hydrology

Article Geothermal Linkage between a Hydrothermal Pond and a Deep Lake: Kuttara Volcano,

Kazuhisa Chikita 1,*, Yasuhiro Ochiai 2, Hideo Oyagi 3 and Yoshitaka Sakata 4

1 Arctic Research Center, University, 001-0021, Japan 2 Japan Petroleum Exploration, Co., Ltd., Tokyo 100-0005, Japan; [email protected] 3 College of Humanities and Sciences, Nihon University, Tokyo 156-0045, Japan; [email protected] 4 Faculty of Engineering, Hokkaido University, Sapporo, 060-8628 Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-11-706-2764

 Received: 24 October 2018; Accepted: 27 December 2018; Published: 6 January 2019 

Abstract: Kuttara Volcano, Hokkaido, Japan, consists of temperate Lake Kuttara and the western Noboribetsu geothermal area. In order to explore geothermal relations between Lake Kuttara and the geothermal area, the heat budget of a hydrothermal pond, Okunoyu, was evaluated, and the heat storage change in the lower layer of Lake Kuttara was calculated by monitoring the water temperature at the deepest point. The lake water temperature consistently increased during the thermal stratification in June–November of 2013–2016. The heat flux QB at lake bottom was then calculated at a range of 4.1–10.9 W/m2, which is probably due to the leakage from a hydrothermal reservoir below the lake bottom. Meanwhile, the heat flux HGin by geothermal groundwater input in Okunoyu was evaluated at 3.5–8.5 kW/m2, which is rapidly supplied through faults from underlying hydrothermal reservoirs. With a time lag of 5 months to monthly mean QB values 2 in Lake Kuttara, the correlation with monthly mean HGin in Okunoyu was significant (R = 0.586; p < 0.01). Applying Darcy’s law to the leakage from the hydrothermal reservoir at 260–310 m below the lake bottom, the time needed for groundwater’s passage through the media 260–310 m thick was evaluated at 148–149 days (ca. 5 months). These findings suggest that the hydrothermal reservoir below lake bottom and the underlying hydrothermal reservoirs in the western geothermal area are both connected to a unique geothermal source in the deeper zone as a geothermal flow system of Kuttara Volcano.

Keywords: Kuttara Volcano; Lake Kuttara; hydrothermal pond; heat budget

1. Introduction There are many volcanic lakes that are geothermally affected by magmatic activity of volcanoes or their eruptions throughout the world, as typified by Lake Nyos, Cameroon, with the lower magma’s pocket [1]. For Crater Lake, Oregon, USA, the US Geological Survey has provided a volcano hazards program for a possible future eruption [2], though the volcano at present seems to be undergoing stable magmatic activity. In New Zealand, the activity of Ruapehu Volcano is explored by using acoustic and seismic signals and estimating the water and heat budgets of Ruapehu Crator Lake [3–6]. Geothermal and hydrothermal systems of volcanic origin are proposed by White and Brannock [7] and Fukutomi et al. [8] from a geophysical viewpoint and by White [9] from a geological viewpoint. Terada and Hashimoto [10] proposed a numerical model for a thermal response of a crater lake to lower geothermal activity. As for other studies for volcanic activity, there are many geochemical investigations about interrelations between hydrothermal lakes and lower magmatic activity [11–15]. However, there are few field studies on estimates of water and heat budgets of hydrothermal lakes or ponds [3,16]. This is because high water temperature at more than 50 ◦C and volcanic heated gases

Hydrology 2019, 6, 4; doi:10.3390/hydrology6010004 www.mdpi.com/journal/hydrology Hydrology 2019, 6, 4 2 of 11

