This study investigates the contribution ratios of different groundwater recharge sources and influence of a dual monsoon system in Kofu basin, central Japan, through the hydrogen and oxygen isotopic analysis of precipitation, river water, and groundwater. The study is focused on the area of the Kamanashigawa and Midaigawa alluvial fans, which are formed by two main rivers. Precipitation isotopic content exhibits significant seasonal variability. Also, river water presents d-excess values higher than annual precipitation at plain areas (9 and 10‰), suggesting that two different air-masses contribute to precipitation, corresponding to the monsoon and pre-monsoon periods. The results of this study allow estimation of relative contributions of different sources to groundwater and influence of a dual monsoon system. The mass-balance analysis of the δ18O to identify the groundwater source indicates the river water contributes 38–100% of the recharge, while precipitation contributes 29–62% in Kamanashigawa alluvial fan. In the case of Midaigawa alluvial fan, river water contributes 77–99% in the northern part; in the southern side, 30–93% of contribution comes from precipitation. The mass-balance analysis of the d-excess indicates pre-monsoon precipitation contributes 46–68% and 39–65% to groundwater of the Kamanashigawa and Midaigawa alluvial fans, respectively.

INTRODUCTION

Environmental tracers such as oxygen and hydrogen stable isotopes (δ18O and δD) are commonly used in hydrological investigations, e.g., to estimate the groundwater recharge, to identify multiple recharge sources, and to investigate the interactions between the surface and groundwater, such as the flow paths and mixing in regional aquifers, advection/diffusion in groundwater aquifers, and the effects of evaporation on groundwater systems (Deshpande et al. 2003; Palmer et al. 2007). The values of δ18O and δD remain constant as long as phase changes or fractionation does not occur along the flow path (Senturk et al. 1970; Perry et al. 1980; Clark & Fritz 1997). However, stable isotope fractionation occurs when the water changes its state (e.g., through evaporation or condensation, raindrop formation, and infiltration in aquifers). Consequently, the isotopic composition varies between the different forms of water (i.e., precipitation, surface water, and groundwater). A comparative analysis of the δ18O and δD composition and spatiotemporal distribution of precipitation, river water, and groundwater provides a useful tool for evaluating recharge mechanism and defining the status of mixed groundwater recharge zones (Chen et al. 2006; Mukherjee et al. 2007; Negrel et al. 2011; Yeh et al. 2011; Liu & Yamanaka 2012; Peng et al. 2012).

To investigate groundwater recharge through stable isotope analysis, the measured δ18O is plotted against δD, producing the ‘local meteoric water line’ (LMWL), which is then compared with the ‘global meteoric water line’ (GMWL, Craig 1961; Gat 1996). The deviations and their characteristics (e.g., slope and intercept) of the LMWL from the GMWL may be used to infer the groundwater recharge mechanism. The LMWL may vary due to evaporation and condensation (e.g., temperature and humidity), terrain environment, altitude, and other reasons. The intercept is also called deuterium excess or d-excess (d = δD − 8.0 δ18O) (Dansgaard 1964). A globally representative d-excess is 10‰ (Craig 1961); however, it may vary depending on the evaporation conditions in the origin of the water source. For example, d-excess is 6.03 in North America and 3.97 in the Tropical Island area (Gat 1996). In Japan, d-excess in different seasons is reported to vary from 14.8 in April–September to 23.8 in October–March, even though the LMWL slope remains approximately the same (Liu & Yamanaka 2012). Comparing the differences in the stable isotopic signatures of groundwater, river water, and local precipitation allows the effect of different sources of groundwater recharge to be identified (Scanlon et al. 2002; Kalbus et al. 2006), whereas comparing the d-excess values of groundwater and precipitation between the different seasons allows identification of the effect of the dual monsoon (Yeh et al. 2011).

Rainwater characteristics in Japan are controlled by monsoon (summer, May–October) and pre-monsoon precipitation (winter, November–April). These two seasonal circulation systems are a part of the larger monsoon circulation in South Asia, and they are caused by seasonal temperature and pressure gradient reversal, which is associated with the wind circulation following the annual northward and southward motion of the sun. Normal precipitation d-excess values for the pre-monsoon and monsoon periods are greater than 20‰ and less than 10‰, respectively (Nakayama et al. 2000). Such seasonal variation is caused by the contribution of the dual moisture sources predominantly from the Pacific Ocean in summer with SE monsoon winds and predominantly from the Sea of Japan with NE monsoon winds in winter (Waseda & Nakai 1983; Araguás-Araguás et al. 1998). Yeh et al. (2011) estimated seasonal contributions of precipitation to groundwater recharge in the Chih-pen Creek basin of eastern Taiwan. However, very few studies that have examined the aspects have investigated both the recharge contribution from different sources and its temporal variability.

Groundwater is an important source of water for industrial, agricultural, and drinking water use in Kamanashgawa and Midaigawa alluvial fans, located in Kofu basin, because this rural area has no dams for surface water use. This basin is located in inland Japan and the distances from the Pacific Ocean and Sea of Japan are about 70 km and 160 km, respectively. The rainwater characteristics in this area are controlled by precipitation during the summer (SE) and winter (NE) monsoons. The SE monsoon operates during the months of May–October and NE pre-monsoon during the months of November–April. Therefore, in a local area such as the Kofu basin, the d-excess for the different seasons is expected to differ from the general values of Japan.

In this context, the study aims to identify the local oxygen and hydrogen stable isotopic characteristics and the relative contribution of the two groundwater sources (precipitation infiltration and river water recharge) and the dual monsoon system (pre-monsoon and monsoon seasons) on groundwater recharge in Kamanashi and Midai alluvial fans of the Kofu basin in the central part of Japan.

MATERIALS AND METHODS

The study area

Kofu Basin, located in central Japan, is drained by the Fuji River and surrounded by steep mountains of ∼2,000 m (Figure 1). The Fuji River originates as the Kamanashi River from the north of the southern Alps and as the Fuefuki River from the north of the Kofu Basin. The Fuji River basin (area = 3,570 km2) has a very complex and fragile geological substratum due to several dislocations in the area. Consequently, many parts of the basin have collapsed; the clastic sediments are transported by the river and accumulate in gentle flow areas. Due to the increased gravel and sand deposition, areas with elevations less than 500 m comprise compound alluvial fans formed by the Kamanashi and Fuefuki River tributaries (MLIT 2001). The average Kamanashi and Fuefuki River streamflows in the central Kofu basin are approximately 10 and 20 m3/s, respectively, whereas that of Fuji River at the basin's outlet is approximately 72 m3/s (Shrestha & Kazama 2007).
Figure 1

Sampling sites of precipitation (squares), groundwater (black dots), and river water (gray dots).

Figure 1

Sampling sites of precipitation (squares), groundwater (black dots), and river water (gray dots).

The Midaigawa and Kamanashigawa alluvial fans (area = 90 km2) were formed in the western Kofu basin (Figure 1) due to sand and gravel deposition by the Midai and Kamanashi River, transported from steep mountain areas. The central and upper parts of the Kamanashigawa alluvial fan consist of gravel, with a depth of 75–95 m; the lower part is composed of clay soil, covered with gravel, with a depth of 15–25 m. The Midaigawa alluvial fan is formed of gravel, transported by the Midai River, without any clay soil. The Midai and Kamanashi riverbeds are raised above the Midaigawa and Kamanashigawa alluvial fans, due to sediment deposition inside the riverbanks (MLIT 2001).

Kofu basin lies in an inland region; therefore, its temperature exhibits extreme variations between summer and winter. The observed metrological data of the Japan Metrological Agency (JMA) (2008) indicates that typically, summers are hot and humid, and winters are cold, with average temperatures of 26 °C and 3 °C, respectively. Annual rainfall in Kofu basin is as little as 1,135 mm in the lower areas, but reaches up to 2,500 mm in the high stands. The entire basin receives a mean annual precipitation of approximately 2,100 mm; approximately 75% of the annual precipitation occurs during the monsoon season from May to October.

