關渡平原土壤砷、鉛污染之空間分佈及成因探討

Abstract

民國九十五年台北市環保局委託臺大生態工程研究中心，針對北投地區關渡平原842公頃農地進行土壤砷含量調查。調查發現關渡平原表、裡土砷濃度大於土壤管制標準值(60 mg kg-1)的面積有128公頃，並集中在大度路以北、大業路以西和新貴子坑溪以東的範圍。其污染成因為早期七星水利會於薇閣國小附近取磺港溪水，並以磺水上圳與磺水下圳引灌至關渡平原北投小組，水中之砷、鉛隨灌溉溝渠漫田擴散於農地中，經百餘年累積造成如今之污染現象。由前述調查掌握了土壤水平污染的範圍，但對污染的深度及濃度分佈成因卻仍未明。故本研究目的係使用X-射線螢光光譜儀測量土壤垂直剖面砷、鉛濃度，了解其濃度分佈，進行初步探討；並利用地理資訊系統整合當地人文歷史與現地土壤污染情況，瞭解關渡平原砷、鉛水平空間分佈之成因。 本研究針對北投小組農田進行調查，沿灌排方向水平之濃度分佈呈指數降低與其它場址相似，而垂直剖面採樣，發現某些樣點砷、鉛濃度在100 cm處出現一異常之峰值現象，與前人文獻中灌溉水所造成土壤砷污染濃度最大值多出現在表層(45 cm以上)之結果不同。本研究參考前人對砷、鉛分配係數的研究，以一維對流—擴延散方程式之解析解與質量守衡的觀念，發現鉛峰值區較不可能為灌溉活動所造成，而在引灌水源其砷、鉛濃度比不變的假設下，100 cm處之砷也並非全由灌溉水帶入。本研究也發現靠近磺水下圳入水口附近之表土砷鉛濃度比值與磺港溪水之比值差異甚大，推論為附近工廠之額外鉛來源干擾所造成。

Over 100 hectare paddy soils found heavily polluted by arsenic at Beitou, Taipei, in 2006 was reported from the Ecological Engineering Research Center of NTU through a project sponsored by the Bureau of Environmental Protection, Taipei City Government. The cause has been identified as long-term irrigated with high As content water which mixed with hot spring water from thermal valley brought the metals into the paddy soils. The result also indicate that there are 128 hectares farmland which surface soil and subsurface soil exceeding the soil pollution control standard of arsenic (60 mg kg-1) with the mean concentration around 110 mg kg-1. Although the boundary of the contaminated site has been delineated, the process of transport heavy metals in horizontal and vertical through soil profiles is unclear. The peak concentration was found at about 100 cm depth in some samples. It is quit different from other studies which the maximum concentration shown up within 45 cm depth. This study applies the analytical solution of 1-D convection-dispersion equation (CDE) to simulate the As and Pb transport in soil profiles in order to check whether the irrigation can cause these phenomenon. The result is negative which means high concentration around 100cm is not from irrigation process. Mass balance on As and Pb also indicate the peak concentration of arsenic and lead in soils are not all from irrigation water.