以參考狀態推估淡水河系營養鹽背景濃度與建立營養鹽基準

Abstract

臺灣現行之河川營養鹽濃度分級標準包含了地面水體分類及水質標準、臺灣河川污染指標，然此二分級為全臺各河系一體適用，未根據各河系不同的自然條件而有所調整；由於各河系的營養鹽背景濃度受地形、地質等因素影響，可能出現背景濃度高於甲類水體水質標準或未(稍)受污染等級濃度的情形，易造成政府機關管理上之困擾。故本研究以淡水河系為研究區，推估淡水河的營養鹽背景濃度，並與國內現行標準及國外分區營養鹽背景濃度比較。 根據淡水河系的自然環境，本研究選擇參考測站第75百分位數法與所有測站第5至第25百分位數法建立淡水河營養鹽基準；其次選用人為干擾模式，以各測站所轄子集水區內之人為干擾因素為自變數、測站營養鹽濃度為應變數進行複迴歸分析，用來推估淡水河營養鹽背景濃度。其中，臺北水源特定區管理局的測站被假設為參考測站，其監測數據用來計算參考測站第75百分位數；所有測站第5至第25百分位數法與人為干擾模式則使用臺北水源特定區管理局與環保署共61個測站之資料。計算淡水河營養鹽背景濃度與建立營養鹽基準之前，本研究針對淡水河的61個測站進行變異數分析，討論淡水河的營養鹽空間變異情形，並分析各測站間營養鹽濃度資料是否具有顯著差異，藉此檢驗水質監測站之代表性。 根據本研究計算結果，淡水河營養鹽背景濃度為：總磷0.027 mg/L、氨氮0.029 mg/L，經由比較可發現淡水河總磷背景濃度高於現行甲類水體水質標準(0.020 mg/L)，然因該標準仍落於總磷濃度基準候選區間內(0.019-0.036 mg/L)，故仍屬可接受之標準；氨氮背景濃度則遠低於甲類水體水質標準(0.1 mg/L)及臺灣河川污染指標未(稍)受污染等級(0.5 mg/L)。主管機關可參考本研究的營養鹽背景濃度估計結果，調整淡水河的營養鹽管理政策。

Uniform regulative or classified standards for river nutrient concentrations, including “surface water classification and water quality standards of Taiwan EPA” as well as “Taiwan river pollution index” (RPI), are adopted for every river basin in Taiwan. However, these nutrient standards may not consider whether different basins need different nutrient standards because of distinct environmental attributes. This may lead to that the background nutrient concentraitions are lower than the concentrations of current standards. Therefore, in this study, the reference conditions of river nutrients will be established to provide authorities to determine the proper criteria or standards for protecting the water quality of the upstream Tamsui River. Meanwhile, the candidate criteria derived from this study will be compared to current Taiwan standards. The USEPA proposed two statistical methods for developing nutrient criteria to States and Tribes. The first method identified the criterion as the 75th percentile of the frequency distribution of nutrient data from reference sites within a region. The second method used the 5th to 25th percentile of the frequency distribution from the general population of nutrient data. To evaluate the applicability in Taiwan of each method, data was assembled from the Taipei Water Management Office (TWMO) and the Taiwan EPA to create a nutrient general population. The data from TWMO in the Taipei Water Source Domain were provided as reference population. Data of the Tamsui River were extracted from the general population and compared to those of the reference sites according to each nutrient component. Furthermore, the land use of subwatersheds controlled by corresponding monitoring sites was examined by multiple linear regressions to verify the disturbance by human activities. Before the background concencentrations of nutrients are predicted, the analysis of variance (ANOVA) was conducted to find out the spatial variations of the nutrient concentrations in the Tamsui River Basin. The results of ANOVA would show whether the nutrient concentrations among 61 sites are significant different. The results of multiple linear regressions indicate the predicted background concentrations of TP and NH3-N in the Tamsui River are 0.027 and 0.029 mg/L. The background TP concentration is higher than the Category A standard of Taiwan EPA, but the Category A standard is still acceptable for the Tamsui River because it is in the interval of candidate criteria. In addition, the background NH3-N concentration is far lower than two current standards. Based on the results of this study, the authorities can adjust the policy of controlling nutrients for the Tamsui River.