濁水溪與屏東平原地下水之硝酸鹽污染

Abstract

沖積扇地層是台灣重要的地下水資源區，其中濁水溪沖積扇及屏東平原沖積扇群，估計 地下水占全台抽用量之80 %。飲水中含硝態氮所造成的危害，於高濃度（>10 mg/l）時可能 造成孕婦流產及新生兒藍嬰症（缺氧致死）；中濃度（1-10 mg/l）時可能會致癌。監測數據顯 示從1991年開始，濁水溪沖積扇扇頂的地下水含有中高濃度的硝態氮，因為扇頂區礫石層透 水性佳且缺乏泥層隔絕，易受農業化肥與生活污水滲入的影響。本研究整理1997－2007年於 濁水溪及屏東平原地下水區的監測數據顯示：濁水溪地區約10 %觀測井的硝態氮濃度偏高， 多位於扇頂補注區，曾超過環保署飲用水限值10 mg/l的井有：東和、坪頂、社寮、古坑、二 水、新光、田中等7口，其中又以新光及古坑井超過限值的次數最多。屏東平原約有32 %的井 硝態氮濃度偏高，其井數較多的原因為屏東平原扇頂礫石層的分布比濁水溪地區更廣。監測 十年間有四口井的濃度曾超過限值：萬巒一、建興一、泰山一及新庄一，又以建興一較嚴重。 本研究也發現：降雨及河川水的補注，因稀釋而使硝酸鹽濃度降低，但在乾季時、因土壤水 份蒸發，造成淺層地下水的硝酸鹽濃度升高。地下水往中下游流動，硝酸鹽因脫硝作用，濃 度降低至檢測下限以下。淺層含水層受硝態氮污染已是全世界所面對的嚴肅課題，台灣亦然， 並且台灣特殊的沖積扇含水層、其扇頂部份缺乏泥層保護，加上人口及農業活動密集，化肥 或生活污水可能進入含水層。目前許多沖積扇地區，其地下水仍是自來水水源之一，當局應 重視此區域性的污染。後續研究應擴大監測的樣本數，例如監測自來水水井及居民自有水井， 以瞭解居民飲用水的硝態氮濃度。 Alluvial fans are the most important aquifers for its abundant reserve of groundwater resource in Taiwan. Groundwater extractions in alluvial fans of Choshui and Pingtung are over 80 % of total yield in Taiwan. Nitrate levels in Choshui alluvial fan aquifers were higher from 1991 because of agricultural N fertilizer application and untreated sewage leakage. High concentration of nitrate (nitrate-N>10mg/l) in drinking water is known to cause methemoglobinemia (blue baby syndrome) in infants while low concentration of nitrate (nitrate-N 1-10mg/l) is a probable carcinogen. In our monitoring period of 1997-2007, about 10 % of wells in Choshui alluvial fan have elevated nitrate level and a higher value of 32 % were detected in Pingtung alluvial fans because of where with a broader fan head recharge area. There are 7 wells in Choshui with nitrate-N concentrations that once exceed the standard for drinking water of 10 mg/l and 4 wells exceeding the standard in Pingtung. Nitrate concentrations in groundwater show seasonal variation that are lower concentrations in rainy season for the dilution of rainfall and stream water and the nitrate concentrations are higher in dry season with the soil water was evaporated. Denitrification is another important process to control the nitrate concentration under the detection limit in groundwaters of middle-lower fan. Nitrate is one of the world’s most common pollutants in groundwater system. During the last 50 years, human activities have dramatically increased inputs of nitrogen to groundwater. We suggest that new comprehensive monitoring program should be implemented to monitor these localized areas on site where nitrate have been detected in groundwater above regulatory guidelines in public-supply wells, domestic wells of local communities and monitoring wells.