/0' J. Hot Spring Sci. /3 31ῌ+*, ,**3ῌ῎  + *,,+0 2,+1,+Changes in Water Chemistry and Color of Mizugama,a Crater Lake of Kusatsu-Shirane Volcano andIts Implications for Volcanic Surveillance
Joyo OSSAKA+ and Yoshikazu KIKAWADA,

Abstract
The phreatic eruption had occurred at the Mizugama crater of Kusatsu-Shirane volcano in March, +310. Before the eruption, the color of the Mizugama crater lake hadchanged from the conventional yellowish brown to bluish color due to the supply of thespring water with high concentration of Fe, from the lake bottom. In the summer of,**/, the Mizugama water changed to yellowish green with the increasing of the Fe,/Fe- ratio in the lake water, while the water color had been brown since the +32*s at least.
After that, signs of increasing volcanic activity have been observed around the Mizugama crater since ,**1. The chemistry and color of the Mizugama water thus may be agood indicator of the volcanic activity of Kusatsu-Shirane volcano.
Key words : Kusatsu-Shirane volcano, Mizugama crater, crater lake, prediction of eruption

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,!"#$%&'  +*, ῌ 2//. ()*+,-. 1 ῌ + , Department ofMaterials and Life Sciences, Faculty of Science and Technology, Sophia University, 1ῌ+ Kioicho,
Chiyoda, Tokyo +*,ῌ2//., Japan.

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and Watanuki, +33* ; R +331

Fig. + (a) The location of Kusatsu-Shirane volcano in Gunma Prefecture, and (b) a map
around the summit of Kusatsu-Shirane volcano. Contours are in meters.

98

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Table + The water chemistry of lake and spring waters in the Mizugama crater before and after the phreatic
eruption in +310.
SamplingdateAtmos. Water
Content (mg/L)
Temp. Temp. pH,,()()Na K Ca Mg T-Fe Fe, Fe- Al- Cl SO., H,SiO-

0-Jul-+300++-Jul-+301.-Aug-+302,3-Jul-+303,.-Jul-+31,3-Jul-+31.
,3-Jul-+31/,-Aug-+311

+.4,,,4/+/4+240+240+34+34/+14.
,+4/+243,*40,*4,+40,14,,42/40,41- ,42,40,403 ,41* 14/,41/42,4-/ ,-42+--Mar-+310
Spring a +-Aug-+310
waterb +-Aug-+310

242,+4*+*4,+243,40 --42 +*4.
,4-+ ,242 24.

,0* +24+ 124* 124* *4*,22 043 --, ,3. -24*+3/ ,40 ,*2*,*2  ,20.

+2+3/+342,,42,42-*/+-/ +41 +//-+2,Lakewater,*41+4, 3.4. -41

+4/ +*+ -40+4, 3/43 *4,3,42 ,/, +/43/42+/42   -34. *4/ 0-1-,42 ,-42 34*  .-40 +-4. -*4,   +-4-   +,42 04- 04/  /,42 34- .-4/  -343 -+4- 240 -140 *41 /+2-+* -** +*4* +1, +4- ,.30/43 //43 /+40.4--04.
.,4/+1*99 

Photo + Photograph of (a) Mizugama before the eruption occurred in +310 (July ,/, +31/), (b) the newexplosion crater formed by the eruption at Mizugama in +310 (March +-, +310), (c) the “black”
spring water found in the new explosion crater in +310, (d) Mizugama on August +, ,**-, (e)Mizugama on July ,3, ,**/ and (f) Mizugama on October ,3, ,**2.

-+310 

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L ,**/ S ijW5L³&L22D

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Table ,The water chemistry of Mizugama lake water in recent years.

Sampling
dateAtmos. Water
Temp. Temp.
()
()

pH,

Mg

+-Aug-,**,3-Jul-,**/,3-Oct-,**2+/4,,-42,4-,4/. ++4* ,4- +*,410 .42 *41 .14,
,41- 14/ -4- 2.4/-40+41,43+24.
+340
-42

Content (mg/L)
Na



K

Ca,

T-Fe Fe, Fe- Al- Cl SO., H,SiO-040 14, ,34. ..41 *4*-,4* -,4* *4* ,*4. *42-,4- +,40 +341 ,34/ +4*2,,.,30,,++4,//40/-4.

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J -+ ++3ῌ+,2.
Takano, B. (+321) : Correlation of volcanic activity with sulfur oxyanion speciation in crater
lake. Science, ,-/, +0--ῌ+0-/.
Takano, B. and Watanuki, K. (+33*) : Monitoring of volcanic eruption at Yugama crater lake
by aqueous sulfur oxyanions. J. Volcanol. Geotherm. Res., .*, 1+ῌ21.

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102

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