Hydrology 2018, 5, x FOR PEER REVIEW 2 of 11 such as water vapor, SO2 and CO2 could damage instruments for monitoring water level and water waterproperties level suchand water as pH, properties electric conductivitysuch as pH, electr andic water conductivity temperature. and water The water,temperature. heat and The chemical water, ◦ heatbudgets and ofchemical Ruapehu budgets Crator of Lake Ruapehu (water Crator temperature Lake (water of less temperature than about of 50 lessC) werethan about estimated 50 °C) for were25 years estimated [3]. However, for 25 years spatial [3]. However, distributions spatial of water distributions temperatures of water in temperatures the lake are not in the shown, lake are and notgroundwater shown, and input groundwater and output input in theand wateroutput budget in the water are not budget estimated. are not In estimated. a volcanic In a volcanic lake, calderaLake Kussharo, lake, Lake Japan, Kussharo, Chikita etJapan, al. [17 Chikita] evaluated et al. groundwater [17] evaluated outflow groundwater and groundwater outflow inflow and groundwaterincluding the inflow input ofincluding geothermal the input water of by geotherm estimatingal water the water by estimating and chemical the water budgets and of chemical the lake. budgetsIn the present of the study,lake. In the the heat present budget study, of a hydrothermalthe heat budget pond of a with hydrothermal water temperature pond with of mostlywater ◦ temperature50–70 C is estimatedof mostly 50–70 over four°C is years.estimated Such over a long-term four years. estimate Such a long-term under heated estimate conditions under heated is very conditionsrare in the world.is very Kuttara rare in Volcano,the world. Hokkaido, Kuttara Japan,Volcano, consists Hokkaido, of the westernJapan, consists Noboribetsu of the geothermal western Noboribetsuarea and eastern geothermal Lake Kuttara area and (Figures eastern1 Lakeand2 ).Kutta In thera (Figure Noboribetsu 1 and geothermalFigure 2). In area,the Noboribetsu in order to geothermalwatch volcanic area, activity, in order seismographs, to watch volcanic clinometers, activity, infrasound seismographs, meters, clinometers, GNSS and infrasound video cameras meters, have GNSSbeen setand by video the Japan cameras Meteorological have been set Agency. by the Kuttara Japan Meteorological Volcano is currently Agency. at theKuttara first levelVolcano for theis currentlyfive volcanic at the alert first levels level (the for Japanthe five Meteorological volcanic alert Agency:levels (the URL Japan http://www.data.jma.go.jp/svd/ Meteorological Agency: URL http://www.data.jma.go.jp/svd/voivois/data/tokyo/STOCK/kaisetsu/English/level.htmls/data/tokyo/STOCK/ kaisetsu/Eng)[18]. Thus, thelish/level.html) volcanic activity [18]. Thus, present the is 2 volcanicrelatively activity stable. present Fukutomi is relatively et al. [16] numericallystable. Fukutomi obtained et al. the [16] heat numerically flux HGin (W/m obtained) by the groundwater heat flux HinputGin (W/ in m a2) hydrothermal by groundwater pond, input Ohyunuma, in a hydrothermal in the Noboribetsu pond, Ohyunuma, area by in estimating the Noboribetsu the water area andby estimatingheat budgets. the water Boehrer and et heat al. [budgets.19] estimated Boehrer the et bottom al. [19] geothermalestimated the heat bottom flux geothermal at the deepest heat points flux atof the four deepest Japanese points volcanic of four lakes, Japanese Shikotsu, volcanic Kuttara, lakes, TowadaShikotsu, and Kuttara, Tazawa, Towa atda 0.29 and W/m Tazawa,2, 1.0 at W/m 0.292 , W/m18.6 W/m2, 1.0 W/m2 and2,0.27 18.6W/m W/m2 ,and respectively. 0.27 W/m This2, respectively. suggests that This of suggests the four that lakes, of Kuttarathe four and lakes, Towada Kuttara are andaffected Towada by relatively are affected high by volcanic relatively geothermal high volcanic activity. geothermal A magneto-telluric activity. A (MT) magneto-telluric survey for internal (MT) surveystructure for of internal Kuttara structure Volcano byof Kuttara Goto and Volcano Johmori by [20 Goto] indicates and Johmori that the [20] zone indicates of relatively that the low zone specific of relativelyresistivity low underlying specific theresistivity bottom underlying at the deepest the point bottom of Lakeat the Kuttara deepest may point be aof hydrothermal Lake Kuttara reservoir.may be aGoto hydrothermal and Johmori reservoir. [20] also Goto pointed and outJohmori that there [20] also is a geothermallypointed out that active there zone is a just geothermally below the western active zoneNoboribetsu just below geothermal the western area. InNoboribetsu this study, the geothermal heat budget area. of a In hydrothermal this study, pondthe heat in the budget Noboribetsu of a hydrothermalarea is continually pond estimated in the Noboribetsu over four years,area is andcontinually then the estimated question ofover how four the years, evaluated and then heat the flux questionby geothermal of how groundwaterthe evaluated input heat flux is related by geot tohermal the bottom groundwater geothermal input heat is related flux in to Lake the bottom Kuttara geothermalis discussed. heat flux in Lake Kuttara is discussed.

FigureFigure 1. 1. LocationLocation of of Kuttara Kuttara Volcano Volcano in in Hokkaido Hokkaido (i (inserted),nserted), Lake Lake Kuttara Kuttara with with the the bathymetry bathymetry (blue(blue isopleths isopleths of of 20 20 m, m, 100 100 m m and and 140 140 m m in in wate waterr depth) depth) and and hydrothermal hydrothermal ponds, ponds, Ohyunuma Ohyunuma and and Okunoyu,Okunoyu, in in the the geothermal geothermal area area (red (red dotted dotted line). line). Th Thee deepest deepest point point of of Kuttara Kuttara is is located located at at site site MD MD (148(148 m m in in depth), depth), and and a a meteorological meteorological st stationation at site KM on the lake shore.