Sampling and chemical analysis

Monthly precipitation samples were collected in the period August 2003–July 2004 at a station (elevation = 250 m) in the Midaigawa alluvial fan (Figure 1, P1). Additionally, to determine the LMWL in this area, event-wise precipitation samples were acquired in the period March 2007–February 2008 at a station (elevation = 308 m) in University of Yamanashi premises, located approximately 6 km east of the Kamanashigawa alluvial fan (Figure 1, P2). All the precipitation samples were collected in polyethylene plastic bottles attached to funnels with an anti-evaporation cap. River water and groundwater samples were collected between January and December 2007 for chemical and isotopic (oxygen and hydrogen) analyses. Sampling was carried out during both wet and dry periods on a bimonthly basis from the rivers (Midai and Kamanashi) and from domestic wells (depth being 20–50 m) (Figure 1).

Hydrogen and oxygen isotope ratios were expressed as δD and δ18O, respectively, where δ = [(Rsample/Rstandard) −1] × 1000 (‰), and R is D/H or 18O/16O in the sampled water (Rsample) or standard mean ocean water (Rstandard). The δ18O and δD were analyzed using an isotope ratio mass spectrometer (IRMS) (Sercon, ANCA20-20, UK) after establishing equilibrium with CO2 gas for 18O and H2 gas for D. Each sample gas was extracted using an auto-sampling system (Sercon, WES, UK). The analytical precision was 0.1‰ for δ18O and 1‰ for δD.

RESULTS

Precipitation isotopic compositions

The δ18O and δD values of precipitation can provide important information about hydrogeological processes and atmospheric circulation. Table 1 presents the results of the stable isotope analyses in the two alluvial fans of Kofu basin, Japan. At P1, the δ18O and δD significantly varies during August 2003–July 2004; the δ18O values range from −14.1 to −6.8‰, and the δD values range from −90 to −42‰. The δ18O and δD in the P2 samples acquired during March 2007–February 2008 also vary; δ18O values range from −18.0 to −0.6‰, and δD values range from −134 to −1‰. However, the variability magnitude differs between the two sites (Table 1). Seasonal changes in δD and δ18O are not expressed in these locations (Figure 2).
Table 1

Stable isotopic composition and metrological data

Location Collected day Rain fall mm Temperature °C δD ‰ δ18O ‰ d-excess ‰ 
P1 Aug-03 291 25.8 −55 −8.3 11 
 Sep-03 164 23.7 −85 −12.0 11 
 Oct-03 72 15.4 −42 −7.2 16 
 Nov-03 157 13 −49 −8.6 20 
 Dec-03 21 6.2 −42 −8.2 23 
 Jan-04 13 2.8 −90 −14.1 22 
 Feb-04 27 5.9 −50 −8.3 13 
 Mar-04 55 8.1 −77 −11.4 15 
 Apr-04 73 15.4 −76 −11.5 16 
 May-04 113 20 −73 −10.5 11 
 Jun-04 111 23.4 −66 −9.5 10 
 Jul-04 49 27.9 −45 −6.8 10 
P2 12-Mar-07 6.9 −43.7 −8.3 22 
 26-Mar-07 10 12.7 −55.1 −8.1 10 
 30-Mar-07 14.2 −26.4 −4.4 
 09-Apr-07 12.3 −5.6 −2.7 16 
 14-Apr-07 10 15.6 −27.4 −5.5 17 
 19-Apr-07 21 9.7 −85.5 −11.9 10 
 26-Apr-07 18 13.7 −55.0 −8.2 13 
 30-Apr-07 11.6 −23.6 −4.3 11 
 07-May-07 30 17.1 −74.2 −10.2 
 11-May-07 17.5 −0.5 −0.6 
 18-May-07 12 14.5 −103.9 −13.6 
 26-May-07 25 16.5 −57.6 −8.4 
 01-Jun-07 10 17.5 −40.0 −6.1 
 09-Jun-07 13 19.3 −38.9 −6.1 10 
 13-Jun-07 16 23.3 −54.8 −8.6 14 
 15-Jun-07 19.3 −55.5 −7.3 
 25-Jun-07 24 22.6 −71.3 −9.9 
 01-Jul-07 13 24.9 −68.9 −9.0 
 02-Jul-07 22.6 −60.8 −8.3 
 05-Jul-07 12 23.8 −77.5 −10.5 
 12-Jul-07 22.4 −44.1 −6.1 
 13-Jul-07 11 22.7 −88.3 −11.3 
 15-Jul-07 199 23.2 −117.0 −15.5 
 18-Jul-07 48 22.7 −82.6 −11.4 
 21-Jul-07 23.4 −38.7 −5.5 
 23-Jul-07 48 26.8 −33.1 −5.4 10 
 31-Jul-07 79 24.7 −75.5 −10.5 
 17-Aug-07 35 29.4 −46.5 −6.8 
 20-Aug-07 44 27.3 −53.5 −7.5 
 26-Aug-07 45 26.7 −79.0 −10.5 
 01-Sep-07 24.2 −51.5 −6.9 
 07-Sep-07 195 25.9 −45.1 −6.2 
 12-Sep-07 107 24.0 −75.4 −10.5 
 25-Sep-07 21 23.6 −22.2 −4.0 10 
 01-Oct-07 59 17.3 −41.1 −7.1 15 
 05-Oct-07 11 17.8 −24.7 −4.7 13 
 16-Oct-07 17.2 −12.9 −3.6 16 
 21-Oct-07 30 14.8 −22.3 −5.5 22 
 30-Oct-07 111 14.7 −124.5 −16.4 
 31-Oct-07 16.7 −15.3 −3.2 10 
 07-Nov-07 10 14.8 −49.5 −7.5 11 
 12-Nov-07 10 13.7 −35.4 −6.2 14 
 04-Dec-07 5.6 −28.9 −5.4 15 
 12-Dec-07 6.5 −33.7 −5.3 
 14-Dec-07 12 8.9 −38.2 −6.9 17 
 23-Dec-07 27 6.2 −109.6 −15.8 16 
 31-Dec-07 40 5.8 −74.2 −11.1 15 
 14-Jan-08 2.5 −31.2 −4.8 
 22-Jan-08 0.3 −115.5 −15.2 
 24-Jan-08 12 2.0 −68.6 −10.2 13 
 04-Feb-08 24 2.5 −134.2 −18.0 10 
 06-Feb-08 0.5 −74.3 −11.2 18 
 11-Feb-08 16 2.5 −72.0 −12.7 29 
 13-Feb-08 2.0 −62.1 −10.1 18 
 27-Feb-08 2.5 −77.3 −11.6 15 
Location Collected day Rain fall mm Temperature °C δD ‰ δ18O ‰ d-excess ‰ 
P1 Aug-03 291 25.8 −55 −8.3 11 
 Sep-03 164 23.7 −85 −12.0 11 
 Oct-03 72 15.4 −42 −7.2 16 
 Nov-03 157 13 −49 −8.6 20 
 Dec-03 21 6.2 −42 −8.2 23 
 Jan-04 13 2.8 −90 −14.1 22 
 Feb-04 27 5.9 −50 −8.3 13 
 Mar-04 55 8.1 −77 −11.4 15 
 Apr-04 73 15.4 −76 −11.5 16 
 May-04 113 20 −73 −10.5 11 
 Jun-04 111 23.4 −66 −9.5 10 
 Jul-04 49 27.9 −45 −6.8 10 
P2 12-Mar-07 6.9 −43.7 −8.3 22 
 26-Mar-07 10 12.7 −55.1 −8.1 10 
 30-Mar-07 14.2 −26.4 −4.4 
 09-Apr-07 12.3 −5.6 −2.7 16 
 14-Apr-07 10 15.6 −27.4 −5.5 17 
 19-Apr-07 21 9.7 −85.5 −11.9 10 
 26-Apr-07 18 13.7 −55.0 −8.2 13 
 30-Apr-07 11.6 −23.6 −4.3 11 
 07-May-07 30 17.1 −74.2 −10.2 
 11-May-07 17.5 −0.5 −0.6 
 18-May-07 12 14.5 −103.9 −13.6 
 26-May-07 25 16.5 −57.6 −8.4 
 01-Jun-07 10 17.5 −40.0 −6.1 
 09-Jun-07 13 19.3 −38.9 −6.1 10 
 13-Jun-07 16 23.3 −54.8 −8.6 14 
 15-Jun-07 19.3 −55.5 −7.3 
 25-Jun-07 24 22.6 −71.3 −9.9 
 01-Jul-07 13 24.9 −68.9 −9.0 
 02-Jul-07 22.6 −60.8 −8.3 
 05-Jul-07 12 23.8 −77.5 −10.5 
 12-Jul-07 22.4 −44.1 −6.1 
 13-Jul-07 11 22.7 −88.3 −11.3 
 15-Jul-07 199 23.2 −117.0 −15.5 
 18-Jul-07 48 22.7 −82.6 −11.4 
 21-Jul-07 23.4 −38.7 −5.5 
 23-Jul-07 48 26.8 −33.1 −5.4 10 
 31-Jul-07 79 24.7 −75.5 −10.5 
 17-Aug-07 35 29.4 −46.5 −6.8 
 20-Aug-07 44 27.3 −53.5 −7.5 
 26-Aug-07 45 26.7 −79.0 −10.5 
 01-Sep-07 24.2 −51.5 −6.9 
 07-Sep-07 195 25.9 −45.1 −6.2 
 12-Sep-07 107 24.0 −75.4 −10.5 
 25-Sep-07 21 23.6 −22.2 −4.0 10 
 01-Oct-07 59 17.3 −41.1 −7.1 15 
 05-Oct-07 11 17.8 −24.7 −4.7 13 
 16-Oct-07 17.2 −12.9 −3.6 16 
 21-Oct-07 30 14.8 −22.3 −5.5 22 
 30-Oct-07 111 14.7 −124.5 −16.4 
 31-Oct-07 16.7 −15.3 −3.2 10 
 07-Nov-07 10 14.8 −49.5 −7.5 11 
 12-Nov-07 10 13.7 −35.4 −6.2 14 
 04-Dec-07 5.6 −28.9 −5.4 15 
 12-Dec-07 6.5 −33.7 −5.3 
 14-Dec-07 12 8.9 −38.2 −6.9 17 
 23-Dec-07 27 6.2 −109.6 −15.8 16 
 31-Dec-07 40 5.8 −74.2 −11.1 15 
 14-Jan-08 2.5 −31.2 −4.8 
 22-Jan-08 0.3 −115.5 −15.2 
 24-Jan-08 12 2.0 −68.6 −10.2 13 
 04-Feb-08 24 2.5 −134.2 −18.0 10 
 06-Feb-08 0.5 −74.3 −11.2 18 
 11-Feb-08 16 2.5 −72.0 −12.7 29 
 13-Feb-08 2.0 −62.1 −10.1 18 
 27-Feb-08 2.5 −77.3 −11.6 15 
Figure 2

