Using geochemical signatures to identify subsurface flow pathways in the intermittent reach of the Yusheng Creek in Taiwan

Abstract

Study region: Yusheng Creek is a mountain stream in central Taiwan where the critically endangered Formosan landlocked salmon (Oncorhynchus masou formosanus) has been reintroduced but remains vulnerable to intermittent streamflow. Study focus: Subsurface flow pathways and groundwater-stream interactions were investigated using end-member mixing analysis (EMMA). Monthly water samples were collected from four observation wells, stream water, and tributaries during two monitoring periods (January-November 2017; October 2021-August 2022). Geochemical parameters and stable isotopes (delta 18O and delta 2H) were analyzed to trace water sources. New hydrological insights: Results reveal contrasting hydrological connectivity between dry and wet years. During dry years, the hyporheic zone persisted within the streambed but remained disconnected from groundwater, while upstream stream water infiltrated and migrated downstream through subsurface pathways. In wet years, the hyporheic zone and groundwater formed a hydraulically connected system, as indicated by similar geochemical signatures. These results highlight the critical role of the hyporheic zone in maintaining longitudinal connectivity in intermittent mountain streams, providing new insights into subsurface flow dynamics in humid subtropical regions.