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FigureFigure 2. Location2. Location of two of two hydrothermal hydrothermal ponds, ponds, Ohyunuma Ohyunuma and Okunoyu, and Okunoyu, and a meteorological and a meteorological station (sitestation M) in (site the catchment,M) in the catchment, upstream ofupstream the small of boilingthe small pond boiling of a squarepond of shape a square (2 m shape× 2 m). (2 Therem × 2 ism). anThere active is fumarole an active with fumarole rising with steam rising and smokessteam and near smokes the summit near the of Mt.summit Hiyori. of Mt. Hiyori. 2. Methodology 2. Methodology 2.1. Field Observations 2.1. Field Observations Locations of Lake Kuttara, hydrothermal ponds in the Noboribetsu geothermal area and observationLocations sites of are Lake shown Kuttara, in Figures hydrothermal1 and2. Lake pond Kuttaras in the (42 Noboribetsu◦29057” N, geothermal 141◦10055” area E) hasand a surfaceobservation area sites of 4.68 are kmshown2 and in meanFigure water 1 and depthFigure of2. 104.9Lake Kuttara m with (42°29 lake level′57″ N, at ca.141°10 258′55 m″ aboveE) has 2 seaa levelsurface abbreviated area of 4.68 as km “asl”. and Vertical mean profileswater depth of water of 104.9 temperature m with lake and electriclevel at conductivityca. 258 m above were sea obtainedlevel abbreviated about twice as per “asl”. month Vertical at the profiles deepest pointof water (site temperature MD; 148 m depth)and electric and at conductivity the other 6 sites were (whiteobtained circles about in Figure twice1 ).per The month vertical at measurementsthe deepest point were (site performed MD; 148 bym usingdepth) a and TCTD at (temperature-the other 6 sites conductivity-turbidity-depth)(white circles in Figure profiler1). The (modelvertical ASTD102, measurements JFE Advantec, were Inc.,performed Japan; accuracies, by using± 0.01a TCTD◦C for(temperature- 0–35 ◦C, ±0.1 conductivity-turbidity-depth) mS/m for 0–200 mS/m, ± 0.3profiler FTU (model for 0–1000 ASTD102, FTU, and JFE ±Advantec,1.8 m for Inc., 0–600 Japan; m, respectively)accuracies, in±0.01 2013–2016. °C for 0–35°C, In order ±0.1 to mS/m know for a temporal 0–200 mS/m, change ±0.3 of FTU water for temperature 0–1000 FTU, in and the ±1.8 bottom m for layer,0–600 eleven m, respectively) temperature in loggers 2013–2016. (HOBO In order TidbiT to v2,know Onset a temporal Computer, change Inc., of USA; water accuracy, temperature±0.2 in◦C) the werebottom fixed layer, at 45 eleven m, 35 m,temperature 25 m, 15 m, loggers 10 m, (HOBO 5 m, 4 m,TidbiT 3 m, v2, 2 m, Onset 1 m andComputer, 0 m above Inc., theUSA; bottom accuracy, of site±0.2 MD °C) by were a mooring fixed at buoy 45 m, system 35 m, 25 [21 m,]. In15 orderm, 10 tom, improve5 m, 4 m, the 3 m, accuracy 2 m, 1 m from and ±0 m0.2 above◦C to the±0.1 bottom◦C, theof loggers’ site MD valuesby a mooring were calibrated buoy system by water[21]. In temperature order to improve from the the accuracy TCTD profiler from ±0.2 by °C using to ±0.1 high °C, correlationsthe loggers’(R2 values= 0.920–0.998 were calibrated, p < 0.01) atby a water range oftemperature 3.6–6.0 ◦C. Atfrom site the MD, TCTD a sediment profiler core by 0.2 using m long high 2 wascorrelations sampled by(R a = portable 0.920–0.998, gravity p < 0.01) core at sampler a range to of determine 3.6–6.0 °C. sedimentaryAt site MD, afacies sediment and core measure 0.2 m the long hydraulicwas sampled conductivity by a portable using the gravity falling core head sampler test in laboratory.to determine A sedimentsedimentary core facies 1.39 m and long measure was also the sampledhydraulic at site conductivity MD in 2009 using by Dr. the Okamura, falling head Kochi test University, in laboratory. Japan, A and sediment the group core “Water 1.39 m and long Cep was inalso Lake sampled Shikotsu”, at site Sapporo, MD in Japan 2009 by [22 Dr.]. Ochiai Okamura, [22] pointed Kochi University, out that the Japan, sediment and corethe group mostly “Water contains and depositsCep in from Lake the Shikotsu”, eruptions Sapporo, of the western Japan volcano[22]. Ochiai (at present, [22] pointed Noboribetsu out that geothermal the sediment area) core and mostly Usu Volcanocontains at 29.4deposits km west from of the Lake eruptions Kuttara. of the western volcano (at present, Noboribetsu geothermal area)The and Noboribetsu Usu Volcano geothermal at 29.4 km area west consists of Lake of Kuttara. Mt. Hiyori, four hydrothermal ponds, Ohyunuma, Okunoyu,The Taisho-jigoku Noboribetsu andgeothermal a small boiling area consists pond (square of Mt. Hiyori, shape of four 2 m hydrothermal× 2 m), the southern ponds, geothermal Ohyunuma, valley,Okunoyu, called Taisho-jigoku “Jigoku-dani” and (Hell’s a small Valley) boiling and thepond Noboribetsu (square shape Spa Townof 2 m with × 2 springm), the points southern of geothermalgeothermal water. valley, Figure called2 shows “Jigoku-dani” a catchment (Hell’s area Valley) upstream and of the the Noboribetsu small boiling Spa pond, Town including with spring the northernpoints of part geothermal of the Noboribetsu water. Figure geothermal 2 shows area. a catchment In Ohyunuma area upstream (42◦3009” of N, the 141 small◦8050” boiling E; surface pond, elevationincluding 240 the m northern asl, water part surface of the area Nobori 1.60betsu× 104 geothermalm2, mean depth area. In 5.7 Ohyunuma m) and Okunoyu, (42°30′9 (42″ N,◦30 141°808” N,′50″ 4 2 141E;◦8 surface057” E; surfaceelevation elevation 240 m 243asl, mwater asl, watersurface surface area 1.60 area × 9.6 10× m10,2 meanm2, mean depth depth 5.7 m) 1.8 and m), theOkunoyu, water 2 2 temperature(42°30′8″ N, was 141°8 recorded′57″ E; everysurface 1 helevation at 0.2 m depth243 m byasl, using water sheath surface type area thermocouples 9.6 × 10 m , mean (temperature depth 1.8 allowance,m), the water±1.5 temperature◦C for a range was of recorded−40–+333 every◦C). 1 h The at 0.2 thermocouple m depth by using sensor sheath in Okunoyu type thermocouples was set at ca.(temperature 10 m distant allowance, from one ±1.5 of geothermally °C for a range bubbled of −40–+333 regions °C). with The thermocoup more than 80le ◦sensorC at the in Okunoyu surface. was set at ca. 10 m distant from one of geothermally bubbled regions with more than 80 °C at the surface. In Ohyunuma, which has no noticeably bubbled region at the surface but two rising steam

