要
JR

Abstract

1. Introduction
The Niseko area is a major resort area in western Hokkaido, Japan, with many companies related to the leisure industry gathered in the area.  Several volcanos such as Mt. Niseko Annupuri and Mt. Iwaonupuri are found in this area.  The volcanic activity is considered to have begun from the early Pleistocene age, and increased markedly in the late Pleistocene (Oba, 1960 ; Igarashi and Yokota, 1970).  A lot of springs are therefore found around the volcanos and are utilized as recreational spas.  Recently, the drilling of hot spring has been actively pursued accompanied by the construction of many accommodation facilities.  Due to concerns regarding this potential drain on spring resources, the Hokkaido government specified parts of the Niseko area as a protected area or a semi protected area from spring resource development in 2020 (Hokkadido, 2020). The quality of the spring water and dominant element components vary by spring in the Niseko area.  There are several types of spring water such as Na-Cl, Mg-SO s , Ca-SO s and Na-HCO 3 (Ohmori and Suzuki, 2020).  In addition, dissolved carbon dioxide (CO 2 ) is a component that has received much interest.  Although there are few reports with detailed information of CO 2 for individual springs in the Niseko area, fans of hot springs have noted the existence of some CO 3 rich springs (e.g. the Kogane spring or Yakushi spring).  It was also reported that a high terrestrial carbon dioxide concentration (94%) was detected from a spring in Niseko and it was suggested that the carbon dioxide originated from magma derived from the mantle (Ono et al ., 1993). Although gaseous carbon dioxide has harmful effects on the human body at high concentra- tions (Langford, 2005), CO 2 in spas has been recognized historically as a useful therapy for patients with arteriopathies (Brandi et al , 2001). Springs with >22.8mmol/kg (1000mgCOヶ/kg) are regarded a medical treatment springs, "carbon dioxide springs" according to Japanese spring regulations.  They are expected to be effective in the improvement of cuts, circulatory disorders, poor circulation and autonomic dysfunction (Maeda, 2015). CO 2 interacts with various physico-chemical factors.  For example, it was reported that the partial pressure of CO 2 could be derived from a formula using alkalinity, pH and Henry's law constant according to the survey of groundwater including some springs (Macpherson, 2009). Temperature causes changes in the solubility of CO 2 in water (Li and Tsui, 1971), and pH changes the chemical form of CO 2 and is shifted according the amount of CO 2 present (Cole and Prailie, 2009).  Thus, CO 2 in springs is likely to be related to both temperature and pH.  However, it is uncertain whether that a general law can be identified for the actual relationship between the CO, concentration and both temperature and pH in springs in which the environment differs from surface waters and groundwaters. In this study, we tried to clarify the CO 2 concentration in springs in the Niseko area and evaluate the relationships between CO 2 and compound factors of temperature and pH in the springs.  In addition, we tried to compare the relationships among springs across Hokkaido.
Materials and methods
2.
The sampling was conducted at a total of 45 sites (43 sites were working or abandoned springs and 2 sites a brook/ swamp) from July to September in 2017–2019.  The map of the sites is shown in Fig. 1. The sampling area covered Kutchan, Niseko, Rankoshi, Kyowa and Iwanai towns. The measurements of temperature, pH, CO 2 and electric conductivity (EC) were performed at sampling.  Temperature, pH and EC were determined by hand-held measuring instruments calibrated beforehand.  Major components and total hydrogen sulfide were measured by conven- tional methods (Ministry of the Environment Government of Japan, Natural Environment Bureau, 2014). CO 2 was determined by the back titration method (see Crossno et al ., 1996).  After 10ml of 0.25 mol/l NaOH, 5 ml of a mixed solution of 1 mol/l potassium sodium tartrate and 1 mol/l trisodium citrate, and small amount of phenolphthalein were added to Ehrenmeyer flask, a 160ml sample was poured carefully into the flask,.  The color of most sample turned red.  In cases in which it did not, the volume of the sample was reduced until it remained a red color. Then, the titration procedure has performed using a volumetric pipet with 0.25mol/l HCl until the red color vanished (pH 8.3).  CO 2 concentration was calculated from the volume of the consumed 0.25mol/l HCl.  If the pH of the spring was below 4, the blank value was subtracted from the initial CO 2 concentration.  The blank value was determined by the same back titration method after the CO 2 was completely removed by boiling. CO 2 , temperature and pH data for the whole of Hokkaido were referenced from the official analysis sheets of 1069 springs issued in 2010-2019.  These analyses were prepared by 10 authorities registered with the Hokkaido government.
