集水區上游南投地區地下水硝酸鹽氮污染潛勢評估

Abstract

集水區上游地下水水質之良窳，直接影響中下游地下水使用之安全，近年來濁水溪沖積 扇上游(南投區域)地下水因農業盛行導致硝酸鹽氮污染趨於嚴重，逐漸影響中、下地下水之 利用，本研究以多變量統計方法之因子分析，評估本區地下水水質結構、特徵及其在空間及 時間上之分佈與變化，並藉由現地水質調查及土地使用情況，研判潛在硝酸鹽氮污染源及可 能之地下水污染途徑。然後結合 DRASTIC 模式，評估與劃定本地區地下水易受污染區，最 後提出永續經營本區地下水使用方案。八項水質包括酸酸值、總硬度、電導度、氨氮、硝酸 鹽氮、鐵、錳及硫化物為因子分析之數據，本區域 2004~2006 年地下水水質因子分析研究果 穩定。鐵錳及硬度因子為台灣地下水水質特徵之ㄧ，皆為地層中溶出，而硝酸鹽氮則為地表 人為活動而導致；南投市之地下水水質呈氧化、酸性及高硝酸鹽氮之水質特徵，雲林斗六之 地下水水質呈還原、中鹼性及低硝酸鹽氮之特徵；DRASTIC 模式評估本區之硝酸鹽氮地下水 之污染潛勢結果，污染潛勢得分在 160 分以上之區域劃定為易受污染區，其中呈現高硝酸鹽 氮汙染潛勢包括埔里鎮、草屯鎮西半部、名間鄉西南側、竹山鎮北側及南投市內零星區域， 皆為南投縣之人口集中或農業盛行之區域。南投縣境內之硝酸鹽氮污染已超過 10 年以上且為 多重污染源，短期內雖未影響濁水溪沖積扇及台中盆地中游地下水區，但本區之地下水水質 條件已不適民生用水之需。鄉鎮市應積極推動地下水污染防治，人口密集之鄉鎮市應加速興 建衛生下水道，農業發達之鄉鎮應管理並輔導農戶減少使用農藥及化肥，以期達成地下水資 源保育與永續利用之目標。 Public concern over groundwater quality has grown significantly in recent years. Agriculture is considered as a major contaminant source of groundwater. Increasing evidence shows that the nitrate contamination exceeds the maximum contaminant level of groundwater in the upper Choushui river alluvial fan of Nantou. The objectives of the study were to apply factor analysis to identify factors that govern the chemistry of groundwater, and to determine the source of nitrate pollutant on its pathway using the groundwater quality data and land use type in this area. These results were integrated using DRASTIC to delineate the groundwater water vulnerability area and formulate the strategy for sustainable use of groundwater resources. Eight hydrochemical parameters including pH, EC, Hardness, NH4+, NO3-, Fe, Mn and S concentrations were considered to evaluate the characteristics of the groundwater by factor analysis. Three main hydrogeochemical factors were determined. Factor 1 has strong positive loadings on Fe, Mn and NH4+ which is a common type of the groundwater hydrochemical environment of Taiwan. Factor 2 has strong loadings on Hardness and EC representing a hardness water property. Factor 3 has strong positive loadings on nitrate and pH which is polluted probably from agriculture, industry and domestic sewage. Redox is the most plausible process determining the groundwater characteristics in this area where the groundwater has oxidized, acidic and high nitrate properties in Nan-Tou city and exhibits reduced, alkaline and low nitrate properties in Dou-Lio. Factor analysis shows the chemistry of groundwater is stable based on the data of 2004-2006. Contamination potential of nitrates is evaluated by DRASTIC. The score exceeds 160 define the scopes of groundwater vulnerable to pollution. The high contamination potential areas of nitrate are Pu-Li, the west of Tsau-Tuen, the southwest of Ming-Jian, the east of Ju-Bei and Nan-Tou city. These areas were agricultural active and densely populated. The nitrate pollution has no immediate threat to the downstream area of Choushui river alluvial fan. However, the present groundwater quality is not suitable for potable water use. In order to achieve sustainable use of groundwater resource, we should plan to remediate the nitrate contamination, construct sewer systems, reduce the use of agrichemicals and protect the areas of high contamination potential.