Using multiple environmental tracers to investigate the relative role of soil and deep groundwater in stream water generation for a snow-dominated headwater catchment

Author

Isabellah V. von Trapp, University of Montana, MissoulaFollow

Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Payton Gardner

Commitee Members

Payton Gardner, Kelsey Jencso, Nancy Hinman

Keywords

groundwater, hillslope hydrology, watershed hydrology, stream water, environmental tracers

Subject Categories

Geochemistry | Hydrology

Abstract

In this study, seasonal fluctuation of environmental tracers in stream flow, soil water, and deep bedrock groundwater were used to constrain the role of deep bedrock groundwater in streamflow generation for a mountainous headwater catchment. Synoptic measurements of stream discharge, ²²²Rn, specific conductivity and major ion concentrations were measured throughout the water year over a 5 km reach of Cap Wallace Creek in the Lubrecht Experimental Forest, Montana, U.S.A. with the intention of understanding groundwater – surface water interactions across spatial and temporal scales. Stage measurements were continually recorded at seven stilling well locations along the reach. Discharge measurements and water samples were taken at these sites throughout the winter, spring, summer, and fall of 2017. Shallow soil and groundwater water level and environmental tracer concentrations from contributing hillslopes were also measured. Dissolved ²²²Rn was used to calculate total subsurface discharge. Multi-component mixing models of major ion chemistry and stream discharge were used to constrain end-member discharge to the stream. Mixing model results were compared to landscape characteristics to identify internal catchment controls on the heterogeneity and duration of subsurface discharge. ²²²Rn modeling suggests that streamflow is dominantly generated by subsurface discharge. End-member mixing analysis indicates that streamflow was partitioned between soil water and groundwater end-members. On average, groundwater comprised 38% of streamflow at the outlet but fluctuated between 26% in the spring and 44% in the early summer. Spatial analyses showed elevation and upslope accumulated area (UAA) to be first-order controls on end-member discharge. Groundwater became a more important component to streamflow at higher catchment scales with lower elevations and higher UAA values, suggesting topography-driven flow. Correlations among landscape and end-member discharge were strong across variable states of catchment ‘wetness,’ indicating that accumulated elevation and catchment area are robust predictors of groundwater discharge across the landscape (r² = 0.52-0.98). These results have implications for understanding the processes controlling seasonal watershed streamflow response to snowmelt and for predicting headwater response to changing climatic conditions.

Recommended Citation

von Trapp, Isabellah V., "Using multiple environmental tracers to investigate the relative role of soil and deep groundwater in stream water generation for a snow-dominated headwater catchment" (2018). Graduate Student Theses, Dissertations, & Professional Papers. 11227.
https://scholarworks.umt.edu/etd/11227