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In Ohyunuma, which has no noticeably bubbled region at the surface but two rising steam areas near the western shore, the thermocouple sensor was fixed near the outlet. The temperature data are managed and supplied by Noboribetsu On-sen Co., Ltd. Okunoyu normally has no outlet, because the water level is dominantly controlled by the water’s intake (ca. 0.023 m3/s) using pipelines. The hot water by the uptake is supplied to the Noboribetsu Spa Town. The data of intake volume and water temperature are also managed and supplied by Noboribetsu On-sen Co., Ltd. In order to estimate the heat budget of Okunoyu, surface inflow, its water temperature and the water level were frequently measured. Meteorology (solar radiation, air temperature, relative humidity, rainfall and wind velocity) was observed at 1 h intervals at site M between Ohyunuma and Okunoyu (Figure2), and at site KM on the shore of Lake Kuttara (Figure1), which are both set at 3 m above ground surface. The missing data were complemented by significant correlations with data of the Noboribetsu AMeDAS station at a distance of 5.4 km south-southwest of site M. Snowfall at site M was assumed to be equal to that at the Noboribetsu station.

2.2. Heat Budget and Geothermal Heat Flux in a Hydrothermal Pond In order to evaluate the geothermal heat flux at the bottom of hydrothermal Okunoyu, the heat budget was estimated by the following equation:     Z 0  ∆G/∆t = ∆ ρwcpT(z) ·A(z)dz /∆t /A0 = Rn − QH − QE + QP + HR + HG + HS (1) −h where ∆G is heat storage change (J/m) of a lake for a budget period ∆t (s), ρw and cp are water density (kg/m3) and specific heat of water (J/kg/K) at temperature T (K) at z (m), vertical coordinate, h is water 2 depth (m) at deepest point, A(z) is the area at z (surface area, A0 at z = 0), Rn is net radiation (W/m ), 2 2 2 QH is sensible heat flux (W/m ), QE is latent heat flux (W/m ), QP is heat flux (W/m ) by direct 2 2 precipitation onto the lake, HR is heat flux (W/m ) by net river inflow, HG is heat flux (W/m ) by 2 net groundwater inflow, and Hs is heat flux (W/m ) by heat conduction at bottom. Net radiation Rn in Equation (1) consists of net shortwave radiation K* and net longwave radiation L*, as given by the following: ∗ ∗ 4 Rn = K + L = (1 − α)K + Ld − Lup = (1 − α)K + Ld − εσTs (2) where K is downward shortwave radiation, α is albedo, Ld and Lup are downward and upward longwave radiations, respectively, ε is emissivity (here, 0.97 for water and 1.0 for air), −8 2 4 σ is Stefan-Boltzmann constant (=5.67 × 10 W/m /K ) and Ts is pond surface temperature (K). Considering observational results of Brandt and Warren [23], α value of water was given as constant of 0.05. Downward longwave radiation Ld was numerically obtained as a function of air temperature, total amount of effective water vapor content and relative sunshine duration [24]. HR, HG and QP in Equation (1) are given as follows:

HR = HRin − HRout = ρwCp{Rin(TRin − T0) − Rout(TRout − T0)}/A0 (3)