Temporal variations of δD, δ18O, d-excess, rainfall amount, and temperature of precipitation at the lower part of Kofu basin: P1 (a) and P2 (b).

Figure 2

Temporal variations of δD, δ18O, d-excess, rainfall amount, and temperature of precipitation at the lower part of Kofu basin: P1 (a) and P2 (b).

On the contrary, d-excess shows distinct seasonal variability. It generally ranges from 10 to 23‰. Relatively higher d-excess values (from 15 to 23‰) are detected corresponding to the pre-monsoon seasons (October–April 2003/2004), whereas they are low (10 to 11‰) for the monsoon seasons (August–September 2003 and May–July 2004) at location P1 (Figure 2(a)). The d-excess value of event-wise precipitation samples also suggests seasonal variability; pre-monsoon seasons (March 2007–April 2007 and October 2007–February 2008) are associated with a high frequency of increased d-excess values (>10‰), whereas the monsoon season (May–September 2007) exhibits a high-frequency of low d-excess values (<10‰) at location P2 (Figure 2(b)). These results are consistent with data reported by other researchers (e.g., Asano et al. 2002; Lee & Kim 2007; Yeh et al. 2011), which indicated that precipitation d-excess in Central Japan, Korea, and Eastern Taiwan has a distinct seasonal variability, high (>15‰) in the dry season and low in the wet season (<10‰).

To illustrate the isotopic data for the different seasons, two LMWLs are plotted using precipitation samples for a 12-month period, collected during August 2003–July 2004, and an additional 55 event-wise samples, collected during March 2007–February 2008 (Figure 3). The LMWLs for the pre-monsoon (October–April) and the monsoon (May–September) seasons are defined by the equations δD = 8.0 δ18O + 14.4 and δD = 8.0 δ18O + 6.8, respectively. The slope of both LMWLs is 8.0, which is identical to that of the GMWL reported by Craig (1961). The LMWLs’ intercept (6.8 for monsoon LMWL and 14.4 for pre-monsoon LMWL) is identical to that of the MWL intercept (7 for monsoon and 16 for pre-monsoon) for the Nara basin in main island of Japan reported by Nakayama et al. (2000), as shown in Figure 3. The δ18O (and δD) annual weighted mean values of the precipitation samples from P1 and P2 are −9.5‰ (−66‰) and −10.1 (−72‰), respectively.
Figure 3

Plot of δD against δ18O for the precipitation samples.

Figure 3

Plot of δD against δ18O for the precipitation samples.

River water isotopic compositions

The bimonthly river water δ18O and δD from both alluvial fans does not present significant temporal variations relative to the values measured in the precipitation samples. The δ18O (δD) is essentially constant in the Midai River at a value of −11.3 ± 0.12‰ (−77 ± 1.1‰) upstream (Table 2, R1) and at −10.9 ± 0.14‰ (−75 ± 1.6‰) downstream (Table 2, R2). In the case of the Kamanashi River, the values are −11.0 ± 0.13‰ (−76 ± 0.9‰) upstream (Table 2, R3), −10.9 ± 0.07‰ (−76 ± 0.7‰) middlestream (Table 2, R4), and −11.0 ± 0.14‰ (−76 ± 0.7‰) downstream (Table 2, R5). Spatial variations of δ18O and δD are insignificant for both rivers, probably due to lack of inflow. Additionally, δ18O and δD are less than the mean values in the precipitation samples (δ18O = −9.5‰ and −10.1‰, δD = −62.5‰ and −7.2‰). The fact that the alluvial fans are located in a range of altitudes (200 to 500 m), lower than those of the average catchment, suggests that altitude may affect δ18O and δD values. The river water d-excess value (11–13‰, Table 2) is higher than the annual precipitation d-excess values (9 and 10‰).

Table 2

Annual mean values and standard deviation of δD, δ18O, and d-excess for bimonthly groundwater and river water samples

