Hydrogeological Study and Tracing of Groundwater Flow Paths in the Beinan River Basin within Eastern Taiwan's Alpine Catchment

The growing demand for water resources is exacerbated by the impacts of climate variations, uneven rainfall distribution, and population growth. In the global inventory of water resources, rivers in mountainous regions contribute significantly to the available water supply. The subsurface aquifer system in these mountainous watersheds plays a crucial role in the hydrological cycle, serving not only as a primary source for downstream rivers or aquifers but also as a vital replenishment source during periods of drought. Due to its remote location and limited manpower, a comprehensive understanding of the hydrological functions of groundwater within the mountain system remains a challenge. Accordingly, this study selected the alpine watershed of Beinan River in eastern Taiwan, characterized by minimal human activities, to delineate groundwater flow paths and evaluate potential contribution of groundwater to water resources to address the existing gaps in understanding. Through long-term streamflow and groundwater level analyses combined with hydraulic tests and tracer experiments, the objective of this study focuses on delineating the subsurface flow paths from weathered soils and regolith to fractured bedrock and characterizing their associated hydraulic properties in this alpine hydrogeological setting. The results show that the main contributor to streamflow is shallow groundwater, particularly during the dry season. Rainfall infiltration is primarily observed in the weathered soils and regolith manifesting as the mountain front recharge (MFR). The groundwater flow in the bedrock is predominated influence by the fractures and its sources can be traced back to distant hillslopes. The water budget within the entire alpine system is preliminary quantified based on the long-term data and hydraulic parameters obtained from the field tests. The results obtained in this study can provide as a reference for developing conceptual models and fundamental frameworks for quantifying the water budget in alpine environments.

Keywords: alpine hydrogeology, groundwater flow path, fractured flow, water budget, Taiwan