HG = HGin − HGout = ρwCp{Gin(TGin − T0) − Gout(TGout − T0)}/A0 (4)  Qp = ρwCpP Tp − T0 (5) 3 2 where Rin and Rout are surface inflow and artificial intake (m /s), HRin and HRout are heat fluxes (W/m ) by surface inflow and intake, TRin and TRout are water temperatures (K) for surface inflow and intake, 3 respectively, Gin and Gout are groundwater inflow and outflow (m /s), HGin and HGout are heat fluxes 2 (W/m ) by groundwater inflow and outflow, respectively, TGin and TGout are temperatures (K) of inflowing and outflowing groundwaters, T0 is standard temperature (K), P is precipitation (m/s) onto lake surface, TP is temperature (K) of precipitation (here assumed to be equal to the wet-bulb temperature). Daily mean values of the meteorological factors and temperatures were utilized for Hydrology 2019, 6, 4 5 of 11 aHydrology budget 2018 period,, 5, x FOR∆t = PEER 86,400 REVIEW s. In case of snowfall onto the pond, latent heat of ice from the negative5 of 11 air temperature to 0 ◦C and fusion heat of the 0 ◦C ice in water are reduced from Equation (5). catchment (Figure 1). Heat◦ flux HGin by groundwater input in Equation (4) is here defined as the Here, T0 = 281.05 K (7.9 C), being equal to the annual mean air temperature at site M, is given, bottom input of geothermal heat ascending through many faults and fractures, containing liquid which corresponds to the temperature of unconfined groundwater under non-geothermal condition in water and volcanic gases mainly composed of water vapor [25]. Hence, the heat flux Hs by heat the catchment (Figure1). Heat flux HGin by groundwater input in Equation (4) is here defined as the conduction in Equation (1) is supposed to be negligibly small, compared with HGin. Unconfined bottom input of geothermal heat ascending through many faults and fractures, containing liquid water groundwater inflow from the surrounding catchment slope in Equation (4) could also be negligibly and volcanic gases mainly composed of water vapor [25]. Hence, the heat flux Hs by heat conduction small because of the muddy bottom deposits [16]. in Equation (1) is supposed to be negligibly small, compared with HGin. Unconfined groundwater Sensible heat flux QH and latent heat flux QE in Equation (1) were numerically obtained by the inflow from the surrounding catchment slope in Equation (4) could also be negligibly small because of following bulk transfer method: the muddy bottom deposits [16]. Sensible heat flux Q and latent heatQ = flux(cρQa inu Equation) ⋅()T − T (1) were numerically obtained by the H H a EH z s z (6) following bulk transfer method:  ρ β  =QH = − (cρaaaHuz)⋅·()()(Ts − T⋅z) − (6) QE lE l  aEuz ez e0 (7)  p  ρaβ  Q = lE = −l · (a u ) · (e − e ) (7) E p E z z 0 where ρa is air density (=1.2 kg/m3), c is specific heat (J/Kg/K) of air under constant pressure, β is ratio 3 whereof waterρa is vapor air density density (=1.2 to kg/m dry ),airc isdensity specific (=0.622), heat (J/Kg/K) aH and of airaE underare dimensionless constant pressure, bulk βtransferis ratio ofcoefficients water vapor for density sensible to heat dry air and density latent (=0.622),heat, respectively,aH and aE are uz is dimensionless wind speed bulk(m/s) transfer at z above coefficients ground forsurface, sensible Tz heatis air and temperature latent heat, (K) respectively, at z, Ts is wateruz is windtemperature speed (m/s) (K) at at surface,z above groundl is latent surface, heat (J/kg)Tz is for air temperatureevaporation, (K) p isat airz ,pressureTs is water (Pa) temperature at z, ez is vapor (K) pressure at surface, (Pa)l atis z latent, and e heat0 is saturated (J/kg) for vapor evaporation, pressure p(Pa)is air at pressure Ts. Here (Pa)z = 3m, at z and, ez is assuming vapor pressure the atmospheric (Pa) at z, andconditione0 is saturated to be neutral vapor at pressure any time, (Pa) aH = at aTE s~. −3 −3 Here1.5 × z10= 3 was m, and given assuming for 1~ < theu3 < atmospheric ~10 m/s [24]. condition to be neutral at any time, aH = aE ~ 1.5 × 10 was given for 1~ < u3 < ~10 m/s [24]. 3. Results and Discussion 3. Results and Discussion Figure 3 shows time series of daily mean water temperature in Ohyunuma and Okunoyu, and dailyFigure mean3 airshows temperature time series and of daily daily precipitatio mean watern at temperature site M in 2013–2016. in Ohyunuma Water andtemperature Okunoyu, in andOhyunuma daily mean varied air temperatureat a range of and 36.9–55.6 daily precipitation °C (mean, 46.6 at site°C), M being in 2013–2016. in phase Waterwith air temperature temperature. in OhyunumaThe temperature varied in at aOkunoyu range of ranged 36.9–55.6 over◦C (mean,42.3–72.2 46.6 °C◦ C),(mean, being 63.0 in phase°C) and with exhibited air temperature. a small Theresponse temperature only to in Okunoyularge rainfalls ranged of overmore 42.3–72.2 than 100◦C mm/day. (mean, 63.0 The◦C) relatively and exhibited high atemperature small response in onlyOkunoyu to large is rainfallsdue to relatively of more than high 100 geothermal mm/day. Theactivity. relatively In fact, high the temperature geothermally in Okunoyububbled region is due toin relativelyOkunoyu high occupies geothermal ca. 35% activity. area Inat fact,the thesurface, geothermally but there bubbled is no regionnoticeably in Okunoyu bubbled occupies region ca.in 35%Ohyunuma area at the except surface, at butthe there near is shore. no noticeably The water bubbled level region of Okunoyu in Ohyunuma temporally except atvaried the near with shore. the Theamplitude water level of ca. of 0.25 Okunoyu m, which temporally means that varied the withsurface the area amplitude and volume of ca. were 0.25 m, approximately which means constant that the surface(9.6 × 10 area2 m2 and and volume 1.7 × 10 were3 m3, respectively). approximately constant (9.6 × 102 m2 and 1.7 × 103 m3, respectively).