Sample ID Fan/Location Latitude Longitude δ18O, Ave ± SD ‰ δD, Ave ± SD ‰ d-excess, Ave ± SD ‰ 
G.W. G1 Kamanashigawa 35 °39′47.42″ 138 °30′17.35″ −10.9 ± 0.08 −76 ± 0.5 12 ± 0.5 
 G2 Kamanashigawa 35 °39′11.83″ 138 °30′39.76″ −11.1 ± 0.09 −77 ± 0.7 12 ± 0.6 
 G3 Kamanashigawa 35 °39′11.82″ 138 °31′32.6″ −10.6 ± 0.15 −74 ± 1.2 11 ± 0.4 
 G4 Kamanashigawa 35 °38′.97″ 138 °30′54″ −10.8 ± 0.11 −76 ± 0.5 11 ± 0.7 
 G5 Kamanashigawa 35 °38′.92″ 138 °31′.52″ −10.6 ± 0.13 −74 ± 1.1 11 ± 0.4 
 G6 Kamanashigawa 35 °38′13.78″ 138 °30′31.25″ −11.0 ± 0.16 −76 ± 1.4 12 ± 0.7 
 G7 Kamanashigawa 35 °37′59.91″ 138 °31′.52″ −10.7 ± 0.06 −74 ± 0.4 11 ± 0.3 
 G8 Kamanashigawa 35 °38′.84″ 138 °32′.99″ −10.3 ± 0.13 −71 ± 0.9 11 ± 0.4 
 G9 Kamanashigawa 35 °37′52.86″ 138 °31′59.97″ −10.4 ± 0.12 −73 ± 0.8 10 ± 0.3 
 G10 Kamanashigawa 35 °37′24.02″ 138 °31′.21″ −10.6 ± 0.04 −74 ± 0.6 11 ± 0.5 
 G11 Kamanashigawa 35 °37′21.18″ 138 °32′.48″ −10.6 ± 0.08 −74 ± 0.6 11 ± 0.3 
 G12 Kamanashigawa 35 °36′49.09″ 138 °31′.48″ −10.9 ± 0.10 −76 ± 0.4 12 ± 0.6 
 G13 Kamanashigawa 35 °36′36.09″ 138 °30′.91″ −10.9 ± 0.10 −76 ± 0.9 11 ± 0.3 
 G14 Kamanashigawa 35 °35′59.68″ 138 °31′.07″ −10.7 ± 0.08 −75 ± 1.0 11 ± 0.5 
 G15 Kamanashigawa 35 °35′.81″ 138 °31′.86″ −10.9 ± 0.07 −76 ± 0.5 11 ± 0.5 
 G16 Kamanashigawa 35 °35′16.07″ 138 °30′.64″ −10.8 ± 0.06 −76 ± 0.8 11 ± 0.5 
 G17 Kamanashigawa 35 °34′.84″ 138 °30′.65″ −10.8 ± 0.14 −75 ± 1.1 12 ± 0.5 
 G18 Kamanashigawa 35 °34′.08″ 138 °31′.27″ −10.9 ± 0.10 −76 ± 0.6 11 ± 0.4 
 G19 Kamanashigawa 35 °36′.13″ 138 °32′.74″ −11.1 ± 0.10 −77 ± 0.6 12 ± 0.9 
 G20 Kamanashigawa 35 °36′.66″ 138 °32′.5″ −11.0 ± 0.12 −76 ± 0.3 12 ± 0.7 
 G21 Kamanashigawa 35 °36′.98″ 138 °33′.42″ −10.9 ± 0.08 −76 ± 0.6 12 ± 0.6 
 G22 Kamanashigawa 35 °35′.51″ 138 °32′.82″ −10.9 ± 0.12 −76 ± 0.9 11 ± 0.5 
 G23 Kamanashigawa 35 °35′.21″ 138 °32′.25″ −10.9 ± 0.06 −75 ± 0.4 12 ± 0.3 
 G24 Kamanashigawa 35 °37′.02″ 138 °30′.64″ −11.0 ± 0.14 −76 ± 1.1 11 ± 0.6 
 G25 Midaigawa 35 °39′.53″ 138 °27′.73″ −10.9 ± 0.07 −75 ± 0.8 12 ± 0.3 
 G26 Midaigawa 35 °39′.64″ 138 °28′.65″ −11.0 ± 0.10 −76 ± 0.9 12 ± 0.7 
 G27 Midaigawa 35 °39′.94″ 138 °29′.86″ −10.8 ± 0.12 −76 ± 1.0 11 ± 1.2 
 G28 Midaigawa 35 °39′.93″ 138 °29′.3″ −10.8 ± 0.12 −75 ± 0.7 11 ± 1.0 
 G29 Midaigawa 35 °39′.71″ 138 °29′.21″ −10.8 ± 0.16 −75 ± 1.2 11 ± 0.9 
 G30 Midaigawa 35 °39′28.54″ 138 °29′1.82″ −10.7 ± 0.14 −75 ± 1.0 11 ± 0.9 
 G31 Midaigawa 35 °38′4.21″ 138 °28′40″ −10.4 ± 0.20 −73 ± 1.4 10 ± 0.8 
 G32 Midaigawa 35 °37′.4″ 138 °27′.69″ −10.2 ± 0.16 −71 ± 1.5 10 ± 0.9 
 G33 Midaigawa 35 °39′.86″ 138 °26′.31″ −10.9 ± 0.06 −76 ± 0.4 12 ± 0.5 
 G34 Midaigawa 35 °38′.97″ 138 °26′.5″ −9.9 ± 0.10 −69 ± 0.9 10 ± 0.6 
 G35 Midaigawa 35 °38′.93″ 138 °27′38.3″ −10.6 ± 0.11 −73 ± 0.9 11 ± 1.3 
 G36 Midaigawa 35 °37′.86″ 138 °29′.4″ −10.6 ± 0.06 −74 ± 0.6 11 ± 0.5 
 G37 Midaigawa 35 °38′.18″ 138 °29′46.31″ −10.8 ± 0.02 −75 ± 0.8 12 ± 0.9 
 G38 Midaigawa 35 °37′.67″ 138 °28′.33″ −10.2 ± 0.16 −71 ± 0.8 11 ± 0.6 
 G39 Midaigawa 35 °37′.35″ 138 °27′.58″ −10.1 ± 0.11 −70 ± 0.8 10 ± 0.9 
 G40 Midaigawa 35 °37′.08″ 138 °28′.57″ −10.1 ± 0.14 −71 ± 0.8 10 ± 0.8 
 G41 Midaigawa 35 °37′.38″ 138 °28′.04″ −10.3 ± 0.10 −72 ± 0.4 10 ± 1.0 
 G42 Midaigawa 35 °36′.93″ 138 °28′.53″ −10.4 ± 0.14 −72 ± 1.1 11 ± 0.7 
 G43 Midaigawa 35 °36′.95″ 138 °29′.05″ −10.5 ± 0.10 −73 ± 0.7 12 ± 1.4 
 G44 Midaigawa 35 °36′.76″ 138 °29′.02″ −10.2 ± 0.05 −71 ± 0.7 11 ± 0.9 
 G45 Midaigawa 35 °35′.22″ 138 °30′.14″ −10.6 ± 0.13 −75 ± 0.5 11 ± 0.8 
 G46 Midaigawa 35 °35′.98″ 138 °27′.12″ −10.0 ± 0.16 −69 ± 0.5 10 ± 0.8 
 G47 Midaigawa 35 °36′.56″ 138 °28′.5″ −10.2 ± 0.09 −71 ± 1.2 11 ± 1.0 
 G48 Midaigawa 35 °35′.21″ 138 °28′.12″ −10.1 ± 0.15 −71 ± 1.5 10 ± 0.9 
 G49 Midaigawa 35 °35′.09″ 138 °29′.33″ −10.3 ± 0.12 −73 ± 1.2 10 ± 0.7 
 G50 Midaigawa 35 °35′.61″ 138 °27′24.22″ −10.2 ± 0.21 −71 ± 1.1 10 ± 0.8 
 G51 Midaigawa 35 °35′.71″ 138 °28′.94″ −10.6 ± 0.10 −74 ± 0.9 10 ± 0.9 
 G52 Midaigawa 35 °35′.72″ 138 °28′.74″ −10.5 ± 0.17 −74 ± 0.9 10 ± 1.2 
 G53 Midaigawa 35 °35′.76″ 138 °29′.49″ −10.4 ± 0.14 −72 ± 1.1 11 ± 1.1 
 G54 Midaigawa 35 °34′.16″ 138 °29′.33″ −10.4 ± 0.11 −72 ± 0.9 11 ± 0.9 
River water R1 Midaigawa Riv. 35 °39′29.65″ 138 °25'34.65″ −11.3 ± 0.12 −77 ± 1.1 13 ± 1.1 
 R2 Midaigawa Riv. 35 °40′.53″ 138 °28'43.03″ −10.9 ± 0.14 −75 ± 1.6 12 ± 0.8 
 R3 Kamanashigawa Riv. 35 °39′.62″ 138 °30'6.92″ −11.0 ± 0.13 −76 ± 0.9 12 ± 1.1 
 R4 Kamanashigawa Riv. 35 °38′.05″ 138 °30'19.13″ −10.9 ± 0.07 −76 ± 0.7 11 ± 0.3 
 R5 Kamanashigawa Riv. 35 °35′38.95″ 138 °30'33.01″ −11.0 ± 0.14 −76 ± 0.7 11 ± 1.0 
