利用地球化學參數瞭解有勝溪斷流河段之水源及其交互作用

Abstract

在氣候變遷的影響下，有許多常流河開始轉變為間歇式流動，然而斷流河段中伏流水層的流動情形仍缺乏研究。有勝溪近年來頻繁發生斷流雖嚴重影響國寶魚櫻花鉤吻鮭之生存，卻提供良好的研究場域。本研究於有勝溪斷流河段河道中的7口觀測井，分別在2017年及2022年，採集月尺度的河水及井水資料，量測其pH、導電度、飽和溶氧百分比、氫氧同位素及各類地球化學參數，進而透過端源混合分析法，企圖瞭解斷流河段中地表水及地下水之水源及其交互作用的情形。研究結果顯示，斷流河段的水源主要來自上游的河水及淺層地下水，在乾年（指2022年）乾季（指10到2月）時，上游河水以伏流水的形式在斷流河段的河床中往下游流動，但無偵測到淺層地下水之訊號；乾年溼季（指3到7月）時，則同時有伏流水及淺層地下水之訊號；溼年（指2017年）不論乾季（指1到5月）或溼季（指6到11月），伏流水與淺層地下水則混合而主導了整條斷流河段的水質。本研究除了清楚地描述了有勝溪斷流河段的水文歷程，也可做為國寶魚棲地復育之科學基礎。Under the impacts of climate change, many perennial rivers are transitioning into intermittent flow, and the flow dynamics of hyporheic zone in the fragmented river reaches remain poorly studied. Despite the frequent flow fragmentation in the Yousheng Creek, influencing the survival of the Formosan landlocked salmons (Oncorhynchus masou formosanus), it provides an excellent research field. In this study, we collected monthly water samples from seven observation wells located along the fragmented reach of the Yousheng River in 2017 and 2022. Variables such as pH, conductivity, dissolved oxygen saturation, hydrogen and oxygen isotopes, and various geochemical concentrations were measured for both river water and well water. By using end-member mixing analysis, we aimed to understand the sources of surface water and groundwater in the fragmented reach and their interactions. The research findings indicate that the water in the fragmented reach primarily originate from the upstream river water and shallow groundwater. During the dry season (from October to February) of the dry year (referring to 2022), the upstream river water flows as hyporheic flow within the riverbed, while shallow groundwater was not detected. In the wet years (referring to 2017) both dry season (from January to May) and wet season (from June to November), hyporheic flow and shallow groundwater mixed and dominated the water quality of the entire fragmented reach of the Yousheng Creek. This study not only provides a clear description of the hydrological processes in the fragmented reach of the Yousheng Creek but also serves as a scientific basis for the restoration of the Formosan landlocked salmon habitat.