3.
Results and discussion
3.1 CO 2 concentrations in springs in the Niseko area
Fig. 2 shows a histogram of CO 2 concentration in springs in the Niseko area.  We found 31% of surveyed springs showed a value below 1 mmol/kg.  The number of springs with a value 1-10mmol/kg decreased with increased CO 2 concentration, although the proportion of springs with a value above 10mmol/kg was relatively high at 16%. The spring with the maximum CO 2 concentration (29.8 mmol/kg) was categorized as a medical treatment spring.
3.2    CO, concentration and compound factors of temperature and pH
To evaluate the relationship between the CO 2 concentration and compound factors of temperature and pH, a scatter plot was made for CO 2 /temperature and CO 2 /pH (Fig. 3).  The value of slope and y-intercept of regression line was 6.0294 and zero, respectively.  The correlation coefficient was 0.7209, which seemed be a relatively high value.  The relative high correlation coefficient value indicated that the CO 2 concentration was related to both temperature and pH for springs in the Niseko area.  Three data sets (the sampling site (SS) 1, SS 2 and SS 3) were outside the 95% prediction interval (see Forthofer et al ., 2007).  This indicated that CO 2 concentrations in the three sites were unexpected in relation to the pH and temperature condi- tions compared to the other springs in the Niseko area.  SS 1 and SS 2 in Fig. 3 were characterized by low pH and high temperature (Table 1).  Although CO 2 concentration tends to be high at a low pH, it may actually become low at a very low pH due to volatilization into the atmosphere. In addition, high temperature lowers the solubility of CO 2 in liquids.  Therefore, the CO 2 concen- trations may tend to be low under the condition of SS 1 and SS2.  SS 3 in Fig. 3 was characterized by a neutral pH, low temperature and low conductivity (Table 1).  Even though the solubility of CO 2 is high at low temperatures, the CO 2 concentration should fall through the exchange from
CO 2 to HCO 3 − besides CO 3 − due to the effect of pH.  Further, a high CO 2 concentration might cause a shift to a lower pH. SS 1 and SS2 are natural volcanic springs with high temperature, low pH and high hydrogen sulfide concentrations, and are located in southern foothills of Mt. Iwaonupuri (Fig. 1).  SS 3 is a natural brook (drifting spring) with low temperature, neutral pH and high hydrogen sulfide concentration, and is located near (about 1 km) the spring which had the highest CO 2 concentration in this study.  It was suggested that the natural springs with dominant Mg-SO, concentrations in the Niseko area were a mixture of water from a volcanic hot spring source and rainwater (Shibata et al ., 2011).  Although no detailed data were available to why the CO 2 concentrations were high at those three sites, we assumed that the source of those springs originally had high CO 2 concentrations derived from volcanic gas and the analyzed spring water showed the initial stage of a CO 2 concentration decrease in the case of SS 1 and SS 2, and a decrease in pH in the case of SS 3. Fig. 4a and 4b show the relationships between CO 2 concentration and temperature and pH, respectively, in springs in the Niseko area.  The springs with temperatures above 42℃ accounted for 76% of all surveyed springs.  Regression analysis did not reveal any significant relation between temperature (x axis) and CO 2 concentration (y axis) ( p >0.05).  The springs with a neutral pH from pH 6.0 to 7.5 accounted for 64% of all surveyed springs, and regression analysis revealed a significant relation between pH (x axis) and CO 2 concentration (y axis) ( b<0.05). These regression analyses indicated that pH rather than temperature was closely related with CO 2 concentration. As shown in Fig. 4a and 4b, none of SS 1, SS2 and SS3 were extraordinary as their data were within the 95% prediction interval.  Therefore, relationships to consider were found in terms CO 2 concentration and the compound factors of temperature and pH, but reasonable relationship were present between CO 2 concentration and temperature and between CO 2 concentration and pH for the three data.
