Effects of the 921 earthquake on the water quality in the upper stream at the Guandaushi experimental forest

Abstract

A powerful 7.3 magnitude earthquake struck Taiwan on September 21, 1999. The stream water chemistry (pH, total alkalinity, conductivity, sodium, potassium, calcium, magnesium, ammonium, fluoride, chloride, sulfate, and nitrate) has been monitored since 1995 at the Guandaushi forestry riparian zone in central Taiwan. Collected data was used as a basis for comparing pre- and post-earthquake impacts. The pH, conductivity, and concentrations of Na, Ca, Mg, SO4, and HCO3 in stream water were lowest during the summer season, when stream water discharge was highest. On the other hand, the lowest concentrations of Cl, NH4, and NO3 in stream water occurred during the winter season, when stream water discharge was lowest. Also, K and F showed very little seasonal fluctuation in concentration. Downward trends in K and Ca were found 14 months prior to the earthquake; although, an upward trend occurred in NH4 at the same time.