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ABSTRACT1 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC 2 Central Geological Survey, MOEA * Corresponding author address: Dr. Sheng-Rong Song, Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC; E-mail: srsong@ntu.edu.twTo evaluate potential monitoring sites as well as useful ions which are capable of serving as earthquake precursors, ten subsurface water bodies in different tectonic domains in southwestern, northern and northeastern Taiwan were selected. They included the deep circulation of hot springs, shallower artesian springs and groundwater. Most of the hot springs clearly show chemical anomalies which correlate with earthquake events during the monitoring periods. Against this, the groundwater does not correspond to any events. Hot springs from deeper reservoirs are superior to artesian springs and groundwater from shallower reservoirs. The artesian spring from the smaller subsurface water body is superior to the groundwater from larger reservoirs. Aside from this, anions, especially chloride, outperform cations as geochemical precursors for earthquake monitoring. It is unambiguous that the major factors that determine the usefulness of chemical anomalies in the waters for earthquake precursory monitoring are the kinds, the depths and the size of reservoirs and the ion species of the subsurface water bodies.

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A phenomenon of mixing blue and brown seawater, called "Yin-Yang Sea", has existed for decades in Liang-Dong Bay. In order to determine the chemical composition and origin of pollutants in the bay, each of 5 water samples from the stream and bay were collected for chemical analyses. Meanwhile, suspension particles in the bay, the sediments on the bottom of bay and the precipitation on the bottom of the stream were also collected for chemical, X-ray diffraction and particle size analyses.The suspension particles are amorphous and are predominantly composed of Fe(OH)3. This is showen by the solubility products of water samples in the bay being higher than the ksp of Fe(OH) 3 . The deposits on the bottom of the stream also show similar results. However, the chemical concentrations of major ions, e.g. Na + , K + , Ca +2 , Mg +2 , Cland SO 4 -2 , are much higher in the bay water than of the stream water. Nevertheless, Fe +2 , Mn +2 , Cu +2 and Si +4 show positive anomalies in the lower stream where the abandoned pits are located. Thus, it is assumed that minerals in the pits, such as pyrite and marcasite, are the main source supplying iron through weathering. Iron released by minerals is carried by the stream and arrives in the bay in hours. Since the oxidation rate becomes very slow in the bay because of the effects of salts and neutral pH values, most of the particles of Fe(OH)3 form in the bay gradually.