Chemical Compositions of Hot Spring Waters in the Tatun Volcanic Area and Their Influence on Stream Waters

• Main Authors: Yaw-Lin Chen, 陳耀麟
• Other Authors: Huann-Jih Lo
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/31371395199507689584

博士 === 國立臺灣大學 === 地質科學研究所 === 90 === According to the ratios of Cl-/SO4-2 and Na+/Ca+2, the springs in the Tatun volcanic area of northern Taiwan are classified into volcanic (both ratios <1) and sedimentary (both ratios >1) types. Chemical concentrations of the spring waters show variations with both time and space. Spatial variations are ascribed to two factors. First, the spring waters of the volcanic type are mainly supplied from volcanic gases and andesitic rocks, while those of the sedimentary one are from volcanic gases, rhyolitic pebbles and brines in the Wuchihshan Formation and seawater. Secondly, field observations show that the flux of volcanic gases and the degree of alteration of rocks vary from spring to spring. On the other hand, temporal variations may be attributed to the contribution of different amounts of meteoric water during different rain events. The values of d34S of the spring waters of the sedimentary type are similar to those of sedimentary materials and imply that reservoirs of these springs may be in the sedimentary rocks. Ionic ratios between these spring water and subsurface hot waters in the Wuchihshan Formation fall in a narrow range, suggesting a simple mixing of the subsurface hot water with water of meteoric origin in the shallow depth. On the other hand, the values of d34S of the spring waters of the volcanic type are similar to those of volcanic materials and suggest that reservoirs of these springs may be in the volcanic rocks. Since most of the springs appear in pyroclastic rocks, it is reasonable to assume that the pyroclastic rocks could be the most potential reservoirs for the springs of the volcanic type because the pyroclastic rocks are loosely consolidated and are generally considered to be more permeable than massive lava flows. Reservoirs in the volcanic rocks are mainly supplied from meteoric water which runs through different paths taking different time to contribute to the springs. The volcanic gases may dissolve in water to contribute chemical components such as SO4-2 and Cl- to the spring reservoirs and may heat up the spring waters as well. Such hot acidic waters may act as major liquid media for hydrothermal alteration of the volcanic rocks. Chemical components such as Na+, K+, Mg+2, Ca+2, Al+3 and Si+4 etc. may release to the spring reservoirs during the hydrothermal alteration of the volcanic rocks in acidic fluids as revealed by laboratory studies. The values of d34S of the spring waters of the sedimentary type suggest that there are other sources except volcanic gases contributing anions. It is considered that their cations may essentially be released from the hydrothermal alteration of rhyolitic pebbles in the Wuchihshan Formation. Brines in the sedimentary rocks may also be the source supplying chemical components. Moreover, seawater may also contribute to the spring waters of Chingshan and Dapu. Distribution patterns of average chemical concentrations among the stream waters are certainly quite different. In NH (Nanhuang-Hsi), average chemical concentrations increase from the up- to down-stream with an abrupt change in the middle-stream. As to BH (Beihuang-Hsi), a generally down-stream-ward decrease of average chemical concentrations of the stream water is apparent. On the contrary, a generally down-stream-ward increase of average chemical concentrations occurs in SH (Swang-Hsi). The discrepancies may essentially be due to the distribution of hot springs and the chemical nature of hot spring waters in each stream system. It is also interesting to see that average chemical concentrations are greatly higher in both NH and BH than in SH. In the area, a great number of hot springs are distributing in NH and BH, but they are rare in SH stream systems. Consequently, it is easy to see that average chemical concentrations are greatly higher in both NH and BH than in SH stream waters.