FigureFigure 3. 3.Daily Daily mean mean water water temperature temperature in Ohyunuma in Ohyu andnuma Okunoyu, and Okunoyu, and daily meanand daily air temperature mean air andtemperature daily precipitation and daily atprecipitation site M in 2013–2016. at site M Thein 2013–2016. date labels The show date “month/day/year”. labels show “month/day/year”.

Figure 4 shows vertical profiles of water temperature and electric conductivity at 25 °C (EC25) at site MD of Lake Kuttara in 2014. The EC25 data were missed on 20 September. After the vertical whole circulation in April, when water temperature is nearest to the Tmd line, the lake is thermally

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Figure4 shows vertical profiles of water temperature and electric conductivity at 25 ◦C (EC25) Hydrology 2018, 5, x FOR PEER REVIEW 6 of 11 at site MD of Lake Kuttara in 2014. The EC25 data were missed on 20 September. After the vertical wholestratified circulation in May–November in April, when [21]. waterThen, temperatureboth water temperature is nearest to and the TEC25md line, temporally the lake increased is thermally at stratified0–45 m above in May–November bottom, especially [21 ].at Then,0–22 m both above water bottom. temperature Under water and EC25pressure temporally of 148 m, increased pure water at 0–45has the m abovemaximum bottom, density especially at 3.688 at °C. 0–22 The m increase above bottom. in water Under temperature water pressure in Figure of 4a 148 should m, pure produce water ◦ haspycnal the maximuminstability, density if it is at pure 3.688 water.C. The However, increase in the water increase temperature in EC25 in Figure (i.e., 4increasea should producein ionic pycnalconcentration) instability, increases if it is pure water water. bulk However, density, which the increase can keep in EC25 the lake (i.e., water increase pycnally in ionic stable concentration) [19]. The increasesincrease in water both bulk water density, temperature which can and keep EC25 the in lake Figure water 4 suggests pycnally that stable the [19 geothermal]. The increase heat in input both wateroccurs temperature by the heat andsupply EC25 from in Figure the underlying4 suggests thatzone. the The geothermal MT survey heat of input Goto occursand Johmori by the heat[20] supplyindicates from that the a underlyingreservoir of zone. low specific The MT resistivity survey of Gotolies at and 260–310 Johmori m [below20] indicates the lake that basin, a reservoir and that of lowsuch specific a reservoir resistivity is horizontally lies at 260–310 limited m belowto the thedeepest lake basin,point and thatits surrounding such a reservoir area. is The horizontally increase limitedin water to temperature the deepest and point EC25 and was its surrounding not noticeably area. observed The increase at the inother water six temperature sites (white andcircles EC25 in wasFigure not 1). noticeably Hence, observedthe geothermal at the otherheat sixis probably sites (white supplied circles infr Figureom the1). hydrothermal Hence, the geothermal reservoir heatunderlying is probably the deepest supplied area. from the hydrothermal reservoir underlying the deepest area.

Figure 4. a b ◦ Figure 4. VerticalVertical distributionsdistributions ofof ((a)) waterwater temperaturetemperature and (b) electric conductivity at at 25 25 °CC at at site site MD. Their distributions at 0–45 m (two-way arrows) above the bottom are deployed in more detail MD. Their distributions at 0–45 m (two-way arrows) above the bottom are deployed in more detail (inserted). The Tmd line indicates a vertical distribution of water temperatures, giving the maximum (inserted). The Tmd line indicates a vertical distribution of water temperatures, giving the maximum density of pure water. density of pure water. The geothermal heat flux at bottom of Lake Kuttara is calculated as that averaged over the period The geothermal heat flux at bottom of Lake Kuttara is calculated as that averaged over the between two profiles by using the temperature profiles in Figure4a. The geothermal heat flux QB is period between two profiles by using the temperature profiles in Figure 4a. The geothermal heat flux then given as follows: QB is then given as follows:   Z h  QB = ∆ ρwCpT dz /∆t (8) 0 QB = Δ 𝜌𝐶𝑇 𝑑𝑧/Δ𝑡 (8) where h is height (m) above bottom (here, = 45 m). Boehrer et al. [19] estimated Q at about 1 W/m2, B which is much larger than that in geothermally inactive volcanic lakes such as Tazawa and Shikotsu where h is height (m) above bottom (here, = 45 m). Boehrer et al. [19] estimated QB at about 1 W/m2, (0.1 W/m2 order) [19] or in ocean (less than 0.1 W/m2)[26,27]. Similarly, the Q values were obtained which is much larger than that in geothermally inactive volcanic lakes such as BTazawa and Shikotsu by using the loggers’ data at 0−45 m above the bottom, after the calibration of loggers’ temperature by (0.1 W/m2 order) [19] or in ocean (less than 0.1 W/m2) [26,27]. Similarly, the QB values were obtained the profiler’s temperature. by using the loggers’ data at 0−45 m above the bottom, after the calibration of loggers’ temperature Figure5 shows time series of (a) daily mean water temperature averaged at 0–45 m above bottom, by the profiler’s temperature. and (b) calculated daily mean (gray line) and monthly mean (black plotted line) heat flux Q at the Figure 5 shows time series of (a) daily mean water temperature averaged at 0–45 mB above deepest point of Lake Kuttara. In June–December, when the lake is thermally stratified, the temperature bottom, and (b) calculated daily mean (gray line) and monthly mean (black plotted line) heat flux QB increase is seen. Two vertical circulations in late December–early January and late April–early May at the deepest point of Lake Kuttara. In June–December, when the lake is thermally stratified, the abruptly decrease and increase the temperature, respectively. The weak stratification in the ice-covered temperature increase is seen. Two vertical circulations in late December–early January and late periods (red arrows in Figure5a; 82 days in 2013, 57 days in 2014 and 31 days in 2016) does not induce April–early May abruptly decrease and increase the temperature, respectively. The weak stratification in the ice-covered periods (red arrows in Figure 5a; 82 days in 2013, 57 days in 2014 and 31 days in 2016) does not induce a noticeable temperature increase. In the winter of 2015, when the lake was not completely ice-covered, the vertical mixing under long weak stratification decreased

