Source variability of sediments in the Shihmen Reservoir, Northern Taiwan: Sr isotopic evidence

Abstract

High turbidity in water reservoirs during the monsoon season has caused serious problems for drinking water quality in Taiwan. In this study, we collected stream waters and streambed sediments, including bed loads and fresh landslide deposits, in the Dahan Stream and analyzed 101 87Sr/ 86Sr ratios in different grain-size fractions in the streambed sediments from the upper catchment and 3 sediment cores from the Shihmen Reservoir, as well as dissolved major and trace elements in stream waters, to identify possible changes of sediment sources in the reservoir. The 87Sr/ 86Sr ratios in stream water and streambed sediments ranged from 0.71296–0.71349 and 0.71714–0.71843, respectively. The large Sr isotopic difference between the paired stream water and streambed sediments are evidence for disequilibrium water/rock interaction in the weathering limited regions. The Sr isotopic compositions in bulk sediments and various grain-size fractions showed no clear spatial variability. However, there is an interesting relationship between grain sizes and Sr isotope ratios, implying effects of mineral sorting during sediment transportation. The illite crystallinity and the illite chemistry index of streambed sediment suggest moderate to strong chemical weathering in this regions. The dissolved constituents in the Dahan stream support that silicate weathering is the predominated controlled mechanism and only minor carbonate dissolution occurred near the Central Mountain Range. The 87Sr/ 86Sr ratios in the sediment cores suggest that modern reservoir sediment is mainly derived from the upper watershed, composed of primarily Oligocene sedimentary rocks. However, the sediment sources have changed significantly since the reservoir built 37 years ago, the 87Sr/ 86Sr ratios were spread widely outside the present-day observations in the Dahan catchment. The homogeneous distribution of 87Sr/ 86Sr in the upper reservoir cores reflected disturbance due to recent turbidity events within the reservoir. These new chemical and isotopic data provide useful spatial and temporal information of weathering sources in a high denudation sub-tropical mountainous watershed.