USING HYDROGEN, OXYGEN, AND TRITIUM ISOTOPES TO IDENTIFY THE HYDROLOGICAL FACTORS CONTRIBUTING TO LANDSLIDES IN A MOUNTAINOUS AREA, CENTRAL TAIWAN

Abstract

Over a 1-year period, 343 samples, including precipitation, creek, pond, and groundwater, were collected from June 2003 to May 2004. Analyses were performed for stable oxygen and hydrogen isotope compositions. Selected samples were also analyzed for tritium. The goal was to identify possible hydrologic factors contributing to a severe landslide in the Li-Shan area, central Taiwan. The isotope characteristics indicate that groundwater from Fu-Shou-Shan farm located up-slope from the landslide area is a major source for slope groundwater, in addition to precipitation. The groundwater is mainly recharged by pond water at Fu-Shou-Shan farm. According to the calculation of a two-end member equation with δ 18O, the contribution of farm groundwater to slope groundwater is significantly higher than that of precipitation, up to a factor of five. The estimated drainage efficiency of the existing system is only 23%. Draining off the slope groundwater in the up-slope region to decrease farm groundwater flow into the slope area is a feasible strategy to effectively reduce the risk of landslide.