| Summary: | 碩士 === 國立臺灣大學 === 生物環境系統工程學研究所 === 95 === This study focused on the analysis of arsenic contents in sediments of the mid-fan and distal-fan of the southern Chou-Shui river alluvial fan. Core samples consisted of silly sand, mud and clay. SiO2 and chrolite were identified as the main components in the core samples by XRD. The average arsenic concentration of core samples is up to 62.35 mg/kg. The arsenic content in aquitards is slightly higher than that in aquifers, and is concentrated in the sequential extraction step of Fe and silicate phases, more than 80% of all the samples. The results from XRF revealed that the arsenic distribution correlates well with iron (oxy)hydroxide and arsenic-bearing pyrite, but poorly correlates with silicate. The main form of iron was amorphous (oxy)hydroxide according to the analysis of XPS and could be the main source of the reduction dissolution mechanism. The groundwater is generally Ca-Na-HCO3 type, with HCO3- as the principle anion. Total arsenic concentrations in the analyzed wells vary between 0.02 and 0.57 mg/L, and are higher than the drinking water standard (0.01mg/L). Arsenic concentrations correlated well with HCO3- and NH4+ contents, but less correlated with TOC and calcium contents. The reaction with organic matters which yields an increase of the concentrations of HCO3- and NH4+ may drive the groundwater toward a more reductive condition. Reductive Fe (oxy)hydroxide is envisaged as the main mechanism for the release of As into groundwater in our study region. Moreover, the replacement of carbonate ion would be another possible mechanism of As release to groundwater.
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