鹿角坑溪與馬槽溪匯流後的離子濃度調節作用

Abstract

酸化對生態系及人類社會帶來許多負面影響，是環境變遷的研究的重要議題之一。人為活動引起的酸化受到廣泛的研究與探討，並逐漸受到控制，然而對生態系統同樣有重要影響的天然的酸化卻較少受到關注。陽明山國家公園的馬槽溪深受火山地質影響而含有高濃度的各種離子，水質高度酸化，但與鹿角坑溪匯流後有明顯改善的現象。前人研究指出，稀釋與沉澱作用是降低離子濃度的主要調節機制，這兩種調節機制在水資源管理上的意涵有很大差異，但少有研究觸及。本研究藉由溪、雨水的成份分析與流量模擬來估算兩溪與匯流後的離子輸出量，利用流量差異推估稀釋作用的效果、輸出量的差異推算各種離子的沉澱量，推導沉澱作用的效果，並探討雨量對兩作用的影響。結果顯示稀釋作用對降低離子濃度的效果為63.7 ± 1.1%，沉澱作用的效果則隨離子種類而不同。雨量是影響兩種作用效果的主要因子，能有效增進兩作用的效果並降低離子濃度、改善水質。大豪雨雖會降低離子濃度但亦會大幅增加離子輸出量及混濁程度，對水質改善並無持續性的作用。乾旱不僅減弱稀釋的效果，也使溪流水更酸而增加鋁的溶解度。氣候變遷的影響使降雨的集中度有增加的趨勢，可能會增加豪雨及乾旱事件的發生而使兩溪匯流後的調節功能降低。Environmental acidification is a critical issue in global change due to its negative effects on social-ecologiacal systems. Anthropogenic acidification has been extensively studied and gradually under control in many parts of the world. However, natural acidification has received much less attention although it also has major effects on ecosystems. The Macao Stream in the volcanic Yangmingshan National Park has high concentration of many metal ions due to volcanic influences. After merging with the Lujiaokeng Stream, the metal ion concentrations are likely regulated through precipitation and dilution. We investigated the relatively contributions of the two processes on reducing ion concentrations by estimating ion flux and streamflow in the two streams and their confluence. The results indicate that dilution accounts for 63.7 ± 1.1% of the reduction of ion concentrations while the effect of precipitation varies among ions. Although rainfall can improve the effects of dilution and precipitation, extreme rainfall events do not contribute to improve water because the flood can lead to high turbidity in the stream. Droughts not only reduced the dilution effect but also limited precipitation because stream water during low flow periods were more acidic, which increases solubility of metal ions. The projected increases in the frequency of torrential rain and drought events will have negative effects on the regulation of ion concentrations in the acidic stream water.