Groundwater chemical anomaly before and after the Chi-Chi earthquake in Taiwan

The Chi-Chi earthquake (M_L = 7.3 and 12 km focal depth) occurred on 21 September 1999 in central Taiwan resulting in deaths totaling over 2300. After the earthquake we collected commercialized bottled water (namely Chingjing water), pumped from wells at Puli, Nantou County, central Taiwan, about 10 km northeast of the epicenter of this earthquake. The anions, e.g., Cl^-, SO₄^2- and NO₃^-, of the bottled water prepared from 1 December 1998 until after the shocks for duration of about two and a half years were analyzed. The analytical data display that the concentrations of both sulfate and nitrate have increased steadily after March 1999, as compared to the constant level measured since December 1998. They reached an excess of 129.9 and 94.7% in April, then dropped in July 1999, and decreased to about 10 and 20% below the constant values, respectively, till after the earthquake. These precursory chemical changes may be attributed to the stress/strain-induced pressure changes in the subsurface water systems, followed by limited precursory geochemical discharges generated by limited changes in the levels of the subsurface reservoirs, thus finally leading to the mixing of deeper aquifer. Then, the groundwater was diluted by the injection of surface water into the subsurface system. This result, therefore, confirms that groundwater chemistry is promising for monitoring earthquakes.