砷污染地區農田土壤與稻作砷含量關係之研究

Abstract

北投關渡平原農地發現有128公頃之表、裡土砷濃度超過現行土壤管制標準，範圍集中在大度路以北、大業路以西和新貴子坑溪以東，係肇因於地熱谷含砷之溫泉水隨灌溉系統進入田間、擴散並滯留於農地。前人調查掌握土壤水平污染情勢與源頭成因，但砷在土壤與作物間的遞移狀況未明，又無機砷已證實為人類致癌物。為此，本研究將針對關渡平原污染區稻田土壤與植體進行砷濃度檢測，藉以評估土壤中砷是否會轉移至水稻，進而透過食物鏈進入人體。 本研究以坪割採樣法採集臺北市北投關渡平原內20塊農地坵塊對應之表土土壤（0－15 cm）與96年度稻作樣本，分別以攜帶式X－射線螢光光譜儀（FP－XRF）與氫化產生器串聯原子吸收光譜儀系統（HG－AAS）檢測農田土壤與水稻植體各部位之總砷濃度。檢測結果為：表土153.2±149.6 mg/kg，稻根203.7±138.0 mg/kg，稻稈4.21±2.01 mg/kg，糙米0.231±0.075 mg/kg（以上數值皆為乾基）。同年2期作稻作砷含量（乾重基準）則為：稻根191.6±160.1 mg/kg，稻稈2.94±1.60 mg/kg，糙米0.045±0.013 mg/kg。 研究發現表土砷含量僅與稻根、稻稈砷含量有統計上之相關性，此外，稻根與稻稈砷含量亦有相關，皆可以線性回歸式表示之。然而，表土與糙米砷含量則無相關。又1、2期作因田間管理與氣候因素，產量與實際生長天數有異，且2期作糙米與稻稈砷濃度皆較1期作為低，稻根則並不隨在田間生長時間增加而增加砷濃度。以國人典型飲食型態進行攝食關渡糙米之砷膳食暴露評估，結果顯示並未超過世界衛生組織（WHO）所建議之最大容許攝食量。

Over 100 hectares paddy soils were heavily polluted by arsenic (As) pollution at Guan-Du Plain, Beitou, resulting from a long-term irrigated water with high arsenic content mixed with hot spring water from Thermal Valley in the past hundred years. Since arsenic had already exist in soils used for rice (Oryza sativa) cultivation, in this research, a survey of arsenic contents of rice and paddy soils has been conducted. The aim of this study is to figure out the effect caused by arsenic-rich soils, such as the accumulation and distribution of arsenic in soil-plant systems, moreover, the probable dietary arsenic exposure through food chains, that is, the arsenic exposure via the soil-crop-human pathway. The top 15 cm soil and rice samples from 20 arsenic-tainted fields within Guan-Du Plain were collected in July and November, 2007. Paired rice and soil samples were surveyed just before harvest, with four samples set per field sampled. The total arsenic concentrations in soil and different parts of rice that is grains, straws, and roots averaged 153.2±149.6, 203.7±138.0, 4.21±2.01, 0.231±0.075 mg/kg dry weight, respectively. Meanwhile, the mean arsenic contents in ratoon rice were 191.6±160.1 mg/kg in roots, 2.94±1.60 mg/kg in straws (leaves and stems were included), and 0.045±0.013 mg/kg in brown rice. All of the above were determined by field portable X-ray fluorescence (FP-XRF) or hydride generator connected with atomic absorption spectrometer system (HG-AAS). Despite the accumulation of arsenic in rice plants attributable to growth on soils contaminated with elevated arsenic levels, there is no significantly statistical relationship between the concentration of arsenic in soils and that in brown rice. However, correlations of soil arsenic levels with roots and straws are statistically significant, while the same in roots and straws, and what’s more, regression of soil arsenic levels with roots is statistically significant. Although the ratoon rice this year contains lower level arsenic due to loose field practice and climate, the arsenic contents in roots from different growing seasons are not significantly different. Finally, based on the typical consumption of Taiwanese people, the daily intake of arsenic from rice has been demonstrated to assess the safety of dietary intake. The result of this demonstration proves to be within and even below the provisional tolerable daily intakes recommended by WHO.