Sample ID Fan/Location Latitude Longitude δ18O, Ave ± SD ‰ δD, Ave ± SD ‰ d-excess, Ave ± SD ‰ 
G.W. G1 Kamanashigawa 35 °39′47.42″ 138 °30′17.35″ −10.9 ± 0.08 −76 ± 0.5 12 ± 0.5 
 G2 Kamanashigawa 35 °39′11.83″ 138 °30′39.76″ −11.1 ± 0.09 −77 ± 0.7 12 ± 0.6 
 G3 Kamanashigawa 35 °39′11.82″ 138 °31′32.6″ −10.6 ± 0.15 −74 ± 1.2 11 ± 0.4 
 G4 Kamanashigawa 35 °38′.97″ 138 °30′54″ −10.8 ± 0.11 −76 ± 0.5 11 ± 0.7 
 G5 Kamanashigawa 35 °38′.92″ 138 °31′.52″ −10.6 ± 0.13 −74 ± 1.1 11 ± 0.4 
 G6 Kamanashigawa 35 °38′13.78″ 138 °30′31.25″ −11.0 ± 0.16 −76 ± 1.4 12 ± 0.7 
 G7 Kamanashigawa 35 °37′59.91″ 138 °31′.52″ −10.7 ± 0.06 −74 ± 0.4 11 ± 0.3 
 G8 Kamanashigawa 35 °38′.84″ 138 °32′.99″ −10.3 ± 0.13 −71 ± 0.9 11 ± 0.4 
 G9 Kamanashigawa 35 °37′52.86″ 138 °31′59.97″ −10.4 ± 0.12 −73 ± 0.8 10 ± 0.3 
 G10 Kamanashigawa 35 °37′24.02″ 138 °31′.21″ −10.6 ± 0.04 −74 ± 0.6 11 ± 0.5 
 G11 Kamanashigawa 35 °37′21.18″ 138 °32′.48″ −10.6 ± 0.08 −74 ± 0.6 11 ± 0.3 
 G12 Kamanashigawa 35 °36′49.09″ 138 °31′.48″ −10.9 ± 0.10 −76 ± 0.4 12 ± 0.6 
 G13 Kamanashigawa 35 °36′36.09″ 138 °30′.91″ −10.9 ± 0.10 −76 ± 0.9 11 ± 0.3 
 G14 Kamanashigawa 35 °35′59.68″ 138 °31′.07″ −10.7 ± 0.08 −75 ± 1.0 11 ± 0.5 
 G15 Kamanashigawa 35 °35′.81″ 138 °31′.86″ −10.9 ± 0.07 −76 ± 0.5 11 ± 0.5 
 G16 Kamanashigawa 35 °35′16.07″ 138 °30′.64″ −10.8 ± 0.06 −76 ± 0.8 11 ± 0.5 
 G17 Kamanashigawa 35 °34′.84″ 138 °30′.65″ −10.8 ± 0.14 −75 ± 1.1 12 ± 0.5 
 G18 Kamanashigawa 35 °34′.08″ 138 °31′.27″ −10.9 ± 0.10 −76 ± 0.6 11 ± 0.4 
 G19 Kamanashigawa 35 °36′.13″ 138 °32′.74″ −11.1 ± 0.10 −77 ± 0.6 12 ± 0.9 
 G20 Kamanashigawa 35 °36′.66″ 138 °32′.5″ −11.0 ± 0.12 −76 ± 0.3 12 ± 0.7 
 G21 Kamanashigawa 35 °36′.98″ 138 °33′.42″ −10.9 ± 0.08 −76 ± 0.6 12 ± 0.6 
 G22 Kamanashigawa 35 °35′.51″ 138 °32′.82″ −10.9 ± 0.12 −76 ± 0.9 11 ± 0.5 
 G23 Kamanashigawa 35 °35′.21″ 138 °32′.25″ −10.9 ± 0.06 −75 ± 0.4 12 ± 0.3 
 G24 Kamanashigawa 35 °37′.02″ 138 °30′.64″ −11.0 ± 0.14 −76 ± 1.1 11 ± 0.6 
 G25 Midaigawa 35 °39′.53″ 138 °27′.73″ −10.9 ± 0.07 −75 ± 0.8 12 ± 0.3 
 G26 Midaigawa 35 °39′.64″ 138 °28′.65″ −11.0 ± 0.10 −76 ± 0.9 12 ± 0.7 
 G27 Midaigawa 35 °39′.94″ 138 °29′.86″ −10.8 ± 0.12 −76 ± 1.0 11 ± 1.2 
 G28 Midaigawa 35 °39′.93″ 138 °29′.3″ −10.8 ± 0.12 −75 ± 0.7 11 ± 1.0 
 G29 Midaigawa 35 °39′.71″ 138 °29′.21″ −10.8 ± 0.16 −75 ± 1.2 11 ± 0.9 
 G30 Midaigawa 35 °39′28.54″ 138 °29′1.82″ −10.7 ± 0.14 −75 ± 1.0 11 ± 0.9 
 G31 Midaigawa 35 °38′4.21″ 138 °28′40″ −10.4 ± 0.20 −73 ± 1.4 10 ± 0.8 
 G32 Midaigawa 35 °37′.4″ 138 °27′.69″ −10.2 ± 0.16 −71 ± 1.5 10 ± 0.9 
 G33 Midaigawa 35 °39′.86″ 138 °26′.31″ −10.9 ± 0.06 −76 ± 0.4 12 ± 0.5 
 G34 Midaigawa 35 °38′.97″ 138 °26′.5″ −9.9 ± 0.10 −69 ± 0.9 10 ± 0.6 
 G35 Midaigawa 35 °38′.93″ 138 °27′38.3″ −10.6 ± 0.11 −73 ± 0.9 11 ± 1.3 
 G36 Midaigawa 35 °37′.86″ 138 °29′.4″ −10.6 ± 0.06 −74 ± 0.6 11 ± 0.5 
 G37 Midaigawa 35 °38′.18″ 138 °29′46.31″ −10.8 ± 0.02 −75 ± 0.8 12 ± 0.9 
 G38 Midaigawa 35 °37′.67″ 138 °28′.33″ −10.2 ± 0.16 −71 ± 0.8 11 ± 0.6 
 G39 Midaigawa 35 °37′.35″ 138 °27′.58″ −10.1 ± 0.11 −70 ± 0.8 10 ± 0.9 
 G40 Midaigawa 35 °37′.08″ 138 °28′.57″ −10.1 ± 0.14 −71 ± 0.8 10 ± 0.8 
 G41 Midaigawa 35 °37′.38″ 138 °28′.04″ −10.3 ± 0.10 −72 ± 0.4 10 ± 1.0 
 G42 Midaigawa 35 °36′.93″ 138 °28′.53″ −10.4 ± 0.14 −72 ± 1.1 11 ± 0.7 
 G43 Midaigawa 35 °36′.95″ 138 °29′.05″ −10.5 ± 0.10 −73 ± 0.7 12 ± 1.4 
 G44 Midaigawa 35 °36′.76″ 138 °29′.02″ −10.2 ± 0.05 −71 ± 0.7 11 ± 0.9 
 G45 Midaigawa 35 °35′.22″ 138 °30′.14″ −10.6 ± 0.13 −75 ± 0.5 11 ± 0.8 
 G46 Midaigawa 35 °35′.98″ 138 °27′.12″ −10.0 ± 0.16 −69 ± 0.5 10 ± 0.8 
 G47 Midaigawa 35 °36′.56″ 138 °28′.5″ −10.2 ± 0.09 −71 ± 1.2 11 ± 1.0 
 G48 Midaigawa 35 °35′.21″ 138 °28′.12″ −10.1 ± 0.15 −71 ± 1.5 10 ± 0.9 
 G49 Midaigawa 35 °35′.09″ 138 °29′.33″ −10.3 ± 0.12 −73 ± 1.2 10 ± 0.7 
 G50 Midaigawa 35 °35′.61″ 138 °27′24.22″ −10.2 ± 0.21 −71 ± 1.1 10 ± 0.8 
 G51 Midaigawa 35 °35′.71″ 138 °28′.94″ −10.6 ± 0.10 −74 ± 0.9 10 ± 0.9 
 G52 Midaigawa 35 °35′.72″ 138 °28′.74″ −10.5 ± 0.17 −74 ± 0.9 10 ± 1.2 
 G53 Midaigawa 35 °35′.76″ 138 °29′.49″ −10.4 ± 0.14 −72 ± 1.1 11 ± 1.1 
 G54 Midaigawa 35 °34′.16″ 138 °29′.33″ −10.4 ± 0.11 −72 ± 0.9 11 ± 0.9 
River water R1 Midaigawa Riv. 35 °39′29.65″ 138 °25'34.65″ −11.3 ± 0.12 −77 ± 1.1 13 ± 1.1 
 R2 Midaigawa Riv. 35 °40′.53″ 138 °28'43.03″ −10.9 ± 0.14 −75 ± 1.6 12 ± 0.8 
 R3 Kamanashigawa Riv. 35 °39′.62″ 138 °30'6.92″ −11.0 ± 0.13 −76 ± 0.9 12 ± 1.1 
 R4 Kamanashigawa Riv. 35 °38′.05″ 138 °30'19.13″ −10.9 ± 0.07 −76 ± 0.7 11 ± 0.3 
 R5 Kamanashigawa Riv. 35 °35′38.95″ 138 °30'33.01″ −11.0 ± 0.14 −76 ± 0.7 11 ± 1.0 

