濁水溪沖積扇南翼地質岩心中砷釋出機制探討

Abstract

本研究針對濁水溪沖積扇南翼包括扇央及扇尾部分岩心之樣本進行分析，樣本為有機質豐富之細沙、泥或黏土等，由XRD之分析指出，其成份以二氧化矽及綠泥石等黏土礦物為主。其中平均砷含量高達62.35 mg/kg，而阻水層岩心中砷濃度略高於含水層，主要集中於鐵及矽酸鹽相，約佔總量80%以上。由XRF之資料顯示，砷之分布與鐵之(氫)氧化物及鐵硫化合物有關，與矽化合物應無較大之關聯性，於XPS之分析得知，鐵化合物應以無晶型之(氫)氧化鐵為主，也可能為還原溶解之釋出機制中砷之主要來源。地下水以Ca-Na-HCO3型態為主，砷濃度範圍從0.02~0.57mg/L，平均值為0.17 mg/L，皆大於飲用水標準(0.01 mg/L)。於扇央與扇尾皆監測出高砷濃度之地下水，而地下水中砷濃度與HCO3- 及NH4+之相關性佳，但與TOC及鈣離子之關係較差，顯示有機質參與反應，其主要之作用為增加地下水之碳酸根及氨氮離子濃度，使得地下水越趨於還原狀態。本區地層中砷之可能釋出機制，應與無晶型之鐵(氫)氧化物還原溶解有關，且碳酸根之取代也是主要反應之一。

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.