蘭陽平原地下水砷之地化特徵及健康風險評估

Geochemical Characteristics of Arsenic in Groundwater of Lanyang Plain and Human Health Risk Assessment

李金靖(Jin-Jing Lee)

指導教授 ： 劉振宇

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.03053

摘要

台灣東北部之蘭陽平原地下水儲量豐富，被大量抽取用在灌溉、養殖、公共、民生等多項用途；然而，地下水中含有的砷濃度卻遠超過我國的飲用水水質標準，直接飲用將有危害人體健康，也可能經由食物鏈的累積，進而對人體健體產生危害。經濟部水利署、環保署及宜蘭縣環保局為監控蘭陽平原地下水水質與水位，分別設立的監測井，水利署從淺層到深層共設置22監測站40口監測井，環保署及環保局則分設置16口及20口的近地表的淺層監測井。為了探究蘭陽平原地下水中砷釋出的機制，本研究使用這些機關2004年測得之水質監測資料進行蘭陽平原地下水的水文地質化學特徵、岩心沉積物分析、含水層的氧化還原帶的劃定及物種組成模擬。根據因子分析 (factor analysis) 與集群分析 (cluster analysis) 的結果，不同深度地下水主要受鹽化因子 (salinization factor) 與砷污染因子 (As-polluted factor) 所影響，鹽化作用分布在蘭陽平原北部沿海地區，係受到淡、海水混合養殖池水 (brackish water) 入滲所致，因為養殖用水大量抽用地下水鹽化作用已從地表逐漸影響到深層地下水。在淺層地下水中砷的發生主要是人為污染入滲引發局部的還原作用所致，而在深層地下水中砷的發生則係因沉積物中的有機物礦化作用 (organic mineralization) 時，造成還原環境而從沉積物中釋出所致。另外，對蘭陽平原中、下游沖積扇之5口地質鑽探井之149個岩心樣本中砷與鐵含量分布與岩心表面之元素化學狀態分析顯示，在海相沉積層中砷與鐵含量呈現正相關，而在非海相沉積層則幾乎無關；掃描式電子顯微鏡 (SEM) 分析顯示砷附著在無結晶型的鐵氧化物，而x光光電子能譜儀 (XPS) 進一布分析鐵的成份指出以FeOOH、FeO與Fe3O4居多，且FeS出現在較深層處，反應出還原情形增強。藉由區別分析 (discriminant analysis) 劃定出蘭陽平原各地下水含水層的氧化還原帶，還原的潛勢是從東向西、由山區向海岸增高，這趨勢與濁水溪沖積扇的潛勢相似，卻與嘉南平原相反，反應出地區性水文地質條件之差異。根據地化模擬結果指出，隨著還原潛勢的增高，主要的物種砷從As5+(氧化帶)轉為As3+(還原帶與過渡帶)，Fe2+與HCO3- 則是Fe與C的主要的物種。綜合蘭楊平原地下水水文地質化學特徵、沉積物岩心分析、氧化還原潛勢及地化模擬結果可知，砷的釋出主要來自沉積物中含砷氫氧化鐵的還原性溶解，在淺層地下水中因人為污染的入滲造成局部水文地質環境的還原狀態改變而砷釋出；在深層地下水中係因沉積物中的有機物礦反應導致砷的釋出。此外，本研究亦使用宜蘭縣環保局1997-1999年家戶飲用水井的水質調查資料進行居民飲用地下水可能的健康危害風險評估，並發展快速推估飲用水井中砷濃度 ≧ 10 μg/L的預警模式，可供後續健康風險管理之用。在健康風險分析方面，經由地理統計方法¬—指標克利金－推估發現有6鄉鎮的家戶水井中砷濃度高於50 μg/L，包括礁溪鄉、宜蘭市、壯圍鄉、五結鄉、冬山鄉及羅東鎮，估算出最高的致癌風險值 (TRs) 是安全值 (10-6) 的2,400倍；進一步推估居民因攝入砷而誘發癌症的年致死數為24例 (每十萬人有5例)，以鄉鎮區分：宜蘭市10例、羅東鎮5例、五結鄉3例、壯圍鄉2例及冬山鄉1例。再者，運用邏輯斯迴歸方法 (logistic regression) 發展出快速預測家戶水井中砷濃度 ≧ 10 μg/L的預警模式，預測的準確率達89.8%且所推估之地下水中砷濃度分布與運用指標克利金方法推估的結果相符，只要檢測地下水中pH值、氨氮與鐵的濃度等三項水質參數，即可立即估算出水中砷濃度 ≥ 10 μg/L的機率。由於蘭陽平原地下水是多目標使用，為建置妥適的地下水使用安全與管理，本研究綜合我國飲用水水質標準、灌溉用水水質標準與養殖用水水質條件將水利署監測井地下水質參數歸納為四種危害指標－鐵與錳危害、氮化合物危害、鹽化危害及砷危害，並運用多變數指標克利金 (multi-variable indicator kriging) 在設定不同的危害機率條件下進行評估，分析結果依四種危害指標衍生出九類危害項目，且在各含水層劃分出安全及潛在危害區域，其中，鐵與錳危害普遍出現在各含水層，並顯示出現深層地下水的安全性較淺層地下水低，中、下游沖積扇地區地下水出現多重複合性危害，安全地水水區域侷限在上游近山區、下游近蘭陽溪與下游北部近山區之部分區域。經由上述研究結果更能提供給政府機對蘭陽平原地下水多目標使用的管理與發展策略的參考。