Isotopic compositions of groundwater

The δD and δ18O values in the groundwater samples do not exhibit any seasonal variation; the standard deviations for the two seasons are below 1.5‰ and 0.2‰, respectively (Table 2). However, relatively wide spatial variation is found in the groundwater annual mean isotope values, with δ18O ranging from −11.0 to −9.9‰ and δD from −76 to −69‰ in the Midai alluvial fan. For the Kamanashigawa alluvial fan, δ18O and δD were in the range of −11.1 to −10.3‰ and −77 to −71‰, respectively (Table 2). In terms of spatial distribution, the δ18O is relatively low toward the Kamanashi River side of both alluvial fans, whereas higher values are detected towards the central and the eastern part of the Midaigawa and the Kamanashigawa alluvial fan, respectively (Figure 4).
Figure 4

Spatial distribution of δ18O values in groundwater samples.

Figure 4

Spatial distribution of δ18O values in groundwater samples.

The annual mean δD and δ18O plots for groundwater, river water, and precipitation are presented in Figure 5, along with the LMWLs for the monsoon and the pre-monsoon seasons. The groundwater isotopic values from both alluvial fans apparently deviate from the LMWLs in both seasons. The regression line's slope is 6.5, different from that of the LMWL.
Figure 5

Plots of δD against δ18O of the groundwater, river water, and weighted mean values for the precipitation samples.

Figure 5

Plots of δD against δ18O of the groundwater, river water, and weighted mean values for the precipitation samples.

The distribution of d-excess in groundwater and river water is shown in Figure 6. Relatively high groundwater d-excess values are detected toward the Kamanashi River side of the alluvial fans. On the other hand, relatively lower d-excess values are measured in the central area of the Midaigawa and in the eastern part of the Kamanashigawa alluvial fan. Previous researchers have reported a positive correlation between rainwater d-excess values and altitude (Rindsberger et al. 1990; Cruz San Julian et al. 1992).
Figure 6

Spatial distribution of the d-excess values in the groundwater samples.

Figure 6

Spatial distribution of the d-excess values in the groundwater samples.

DISCUSSION

Precipitation and river water isotopic differences

The results of the present study illustrate that precipitation values δ18O and δD exhibit a wide temporal variability (Figure 2), unlike river water and groundwater δ18O and δD values (Table 2). The isotopic fractionation by the rainfall recharge significantly affects the groundwater isotopic values. However, the amplitude of the seasonal fraction for the isotopic values in groundwater is expected to decrease with increasing residence time (Maloszewski & Zuber 1993; Asano et al. 2002). Asano et al. (2002) reported the relationship between the estimated residence time of the spring water and amplitude of the isotopic fractionation. It shows, in the case of groundwater with more than 1 year residence time, the isotopic amplification pattern will not have a significant regression. In addition, Mizutani et al. (2001) hypothesized that if the groundwater system is predominantly recharged through infiltration of precipitation, the groundwater isotopic composition will directly reflect the isotopic values of precipitation.

The two-river water (Midai and Kamanashi River) is represented by depleted isotopic composition rather than the weighted average of isotopic composition of precipitation at plain areas. The maximum altitudes of the Midai and Kamanashi River watersheds are approximately 2,400 m and 2,500 m, respectively. Therefore, it is possible that the different isotopic values between river water and precipitation result from this altitude difference. This hypothesis is further corroborated by previously reported evidence such as the depletion in surface water annual mean δ18O and δD values compared with the precipitation weighted mean in alluvial fans; and the decreasing of surface water 18O and D content with increasing altitude (Dansgaard 1964; Siegenthaler & Oeschger 1980; Poage & Chamberlain 2001; Yeh et al. 2011; Liu & Yamanaka 2012). Waseda & Nakai (1983) also indicated the altitude effect as an important factor determining the surface and meteoric waters’ isotopic compositions in central and northeast Japan, and they reported an average altitude effect on surface water δ18O and δD values in those areas as −0.25‰ and −2.0‰ per 100 m, respectively.

In the plain area, river water d-excess exhibits a higher value (11–13‰) than the annual precipitation (9 and 10‰), suggesting the evaporation effects on different elevations or two air-masses contributed to the precipitation event, one with a monsoon precipitation (low d-excess) and one with a pre-monsoon precipitation (high d-excess).

Seasonal variation of d-excess values of precipitation samples

The d-excess is defined as the excess deuterium that cannot be accounted by equilibrium fractionation between water and vapor. Since condensation is most often an equilibrium process, d-excess is an indicator of kinetic fractionation during evaporation, governed by molecular diffusivity of isotopic molecular species (Dansgaard 1964; Clark & Fritz 1997). Although temperature and wind speed can influence the kinetic fractionation, relative humidity is the most important factor. The d-excess of the GMWL has a value of ∼10 representing evaporation at ∼85% relative humidity. However, the d-excess value in the regional precipitation can be greater than ten if the evaporation in the source region takes place under lower humidity (Gat & Carmi 1970).

The d-excess in precipitation was generally higher in winter (d > 10) and lower in summer in Japan. Dansgaard (1964) and Waseda & Nakai (1983) have stated that the extremely high d-excess that appeared in Japanese precipitation during the winter season resulted from rapid evaporation induced by dry, continental air masses brought from the Sea of Japan. In the case of the meteoric condition, the winter season dominates NE pre-monsoon winds. The resultant precipitation will have high d-excess in Japan. Katsuyama et al. (2015) have studied a seasonal variation of d-excess values of precipitation at three locations crossing a main island of Japan; Sea of Japan side (Tottori prefecture), Sea of Japan side (Shiga prefecture), and Pacific Ocean side (Nara prefecture). The climate conditions are clearly different among these stations. In the Sea of Japan side (Tottori prefecture), much snow falls from December to March with low air temperatures. In Shiga, there is less snowfall but much more rainfall during summer. Summer rainfall is more plentiful in the Pacific Ocean side (Nara prefecture). However, similar sinusoidal d-excess variations in precipitation were repeated at these three stations, i.e., higher during winter and lower during summer. The sinusoidal pattern is caused by the contribution of the dual moisture sources predominantly from the Pacific Ocean in summer and predominantly from the Sea of Japan in winter (Waseda & Nakai 1983; Araguás-Araguás et al. 1998). Tase et al. (1997) also reported that this seasonal pattern was commonly observed at six stations in Kanto, Shikoku, and Kyushu regions in Japan. Hence, it can be inferred that dominant moisture sources are a significant influence on the seasonal variation of d-excess values in precipitation in our study area as well.

Identification of groundwater recharge sources and their mixing

Various recharge sources for alluvial aquifer systems are identified by groundwater and recharge isotopic and chemical characteristics (Vanderzalm et al. 2011). Plotting δ18O versus δD (Figure 5) shows that groundwater samples from the Midaigawa and Kamanashigawa alluvial fans fall between river water and weighted average precipitation, indicating a possibility of recharge from both sources and their mixing in the aquifer.

