equations (9) and (10) ...
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Full-text available
  • Jun 2020
Hydrogeological properties can change in response to large crustal earthquakes. In particular, permeability can increase leading to coseismic changes in groundwater level and flow. These processes, however, have not been well-characterized at regional scales because of the lack of datasets to describe water provenances before and after earthquakes. Here we use a large data set of water stable isotope ratios (n = 1150) to show that newly formed rupture systems crosscut surrounding mountain aquifers, leading to water release that causes groundwater levels to rise (~11 m) in down-gradient aquifers after the 2016 Mw 7.0 Kumamoto earthquake. Neither vertical infiltration of soil water nor the upwelling of deep fluids was the major cause of the observed water level rise. As the Kumamoto setting is representative of volcanic aquifer systems at convergent margins where seismotectonic activity is common, our observations and proposed model should apply more broadly.
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Coseismic Groundwater Drawdown Along Crustal Ruptures During the 2016 M w 7.0 Kumamoto Earthquake
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Groundwater level changes after earthquakes provide insight into changes in hydrogeological properties such as permeability and pore pressure. Quantifying such changes, both their location and magnitude, is usually hindered by limited data. Using extensive high-resolution water level monitoring records, we provide direct evidence of significant groundwater drawdown (4.74 m maximum) over a 160 km2 area along crustal ruptures after the Mw 7.0, 2016 Kumamoto earthquake. Approximately 106 m3 of water disappeared within 35 minutes after the main shock. The loss of water was not caused by static-strain driven pore-pressure decrease nor by releasing of water through structural pathways, but most likely by water transfer downwards through open cracks. Such changes may impact the security of water resources, the safety of underground waste repositories, and contaminant transport in seismically active areas.
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Recovery of Lost Nexus Synergy via Payment for Environmental Services in Kumamoto, Japan
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Full-text available
  • Mar 2019
The objective of this study is to characterize and quantify the “trans-spatial nexus synergy” benefit of subsidized water ponding in the water-energy-food nexus in Kumamoto, Japan. After years of decreased rice production in upstream areas and associated declines in groundwater levels, the Kumamoto city government implemented a subsidy program whereby farmers in the Shira River basin receive payments to water their fields, which provides valuable groundwater recharge to downstream Kumamoto city. We quantify the economic benefits of this subsidy program, which include avoided additional energy costs to obtain scarcer levels of groundwater, as well as net revenue from the crops in the Shira River basin that would otherwise not be grown in the absence of the subsidy. These annual benefits can be combined and compared to the annual cost of the government subsidy. We also calculate potential historical losses that may have occurred in the region as a result of land use transitions from rice farming to urban use, which disrupted the nexus synergy between the watered fields and the groundwater table.
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Earthquake-induced structural deformations enhance long-term solute fluxes from active volcanic systems
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Full-text available
  • Oct 2018
Evidence for relationships between seismotectonic activity and dissolved weathering fluxes remains limited. Motivated by the occurrence of new springs emerging after the 2016 Kumamoto earthquake and supported by historical groundwater data, this study focuses on the long-term effect of near-surface structural deformation on the contribution of deep, highly saline fluids to the solute fluxes from the Aso caldera, Kyushu, Japan. Available hydrologic and structural data suggest that concentrated, over-pressured groundwaters migrate to the surface when new hydraulic pathways open during seismic deformation. These new springs have a hydrochemical fingerprint (including δDH2O, δ18OH2O, δ7Li, δ11B, δ18OSO4, and δ34SSO4) indistinguishable from long-established confined groundwater that likely reflects a mixture of infiltrated meteoric water with high-sulfate hydrothermal fluids. A comparison of historical hydrochemistry data and patterns of past seismicity suggests that discharge of deep fluids is associated with similar deformation structures to those observed during the Kumamoto earthquake, and that seismic activity plays an important role over historic timescales in delivering the majority of the solutes to the caldera outlet, sustaining fluxes that are amongst the world's highest. This upwelling mechanism might be relevant for other systems too, and could contribute to the over-proportional share of active volcanic areas in global weathering fluxes.
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Evaluations of the downward velocity of soil water movement in the unsaturated zone in a groundwater recharge area using δ18 O tracer: the Kumamoto region, southern Japan
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Full-text available
  • Jun 2018
Water and substances from the surface infiltrate the unsaturated zone before reaching groundwater. Yet, little study has been done on the unsaturated zone due to the difficulty of sampling. A lot of studies have been carried out on the top soil down to a depth of one metre and on shallow aquifers because they are easily accessible for sampling. The unsaturated zone of the Kumamoto region recharge areas is important due to concerns about groundwater pollution from agriculture. The aim of this study was to estimate the downward velocity of soil water movement through the unsaturated zone and the recharge rate using δ18O as a tracer. Five sampling sites were selected and a core was taken from each site. The cores were cut into 0.1 m pieces and soil water was extracted from each to analyze for δD and the δ18O content. Average δD and δ18O compositions of soil water were similar to the isotopic compositions of summer precipitation. Annual average recharge rate and the downward velocity of soil water in each site were estimated by fitting a vertical δ18O profile pattern to a precipitation δ18O time series as a theoretical water displacement flow model for recharge. An estimated annual average recharge rate in the recharge area ranged from 745 to 1058 mm/yr with the annual average downward velocity of 1.37 to 2.34 m/yr. Based on the estimated downward velocity, the infiltration time for soil water to reach the aquifer was determined as ranging from 9 to 24 years, which corresponds with previous groundwater age estimations presented in an earlier published study on the same area. It was assumed that contaminants will reach aquifers in 9 to 25 years if the effects of diffusion and microbiological reaction are not taken into account.