3.3 Comparison with springs across the whole of Hokkaido
The value of slope and correlation coefficient ( r ) were modified from 6.0294 to 6.1746 and from 0.7209 to 0.9616, respectively, on excluding the data for SS 1-3 (Fig. 5a).  The regression line is also shown in Fig. 5a based on 918 springs in Hokkaido Prefecture, Japan, without 151 springs outside the 95% prediction interval.  The regression line slope on the Niseko springs was about twice that for the springs across the whole of Hokkaido.  This means that pH was lower and temperature was higher under the same CO 2 concentration conditions in the springs in Niseko area compared with those across the whole of Hokkaido.  There are a lot of non-volcanic, deep drilled springs in Hokkaido.  Such springs tend to have high pH and moderately high temperature, thus contributing to the low value of the regression line slope in Fig. 5a.  The correlation coefficient of the regression line for the whole of Hokkaido was lower than that for the Niseko area.  This indicated that the slope of the regression line varied for each spring area in Hokkaido. In addition to the springs in Hokkaido, we tried to confirm the regression line of the springs in the Beppu spring area in southern Japan (Fig. 5b), calculated from data listed previously (Yang, 2021).  The slope of the regression line was higher at 7.7972 (n = 48, r = 0.9851 when the four springs outside the 95% prediction interval were excluded) than the springs in the Niseko area.  The Beppu springs are volcanic and located in one of the largest geothermal fields in Japan (Komatsu et al ., 2012).  The high value of the slope might relate with the origin of CO 2 such as magma.  The high temperatures above 48℃ were recorded at the four sites outside 95% prediction interval, (Table 2).  The three springs of those sites were acid and weak acid springs. The dominant ions were Mg and SO 4 at the two sampling sites.  The data of spring site with the high temperature and acidity or weak acidity may often lie outside the 95% prediction interval from the examples in the Niseko and Beppu areas.
3.4 Notes for the group assessment in this study
The least square method determined the regression line generally requires to minimalize the residual sum of squares for the parameters in y axis regarding that the parameters in x axis is error-free.  If the parameters in x axis and y axis replace each other, there is a little change in the correlation coefficient and the difference sometimes occurs with the data outside 95% predic-
tion interval.  Therefore, which to select between x axis and y axis has to be considered for the parameters of CO 2 /pH and CO 2 /temperature.
4.
Conclusion
CO 2 concentrations were determined for the springs in Niseko area. Relatively high concen- tration above 10mmol/kg occupied 16% of 45 surveyed springs and the maximum concentration was 29.8mmol/kg.  A scatter plot was made for CO 2 /temperature and CO 2 /pH to clarify the relationship of CO 2 concentration and combined factor of temperature and pH.  It showed a good correlation with 0.9616 of correlation coefficient on the exclusion of three data outside the 95% prediction interval.  The value of regression line' s slope of springs in Niseko area was quite higher than springs in whole Hokkaido and was sometimes lower than springs in Beppu area. In this study, the relationship among CO 2 , and combined factor of temperature and pH could be assessed easily from a scatter plot without the simulations based on complicated calculations found in previous reports.  In addition, the data with a too high CO 2 concentration in relation to temperature and pH was able to be found by the establishment of a 95% prediction interval. We speculated that the slope of the regression lines had generally high values in the volcanic spring groups, although insufficient survey examples were available.  As we speculated that the value of the slope is related to the CO 2 source, further investigation is needed to clarify the relationship between CO 2 concentration and compound factors of temperature and pH in various spring areas.
Acknowledgements
We thank to the administrators of spring well for collecting the spring water samples.  In addition, we are also thankful to the anonymous referees for the valuable comments.  This study was supported financially by the Hokkaido government.