The greater negativity of the δ18O in the shallow groundwater samples toward the Midai River side in the Kamanashigawa alluvial fan (Figure 4) and the virtually identical δ18O in the groundwater and river water toward the riverside of the Midai and Kamanashi Rivers in the Midaigawa alluvial fan, both suggest groundwater recharge from the rivers. On the other hand, samples most enriched in δ18O and δD, having values of −9.9‰ and −69‰, respectively, are found in the groundwater (G34), well located at an altitude (490 m) higher than the riverbed perimeter (460 m). These values are close to the annual mean δ18O and δD values of precipitation in the Midaigawa alluvial fan (−9.5‰ and −66‰), suggesting that precipitation is the primary source of groundwater at G34. Such high δ18O values of groundwater were also detected in the eastern part of the Kamanashigawa alluvial fan and southern part of the Midaigawa alluvial fan. This isotopic distribution suggests that the groundwater in this area might be recharged through precipitation infiltration.

From the above discussions, it is clear that groundwater in the alluvial fans is recharged from both river water drained from mountain and direct precipitation infiltrates; however, each source's contribution may spatially vary within the study area. To reveal any mixing of the different sources of groundwater in the aquifer, d-excess values of annual precipitation, river water, and groundwater are plotted against δ18O (Figure 7). The values of the groundwater samples plotted between those of precipitation and river water samples suggests that the d-excess varies according to mixing between river water and annual precipitation.
Figure 7

Plot of d-excess and δ18O values of the groundwater, river water, and precipitation samples.

Figure 7

Plot of d-excess and δ18O values of the groundwater, river water, and precipitation samples.

Contribution of the different sources and timing of the groundwater recharge

The relative contributions to the groundwater recharge from the two distinct sources (i.e., precipitation and river water), which are important in water-budget studies, are inferred through the mass-balance analysis of the groundwater, river water, and precipitation isotopic values corresponding to different seasons from their d-excess values. To estimate the contribution of each source, this study considers a mass-balance of δ using a simple linear mixing model shown in Equations (1) and (2): 
formula
1
 
formula
2
Equations (1) and (2) can convert to Equation (3) as below: 
formula
3
where Xriver water and (1 – Xriver water) are the contribution of river water and precipitation infiltration, respectively. We used averaged δ18O values of Kamanashigawa River and Midaigawa River water for δriver water (Table 3). For the δprecipitation we used averaged δ18O values of precipitation samples at P1 and P2 (Table 3). Calculations based on Equations (1) and (2) in the wells considered in this study indicate that the contribution of river water from mountain watersheds to groundwater recharge ranges from 38% to 100% in the Kamanashigawa alluvial fan and from 7% to 99% in Midaigawa (Table 4). Although this suggests a relatively higher contribution of river water (originating from the mountain watershed) to the groundwater recharge in the Kamanashigawa alluvial fan, the wider range of contribution in different locations exemplifies their non-uniform distribution throughout the alluvial fans. Cluster analysis of the contributions from the two sources within the study area (Figure 8) demonstrates that in the central part of Kamanashigawa (locations G3, G5, G7, G8, G9, G10, and G11), precipitation contributes 29–62%; its contribution tends to increase with the distance from the Kamanashi River. In the case of Midaigawa, river water and precipitation recharge differs greatly between the different regions. The northern region (within 2 km from the Midai and Kamanashi rivers, locations G25, G26, G27, G28, G29, G30, G33, and G37) receives 77–99% from river water, whereas the southern region (adjacent to Kushigata Mountain, G32, G38, G39, G40, G41, G42, G44, G46, G47, G48, and G50) receives 51–85% from precipitation.
Table 3

Averaged values of isotope data of river water and precipitation samples

  Kamanashi alluvial fan Midai alluvial fan 
Xriver water δ18O (‰) −11.0 −11.1 
Xprecipitation δ18O (‰) −9.8 −9.8 
Xpre-monsoon d-excess 14.4 14.4 
Xmonsoon d-excess 6.8 6.8 
  Kamanashi alluvial fan Midai alluvial fan 
Xriver water δ18O (‰) −11.0 −11.1 
Xprecipitation δ18O (‰) −9.8 −9.8 
Xpre-monsoon d-excess 14.4 14.4 
Xmonsoon d-excess 6.8 6.8 
Table 4

Calculated values of contribution for river water and precipitation infiltration in groundwater samples and also pre-monsoon and monsoon contributions