關鍵字

砷 ； 地化特徵 ； 健康風險 ； (多變數)指標克利金(MVIK) ； 邏輯斯迴歸 ； 掃描式電子顯微鏡 (SEM) ； X光光電子能譜儀 (XPS)

並列摘要

Elevated arsenic (As) in groundwater has been found in the Lanyang plain of northeastern Taiwan. The 2004 monitoring data of groundwater quality by the WRA (22 drilling stations with 40 wells), EPA (15 wells), and EPB (16 wells) were adopted to explore hydrogeochemical characteristics, mechanisms of As release to groundwater, As speciation and redox zonation of aquifer in the Lanyang Plain. Boundaries of groundwater of various depths were affected by water salinization in fishponds and As pollution was first delineated by factor analysis and cluster analysis. Large extraction of groundwater by aquaculture was a major cause of salinization, which gradually expanded from uppermost aquifer to lower aquifer. The pollutants (TOC and NH4+-N) from human activities infiltrated to shallow groundwater, triggering reducing reactions, and release As. The presence of As in deep aquifers results from the degradations of organic matter in sediment, causing groundwater progressively be more reductive. Furthermore, redox zonation and As speciation in aquifers were evaluated by using discriminant analysis and geochemical modelling (PHREEQC). The reducing conditions tend to increase from the mountainous area to the coastal area in aquifers of the Lanyang plain. Additionally, analytical results of a total of 149 geological core samples from 5 drilling wells located at mid- and distal-fan area, indicating a positive correlation of As and Fe contents in marine sequences. Surface analysis of core sample surface were performed by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), As was adsorbed onto/copricipitated with noncrystalline Fe hydroxides. The reductive dissolution of As-rich iron oxyhydroxide was postulated to be the major release mechanism of As into groundwater. Additionally, a total of 929 residential well water quality in the Lanyang Plain surveyed from 1997 to 1999, were adopted to evaluate the potential risk to human health and population mortality. The As concentrations estimated by indicator kriging (IK) in groundwater were high ( > 50 μg/L) at six townships−Jiao-Si, Yi-Lan, Juang-Wei, Wu-Jie, Don-Shan and Lou-Don. The estimated TR values exceeded ten times of the safe value (10-6) in the As-affected townships. The highest TR values were 2,400 times higher than the safe value. Most annual mortalities due to As-induced internal cancers occurred in the Yi-Lan township (10 cases), Lou-Don (5 cases), Wu-Jei (3 cases), Jhung-Wei (2 cases) and Don-Shan (1 case), and the highest number of mortalities per year in the study area is 24 (five fatalities per 100,000 persons). Moreover, a parsimonious model was developed to predict occurrence probability of As concentration ≧10 μg/L in groundwater by logistic regression. The model parameters were based on pH, and NH4+-N and Fe concentrations which are correctively associated with As occurrence. The percentage accuracy of total correct classification for the model was 89.8%. The model can be applied to inform local resident on potential exposure risk of As-affected groundwater. Finally, a probability map of groundwater resources for multi-purpose uses (irrigation, aquaculture and drinking water) was assessed using multiple variable indicator kriging (MVIK). The model was based on strictest criteria of multi-purpose uses, and hydrochemical parameters were classified four main hazard categories – saline hazard, nitrogen hazard, As hazard and Fe-Mn hazard. Analytical results demonstrate that deep aquifer has a high hazard rating and is less safe than the shallow aquifer. The Fe-Mn hazard in Lanyang Plain groundwater is present in most aquifers, and is partially combined with other hazards, such as the nitrogen hazard and the As hazard, thereby forming other hazards. Safe and potentially hazardous groundwater regions for multi-purpose uses were delineated according to estimated probabilities of 0.25, 0.5 and 0.75. Thus, a zonal management plan based on safe groundwater use is formulated. This plan is useful to local governments in developing groundwater resources in the Lanyang Plain.

並列關鍵字

Arsenic (As) ； geochemical modelling ； health risk ； indicator kriging ； logistic regression ； scanning electron microscopy (SEM) ； X-ray photoelectron spectroscopy (XPS) ； multi-variable indicator kriging (MVIK)

參考文獻

DOH, 2002. Prevention and Cure to Cancer: Liver Cancer. Department of Health, Taiwan (ROC) (http://www.doh.gov.tw/).

Acharyya, S.K., Chakraborty, P., Lahiri, S., Raymahashay, B.C., Guha, S., Bhowmik A., 1999. Arsenic poisoning in the Gangs delta. Nature 401, 545.

Acharyya, S.K., Lahiri, S., Raymahashay, B.C., Bhowmik, A., 2000. Arsenic toxicity of groundwater in parts of the Bengal basin in India and Bangladesh: the role of Quaternary stratigraphy and Holocene sea-level fluctuation. Environ. Geol. 39, 1127-1137.

Adams, S., Titus, R., Pietersen, K., Tredoux, G., Harris, C., 2001. Hydrochemical characteristics of aquifers near Southerland in the Western Karoo, South Africa. J. Hydrol. 241, 91-103.

Aelion, C.M., Conte, B.C., 2004. Susceptibility of residential wells to VOC and nitrate contamination. Environ. Sci. Technol. 38, 1648-1653.