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a noticeable temperature increase. In the winter of 2015, when the lake was not completely ice-covered, water temperature. The monthly mean heat flux QB was evaluated at 2.8–10.7 W/m2 for the periods the vertical mixing under long weak stratification decreased water temperature. The monthly mean of the noticeable increase in water temperature.2 The daily mean QB exhibits large fluctuations, and heat flux QB was evaluated at 2.8–10.7 W/m for the periods of the noticeable increase in water thus the monthly mean values are here related to monthly mean geothermal heat flux in Okunoyu temperature. The daily mean QB exhibits large fluctuations, and thus the monthly mean values are fromhere the relatedheat budget to monthly estimate. mean geothermal heat flux in Okunoyu from the heat budget estimate.

FigureFigure 5. Temporal 5. Temporal variations variations ofof( a()a daily) daily mean mean water water temperature temperature averaged averaged over 0–45 over m above 0–45 bottom m above at site MD and (b) calculated daily mean (gray line) and monthly mean (black plotted line) heat flux bottom at site MD and (b) calculated daily mean (gray line) and monthly mean (black plotted line) QB. Red arrows indicate completely ice-covered periods in Lake Kuttara. heat flux QB. Red arrows indicate completely ice-covered periods in Lake Kuttara. Figure6 shows daily mean heat fluxes at water surface of Okunoyu in Equation (1), and evaluated heatFigure flux 6H Ginshowsby groundwater daily mean input heat in fluxes Equation at (4).water In the surface calculation of Okunoyu of HGin, ∆G /in∆ tEquationin Equation (1), (1) and 3 evaluatedwas approximately heat flux HGin given by groundwater as ρw cp·V·(∆ Tinputav/∆t )/inA Equation0, where V (4).is waterIn the volume calculation (m ), ofA0 HisGin surface, ∆G/∆t in 2 Equationarea (m(1)) was and apprTav isoximately water temperature given as (K)ρw c averagedp∙V∙(∆Tav/∆ overt)/A the0, where pond. V Here, is water the measured volume (m water3), A0 is surfacetemperature area (m2) was and assumed Tav is water to represent temperatureTav. This (K) is averaged based the over fact thatthe pond. the measuring Here, the point measured is located water near one of the geothermally bubbled regions and an airborne IR survey of Terada et al. [28] above temperature was assumed to represent Tav. This is based the fact that the measuring point is located nearOkunoyu one of the deployed geothermally nearly bubbled a uniform regions surface and water an airborne temperature. IR survey At the of water Terada surface, et al. upward [28] above longwave radiation L and latent heat flux Q are remarkably large, reflecting the high water Okunoyu deployed nearlyup a uniform surface waterE temperature. At the water surface, upward temperature in Figure3. Heat flux by river inflow HRin in Equation (3) was then neglected because the longwave radiation Lup and latent2 heat flux QE are remarkably large,2 reflecting the high water magnitude HRin = 100–300 W/m is much smaller than HRout = 3–6 kW/m by the intake. The heat flux temperature in Figure 3. Heat flux by river inflow HRin in Equation (3) was then neglected because HGout by groundwater outflow in Equation (4) was also neglected, because muddy deposits at bottom the magnitude HRin = 100–300 W/m2 is much smaller than HRout = 3–6 kW/m2 by the intake. The heat is impermeable [16]. Geothermal heat flux HGin by groundwater inflow was evaluated at a range of flux 3.5–8.5HGout by kW/m groundwater2, which completely outflow in corresponds Equation to (4) 3.4–8.2 was also MJ/day. neglected, because muddy deposits at bottom is impermeable [16]. Geothermal heat flux HGin by groundwater inflow was evaluated at a range of 3.5–8.5 kW/m2, which completely corresponds to 3.4–8.2 MJ/day.

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Figure 6. Daily mean heat fluxes calculated for Okunoyu in 2013–2016. See the text for the symbols. Figure 6. Daily mean heat fluxes calculated for Okunoyu in 2013–2016. See the text for the symbols.