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Groundwater age determination using 85 Kr and multiple age tracers (SF 6 , CFCs, and 3 H) to elucidate regional groundwater flow systems
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  • Aug 2017
Study region: The Kumamoto area (945 km2) in the south of Japan, where almost 100% of the drinking water is dependent on groundwater. Study focus: Simultaneous measurement of groundwater dating tracers (85Kr, chlorofluorocarbons [CFCs], sulphur hexafluoride [SF6], and 3H) was performed in the Kumamoto area, to elucidate the regional groundwater flow system and obtain improved estimates of groundwater ages. The groundwater ages were estimated from the 85Kr concentrations in nine locations from three areas: along two major groundwater flow lines (A–A’ and B–B’); and a high-nitrate-input recharge area (C area). New hydrological insights for the region: The groundwater ages could not be estimated using CFCs or SF6, particularly in the urban areas because of artificial additions to the concentration over almost the entire study area. However, even in these regional circumstances, apparent ages of approximately 16, 36, and not less than 55 years were obtained for three locations on the A–A’ line (recharge area, discharge area, and stagnant zone of groundwater, respectively) from 85Kr measurements. This trend was also supported by lumped parameter model analysis using a time series of 3H observations. In contrast, along the B–B’ line, the groundwater age of not less than 55 years at three locations, including the recharge to discharge area, where CFCs and SF6 were not detected, implies old groundwater: this is also the area in which denitrification occurs. In the C area, very young groundwater was obtained from shallow water and older groundwater was detected at greater depths, as supported by the long-term fluctuations of the NO3−–N concentration in the groundwater. The results of this study can be effectively used as a “time axis” for sustainable groundwater use and protection of groundwater quality in the study area, where groundwater accounts for almost 100% of the drinking water resources.
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Understanding nitrate contamination based on the relationship between changes in groundwater levels and changes in water quality with precipitation fluctuations
Article
  • Dec 2018
  • SCI TOTAL ENVIRON
There are growing concerns about nitrate contamination in Kumamoto City, where >700,000 people completely depend on groundwater as a source of drinking water. We found that some groundwater samples showed considerably different nitrate concentrations although their sampling locations were close to one another, and we speculated that this phenomenon was due to the differences in subsurface geological properties. In order to verify this hypothesis, we carried out temporally intensive long-term monitoring of the groundwater levels and water qualities at three of the closely related sampling wells, and the results revealed that the changes in water level and water quality were different at each well. The water level at well T1, where nitrate concentrations ranged from 12 to 26 mg N/L, showed a significantly sensitive and unique response to heavy rain, which indicated that the subsurface at this site might be highly permeable; this would have allowed for the influent water to easily reach the groundwater aquifer over a short period. However, wells T2 and T3, which were located within 0.6 and 1.9 km from well T1, respectively, had nitrate concentrations that were lower than that in well T1 (4.5–8.0 mg N/L) and showed only gradual responses to heavy rain. These observations suggest that the highly permeable subsurface properties in the vicinity of well T1 contributed to the more serious nitrate contamination in well T1 than those at wells T2 and T3. This study demonstrates the importance of temporally intensive, long-term monitoring for capturing changes in groundwater level and water quality with precipitation fluctuations, and we showed how this approach can lead to a better understanding of the nitrate contamination situation.
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Estimating silicate weathering timescales from geochemical modeling and spring water residence time in the Kirishima volcanic area, southern Japan
Article
  • Apr 2018
  • CHEM GEOL
Kyushu Island in southern Japan is located within an area subjected to one of the highest rates of chemical weathering in the world. This area is purported to have the highest discharge rate of dissolved silica and other cations within the Japanese Island arc. To understand the timescale of chemical weathering in such a weathering ‘hot spot’, details of water–rock interaction are characterized using a conventional geochemical model for the Kirishima volcanic area. The model is focused on source water (spring water) with different residence times previously estimated by chlorofluorocarbons (CFCs) concentration analysis. The geochemical and geological features of the spring water samples suggest that the primary reactant minerals are pyroxenes and plagioclase. It was also found from activity-diagram analysis that kaolinite and Ca-smectite are the major precipitable secondary minerals from the system. The occurrence of these reactions was quantitatively confirmed by stoichiometric calculations of measured dissolved ions. In combination with previous estimates of spring water residence ages, it is concluded that saturation of dissolved silica and simultaneous transition of phase precipitation from kaolinite to Ca-smectite starts in ca. 20 years after aqueous recharge. Our findings imply the usefulness of geochemical data as a function of water-rock interaction timescale in certain field. In future work, these findings can be compared to data from other areas to improve understanding of global heterogeneity in chemical weathering rates.
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