利用稀土元素及多變量分析研究 蘭陽平原地下水之砷汙染

Study of groundwater arsenic pollution in Lanyang Plain using rare earth elements and Multivariate Analysis

詹士珍(Shih-Chen Chan)

指導教授 ： 呂學諭

國立中正大學/理學院/地球與環境科學系應用地球物理與環境科學研究所/碩士(2014年)

摘要

本研究於蘭陽平原之地下水進行各項陽離子分析、多變量統計分析，來對該區地下水的化學環境及特性進行解釋判讀，也將稀土元素特徵地質方法應用於本研究中做為一個有利的重金屬示蹤劑，以了解此地下水環境之成因及砷之流佈。
砷含量於此地含水層各抽水井濃度皆不容忽視。第一含水層以蘭陽溪北部宜蘭市、蘭陽溪下游東南部五結監測井地下水為大宗，與當地掩埋場汙水滲漏及工業區發展有密切關聯。第二、第三含水層濃度主要與強還原環境有極大相關性，當中五結、龍德、利澤三口深井濃度為最高，蘭陽溪以北的深井也有受砷汙染的現象，這些地下水鐵含量高、代表著此地曾含有豐富且吸附力強的鐵水合物，且沉積物年齡多處於砷含量豐富的第四紀末次冰期，而中央山脈之泥岩、變質雜岩區的黃鐵礦於水－岩作用時造成蘭陽平原東南端的砷含量遠高於北區。多變量分析分析結果與上述砷之富集大致相符。
稀土元素特徵之趨勢上，呈現由蘭陽沖積扇扇頂粗顆粒高溶氧之氧化地下水至蘭陽溪下游細顆粒強還原地下水及過渡區存在富集現象的程度差異，隨地下水向下游流動，氧化還原電位漸漸降低，凸顯輕、重稀土元素在脫附及再吸附之化學特性不同在化學行為上的差異，而酸鹼值由酸至鹼的改變造成陰離子團與陽離子之分異，使蘭陽溪下游輕稀土元素逐漸貧乏、砷濃度反而提升；鈰正異常分布在數口強還原環境且深度較深的水井，與砷呈現正相關，其他呈還原環境之水井則大多為負異常的現象，因鈰異常主要反應著過去的地下水歷史，若地下水年紀較輕則異常現象則不易觀察。

關鍵字

砷

並列摘要

The study area, Lanyang Plain in the northeastern Taiwan, has highly developed agriculture and aquaculture, which consume over 70% of the water supplies. Groundwater is frequently considered as an alternative water resource. However, the serious arsenic pollution of groundwater in Lanyan Plain should be well studied to ensure the safety of groundwater usage.
In this study, 39 groundwater samples were collected. The chemical results demonstrate that the pH values of groundwater at downstream of Lanyang River are about 6.5 to 8.0 while those sampled from upstream area are relatively acid. The redox potential (ORP) in all samples appear strong reducing condition, except for some groundwater sampled from upper alluvial fan with ORP in the range of 0 to 100. The groundwater samples with higher electric conductivity were collected from the coastal area, which demonstrates that groundwater salinization is a considerable process to degrade the groundwater quality.
The groundwater samples collected from the first aquifers have high arsenic contents at WJ and YN. It could result from sanitary landfill site and developing of industrial zone. The samples located northern and southeastern from second and third aquifer have much lower ORP and cause the higher concentration of arsenic. However, the pyrite in rocks from Central Mountain Range might be one of the major arsenic sources. On the other hand, the groundwater from the northern area has relatively lower arsenic concentrations of about 20 to 91 ppb, which could be controlled by the low pyrite content in the rock from Hseishan Range. As many previous researches described, the high arsenic content can be found in the aquifers deposited in Quaternary last glacial period. In addition, some groundwater samples demonstrate significant artificial contamination from agriculture and/or industry. The results of multivariate statistical analysis on hydrochemical data also support the above derivation.
Comparing the rare earth elements pattern upstream to downstream of Lanyang River, Light rare earth elements are depleted and concentration of arsenic increases gradually. It could be affected by the change of pH. Negative cerium anomaly is observed in most groundwater, because of the geological environment. Positive cerium anomaly is related to reducing condition and dissolution of minerals.

並列關鍵字

arsenic

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