Figure7 shows a relation between monthly mean QB (Kuttara) delayed by 5 months and monthly B meanFigureH Gin7 shows(Okunoyu). a relation Giving between a time lag monthly of 5 months) mean to QQB,(Kuttara) there exists delayed a significant by 5 correlation months and 2 2 monthly(R = 0.586,mean pH

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FigureFigure 7. 7.Relation Relation between between monthly monthly mean meanQ QBBin in Lake Lake Kuttara Kuttara and and monthly monthly mean meanH HGinGin in Okunoyu.Okunoyu.

TheTheQ QBBvalues values areare delayeddelayed byby 55 monthsmonths forforH HGinGin..

4.4. Conclusions Conclusions KuttaraKuttara Volcano,Volcano, Hokkaido, Hokkaido, Japan,Japan, consistsconsists ofof thethe westernwestern geothermalgeothermal areaarea andand easterneastern LakeLake Kuttara.Kuttara. The The heat heat flux flux by geothermalby geothermal groundwater groundwa inputter input was evaluated was evaluated from the from heat the budget heat estimate budget forestimate a hydrothermal for a hydrothermal pond, Okunoyu, pond, Okunoyu, in the geothermal in the geothermal area, and area, compared and compared with geothermalwith geothermal heat fluxheat at flux the at bottom the bottom of Lake of Lake Kuttara. Kuttara. During During the the thermal thermal stratification stratification of of the the lake, lake, the thelower lower layerlayer ◦ increasedincreased both both the the water water temperature temperature andandelectric electric conductivityconductivity atat 2525 °CC (EC25).(EC25). ByBy usingusing thethe MTMT method,method, Goto Goto and and Johmori Johmori [20 [20]] pointed pointed out out that that a hydrothermala hydrothermal reservoir reservoir lies lies at 260–310 at 260–310 m below m below the lakethe lake bottom bottom and and hydrothermal hydrothermal reservoirs reservoirs are justare just below below the the western western geothermal geothermal area area as theas the zone zone of relativelyof relatively low low specific specific resistivity. resistivity. Hence, Hence, the the increase increase in in both both water water temperature temperature and and EC25 EC25 inin LakeLake KuttaraKuttara suggestssuggests thatthat geothermalgeothermal heatheat isis suppliedsupplied fromfrom aa hydrothermalhydrothermal reservoirreservoir underlyingunderlying thethe bottom.bottom. GivingGiving aa timetime laglag ofof 55 monthsmonths toto thethe geothermalgeothermal heatheat fluxfluxQ QBB atat lakelake bottombottom calculatedcalculated monthlymonthly by by monitored monitored water water temperature, temperature, the the heat heat flux fluxH HGinGin byby geothermalgeothermal groundwatergroundwater inputinput inin OkunoyuOkunoyu isis significantly significantly correlatedcorrelated withwithQ QBB.. AA supposedsupposed calculationcalculation byby meansmeans ofof thethe Darcy’sDarcy’s lawlaw givesgives 148–149 148–149 daysdays asas aa timetime necessarynecessary forfor thethe passagepassage ofof geothermalgeothermal waterwater inin thethe mediamedia betweenbetween thethe lake lake bottom bottom and and the the underlying underlying hydrothermalhydrothermal reservoir. reservoir. The The geothermalgeothermal leakageleakage inin LakeLake KuttaraKuttara couldcould thus thus be be connected connected toto thethe western western geothermalgeothermal areaarea byby thethe deeperdeeper geothermalgeothermal zonezone ofof KuttaraKuttara Volcano,Volcano, and and the the magnitude magnitude and and variation variation of ofQ QB Bmay may become become an an indicator indicator for for volcanic volcanic activity. activity.

AuthorAuthor Contributions: Contributions:K.C., K.A. Y.O. Chikita, and Y.S. Y. participatedOchiai and inY. allSakata the filed part surveysicipated to in set all and the manage filed surveys field instruments; to set and H.O.manage set somefield instruments; instrument at H. Lake Oyagi Kuttara set some and instrument suppliedmany at Lake data; Kuttara K.C. and conceived supplie andd many designed data; K.A. the Kuttara Chikita Volcanoconceived research and designed project, andthe Kuttara wrote the Volcano paper. research project, and wrote the paper. Funding: Part of this research was funded by the Earthquake Research Institute, the University of Tokyo Joint Funding: Part of this research was funded by the Earthquake Research Institute, the University of Tokyo Joint Usage/Research Program. Usage/Research Program. Acknowledgments: We would like to express gratitude to graduate students of Laboratory of Physical Hydrology, DivisionAcknowledgments: of Natural History We would Sciences, like Graduate to express School gratitude of Science, to grad Hokkaidouate students University, of forLaboratory their great of help Physical in the fieldHydrology, surveys. Division We are also of Natural indebted History to Noboribetsu Sciences, On-sen, Gradua Co.,te School Ltd. and of Science, members Hokkaido of the group University, “Water andfor their Cep ingreat Lake help Shikotsu” in the field for their surveys. welcome We are data also supply. indebted to Noboribetsu On-sen, Co., Ltd. and members of the group Conflicts“Water and of Interest: Cep in LakeThe authorsShikotsu declare” for their no conflictwelcome of data interest. supply. Conflicts of Interest: The authors declare no conflict of interest. References

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