1. Summary and Recommendation
Isotope Hydrologic Variations of Choshuichi Alluvial Aquifers After the Chi-Chi Earthquake
Chung-Ho Wang1, Ming-Kuo Lee2 and Chi-Yuen Wang3
1. Institute of Earth Sciences, Academia Sinica, Nankang, Taipei, Taiwan 11529, ROC 2. Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA 3. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA
Introduction
The aim of this study is to assess the effect of Chi-Chi Earthquake on the hydrogen and oxygen isotope compositions of groundwaters in the Choshuichi Alluvial Fan, central Taiwan.
Study Area
Upper: Groundwater monitoring network of Water Resources Bureau in the Choshuichi Alluvial Fan. Stations with open circle are wells with anomalous isotope variations after the main Chi-Chi Earthquake. Dashed lines are the boundaries of proximal, middle and distal regions defined in reports of the Water Resources Bureau of Taiwan. Two subdivisions can be divided in terms of subsurface hydrogeology and groundwater flow lines: the northern Changhua area and southern Yunlin area, separated by the Choshui Chi.
Lower: General hydrogeological framework of Chochuichi Alluvial Fan. F1, F2 and F3 denote unconfined and confined aquifers, respectively.
Sample Anomaly
Comparisons are made for two sets of groundwater samples (180 each) that were collected before (Jan, Feb, Mar, 1999) and after (Oct, Nov, Dec, 1999) the Chi-Chi Earthquake (September 21, 1999; Mw=7.5).
The seasonal variations (< two analytical precisions) are subtracted before evaluating the isotope anomalies caused by the Chi-Chi Earthquake.
Results and Discussion
In general, the mean stable isotopes show relatively depleted values after the main Chi-Chi Earthquake for all three regions and most aquifers in the Choshui Alluvial
Hydrogen and oxygen isotope compositions of five isotope-anomalous groundwater samples in the proximal region. Hexagonal symbols are samples before the earthquake; open squares represent those after the earthquake. Arrow indicates the isotope-shifting direction. Solid line is the local meteoric water line (LMWL) and solid triangles are those of neighboring local river waters. Four isotope-depleted-shifting groundwater samples can be explained by mixing with neighboring river water samples. The WT(I) sample would be modified with a surface water of relatively enriched isotope values.
The profile from Ershui (ES) to Shikong (CG) section with lithology and oxygen isotope values of respective groundwaters collected after the earthquake. Samples with anomalous isotope variations are shown by open square (depleted effect) or open triangle (enriched effect) symbols for each screen of six stations. The arrows are the possible flow paths of groundwaters during the earthquake by evaluating the shifted isotope values with adjacent water samples. Some vertical mixing between upper and lower aquifers are implied by the homogeneous values after the earthquake. The ruptures generated in the aquitard by the earthquake stress probably caused the vertical mixing between upper and lower aquifers
Summary and Recommendation
About one fourth or more groundwater samples collected in this study show significantly anomalous isotopic changes after the main Chi-Chi Earthquake and the affected percentages are getting higher for the well-confined regions away from the epicenter. More groundwater samples showed anomalous changes in the northern Changhua area.
For unconfined aquifers, surface and neighboring river waters are the possible main sources for the isotope changes during the earthquake.
For confined aquifers, groundwater samples in the northern Changhua area carry more anomalous isotope signals relative to those of the southern Yunlin area. Horizontal transmission expedited by the earthquake stress and minor vertical mixing between aquifers are the main mechanisms of water isotope variations.
A further work is needed to fully integrate the isotopic data with other chemical, geophysical and hydrological information to provide a deep insight of the earthquake effect on the hydrology and beget a more integral hydrogeology framework in the Choshuichi Alluvial Fan.