    Xriver water Xprecipitation Xpre-monsoon Xmonsoon 
ID Fan/Location Latitude Longitude 
G1 Kamanashigawa 35 °39′47.42″ 138 °30′17.35″ 93 67 33 
G2  35 °39′.83″ 138 °30′39.76″ 100 66 34 
G3  35 °39′.82″ 138 °31′.6″ 67 33 56 44 
G4  35 °38′.97″ 138 °30′54″ 85 15 56 44 
G5  35 °38′27.92″ 138 °31′47.52″ 70 30 54 46 
G6  35 °38′.78″ 138 °30′.25″ 100 66 34 
G7  35 °37′.91″ 138 °31′.52″ 71 29 56 44 
G8  35 °38′.84″ 138 °32′.99″ 38 62 58 42 
G9  35 °37′.86″ 138 °31′.97″ 53 47 46 54 
G10  35 °37′24.02″ 138 °31′3.21″ 67 33 56 44 
G11  35 °37′21.18″ 138 °32′.48″ 68 32 50 50 
G12  35 °36′.09″ 138 °31′.48″ 93 63 37 
G13  35 °36′.09″ 138 °30′.91″ 91 61 39 
G14  35 °35′59.68″ 138 °31′.07″ 78 22 57 43 
G15  35 °35′28.81″ 138 °31′.86″ 91 61 39 
G16  35 °35′16.07″ 138 °30′.64″ 85 15 53 47 
G17  35 °34′45.84″ 138 °30′24.65″ 83 17 62 38 
G18  35 °34′.08″ 138 °31′.27″ 92 59 41 
G19  35 °36′27.13″ 138 °32′.74″ 100 64 36 
G20  35 °36′.66″ 138 °32′.5″ 100 68 32 
G21  35 °36′.98″ 138 °33′.42″ 93 64 36 
G22  35 °35′.51″ 138 °32′34.82″ 89 11 57 43 
G23  35 °35′.21″ 138 °32′.25″ 91 64 36 
G24  35 °37′.02″ 138 °30′.64″ 97 60 40 
G25 Midaigawa 35 °39′53.53″ 138 °27′.73″ 88 12 63 37 
G26  35 °39′59.64″ 138 °28′40.65″ 99 62 38 
G27  35 °39′.94″ 138 °29′.86″ 85 15 53 47 
G28  35 °39′.93″ 138 °29′35.3″ 85 15 61 39 
G29  35 °39′3.71″ 138 °29′30.21″ 82 18 57 43 
G30  35 °39′28.54″ 138 °29′.82″ 77 23 58 42 
G31  35 °38′4.21″ 138 °28′40″ 49 51 46 54 
G32  35 °37′.4" 138 °27′54.69″ 34 66 45 55 
G33  35 °39′.86″ 138 °26′38.31″ 95 65 35 
G34  35 °38′28.97″ 138 °26′12.5″ 93 41 59 
G35  35 °38′16.93″ 138 °27′38.3″ 64 36 61 39 
G36  35 °37′43.86″ 138 °29′51.4″ 70 30 55 45 
G37  35 °38′32.18″ 138 °29′46.31″ 85 15 64 36 
G38  35 °37′48.67″ 138 °28′44.33″ 37 63 52 48 
G39  35 °37′0.35″ 138 °27′24.58″ 22 78 46 54 
G40  35 °37′10.08″ 138 °28′26.57″ 26 74 44 56 
G41  35 °37′.38″ 138 °28′50.04″ 38 62 47 53 
G42  35 °36′.93″ 138 °28′12.53″ 49 51 51 49 
G43  35 °36′.95″ 138 °29′56.05″ 61 39 64 36 
G44  35 °36′10.76″ 138 °29′15.02″ 29 71 50 50 
G45  35 °35′.22″ 138 °30′15.14″ 70 30 50 50 
G46  35 °35′.98″ 138 °27′12.12″ 15 85 47 53 
G47  35 °36′.56″ 138 °28′19.5″ 34 66 50 50 
G48  35 °35′.21″ 138 °28′39.12″ 23 77 39 61 
G49  35 °35′18.09″ 138 °29′0.33″ 44 56 42 58 
G50  35 °35′.61″ 138 °27′24.22″ 30 70 49 51 
G51  35 °35′.71″ 138 °28′20.94″ 66 34 46 54 
G52  35 °35′13.72″ 138 °28′31.74″ 57 43 47 53 
G53  35 °35′.76″ 138 °29′46.49″ 52 48 55 45 
G54  35 °34′38.16″ 138 °29′21.33″ 52 48 54 46 
    Xriver water Xprecipitation Xpre-monsoon Xmonsoon 
ID Fan/Location Latitude Longitude 
G1 Kamanashigawa 35 °39′47.42″ 138 °30′17.35″ 93 67 33 
G2  35 °39′.83″ 138 °30′39.76″ 100 66 34 
G3  35 °39′.82″ 138 °31′.6″ 67 33 56 44 
G4  35 °38′.97″ 138 °30′54″ 85 15 56 44 
G5  35 °38′27.92″ 138 °31′47.52″ 70 30 54 46 
G6  35 °38′.78″ 138 °30′.25″ 100 66 34 
G7  35 °37′.91″ 138 °31′.52″ 71 29 56 44 
G8  35 °38′.84″ 138 °32′.99″ 38 62 58 42 
G9  35 °37′.86″ 138 °31′.97″ 53 47 46 54 
G10  35 °37′24.02″ 138 °31′3.21″ 67 33 56 44 
G11  35 °37′21.18″ 138 °32′.48″ 68 32 50 50 
G12  35 °36′.09″ 138 °31′.48″ 93 63 37 
G13  35 °36′.09″ 138 °30′.91″ 91 61 39 
G14  35 °35′59.68″ 138 °31′.07″ 78 22 57 43 
G15  35 °35′28.81″ 138 °31′.86″ 91 61 39 
G16  35 °35′16.07″ 138 °30′.64″ 85 15 53 47 
G17  35 °34′45.84″ 138 °30′24.65″ 83 17 62 38 
G18  35 °34′.08″ 138 °31′.27″ 92 59 41 
G19  35 °36′27.13″ 138 °32′.74″ 100 64 36 
G20  35 °36′.66″ 138 °32′.5″ 100 68 32 
G21  35 °36′.98″ 138 °33′.42″ 93 64 36 
G22  35 °35′.51″ 138 °32′34.82″ 89 11 57 43 
G23  35 °35′.21″ 138 °32′.25″ 91 64 36 
G24  35 °37′.02″ 138 °30′.64″ 97 60 40 
G25 Midaigawa 35 °39′53.53″ 138 °27′.73″ 88 12 63 37 
G26  35 °39′59.64″ 138 °28′40.65″ 99 62 38 
G27  35 °39′.94″ 138 °29′.86″ 85 15 53 47 
G28  35 °39′.93″ 138 °29′35.3″ 85 15 61 39 
G29  35 °39′3.71″ 138 °29′30.21″ 82 18 57 43 
G30  35 °39′28.54″ 138 °29′.82″ 77 23 58 42 
G31  35 °38′4.21″ 138 °28′40″ 49 51 46 54 
G32  35 °37′.4" 138 °27′54.69″ 34 66 45 55 
G33  35 °39′.86″ 138 °26′38.31″ 95 65 35 
G34  35 °38′28.97″ 138 °26′12.5″ 93 41 59 
G35  35 °38′16.93″ 138 °27′38.3″ 64 36 61 39 
G36  35 °37′43.86″ 138 °29′51.4″ 70 30 55 45 
G37  35 °38′32.18″ 138 °29′46.31″ 85 15 64 36 
G38  35 °37′48.67″ 138 °28′44.33″ 37 63 52 48 
G39  35 °37′0.35″ 138 °27′24.58″ 22 78 46 54 
G40  35 °37′10.08″ 138 °28′26.57″ 26 74 44 56 
G41  35 °37′.38″ 138 °28′50.04″ 38 62 47 53 
G42  35 °36′.93″ 138 °28′12.53″ 49 51 51 49 
G43  35 °36′.95″ 138 °29′56.05″ 61 39 64 36 
G44  35 °36′10.76″ 138 °29′15.02″ 29 71 50 50 
G45  35 °35′.22″ 138 °30′15.14″ 70 30 50 50 
G46  35 °35′.98″ 138 °27′12.12″ 15 85 47 53 
G47  35 °36′.56″ 138 °28′19.5″ 34 66 50 50 
G48  35 °35′.21″ 138 °28′39.12″ 23 77 39 61 
G49  35 °35′18.09″ 138 °29′0.33″ 44 56 42 58 
G50  35 °35′.61″ 138 °27′24.22″ 30 70 49 51 
G51  35 °35′.71″ 138 °28′20.94″ 66 34 46 54 
G52  35 °35′13.72″ 138 °28′31.74″ 57 43 47 53 
G53  35 °35′.76″ 138 °29′46.49″ 52 48 55 45 
G54  35 °34′38.16″ 138 °29′21.33″ 52 48 54 46 
Figure 8

Spatial distribution of groundwater recharge sources contribution ratios.

Figure 8

Spatial distribution of groundwater recharge sources contribution ratios.

To identify the seasonal variability between the contribution between the two sources (i.e., pre-monsoon and monsoon), a mass-balance of mean d-excess values using Equations (4) and (5) is considered: 
formula
4
 
formula
5
Equations (4) and (5) can convert to Equation (6) as below: 
formula
6
where Xpre-monsoon and (1 – Xpre-monsoon) are the contribution of pre-monsoon and monsoon precipitations, respectively. The d-excess values are using the intercept of LMWL in pre-monsoon and monsoon for dpre-monsoon and dpre-monsoon, respectively (Table 3). On the basis of this equation, the estimated contribution ratios of pre-monsoon and monsoon precipitations to the groundwater are presented in Figure 9. The precipitation during the pre-monsoon and monsoon seasons contributes 46–68% and 32–54%, respectively, in groundwater recharge of the Kamanashigawa alluvial fan. In the case of Midaigawa, the contributions in the pre-monsoon and monsoon seasons are 39–65% and 35–61%, respectively (Table 4).
Figure 9

Spatial distribution of the contribution ratios of the monsoon and pre-monsoon precipitation for groundwater.

Figure 9

Spatial distribution of the contribution ratios of the monsoon and pre-monsoon precipitation for groundwater.

CONCLUSIONS

The present study reveals the influence of the presence of two sources and seasonal variability on groundwater recharge in the alluvial fans of the Kofu basin, Japan. The alluvial fans are recharged from two sources: the river water drained by mountainous catchments and direct infiltration of precipitation in the plain area. The isotopic data indicate that river waters are isotopically lighter, whereas altitude effects become evident in the precipitation isotope values. In the Kamanashigawa alluvial fan, river water contributes 38–100% of the recharge, while precipitation contributes 29–62% in the central part and increases its contribution with distance from the Kamanashi River. In the case of Midaigawa, river water contributes 77–99% in the northern part, while in the southern side 30–93% of contribution comes from precipitation. Comparing the d-excess with δ18O values reveals mixing of the river water with precipitation in the alluvial fan aquifer. Variation in precipitation and river water d-excess values, in the plains of the study area, suggests that pre-monsoon precipitation in the mountain areas (e.g., in the form of snow) might contribute more in recharge. The mass-balance analysis of the d-excess in groundwater and precipitation of different seasons (i.e., pre-monsoon and monsoon) indicates pre-monsoon precipitation contributes 46–68% and 39–65% to the groundwater recharge of Kamanashigawa and Midaigawa alluvial fans, respectively.

Notably, the pre-monsoon precipitation contribution to the groundwater recharge is more than 39%, while the total precipitation on the plain area in the same season is only 25% of the annual precipitation. This requirement could be met by pre-monsoon precipitation in the mountain area, which is carried by the river and recharged in the form of river water. Therefore, even if the annual precipitation amount in the plain area of the alluvial fans is quite small, the pre-monsoon precipitation could play an important role in recharging the groundwater, especially in the case of mountainous watersheds.

ACKNOWLEDGEMENTS

We are grateful to all members of the Water Quality Research Group of Interdisciplinary Center For River Basin Environment, University of Yamanashi. This study was supported by the Global Centres of Excellence (GCOE) program and Grant-in-Aids for Scientific Research (C) (No. 25630226) of the Japan Society for Promotion of Science (JSPS).

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