Quantifying Discharge to a Subalpine Stream Using Physical and Geochemical Methods

Abstract:

The goals of this study were to identify locations or reaches where groundwater enters streams and to quantify groundwater influx along two subalpine streams in the Tahoe Basin of the Sierra Nevada, California. Radon, a naturally-occurring, dissolved gas isotope found in surface water only in proximity to groundwater inputs, was measured along a 3 km reach of Squaw Creek and a 1 km reach of Martis Creek. A mass balance model of stream radon activity that considered only groundwater discharge as a radon source, and gas emanation as a radon sink was used to fit observed radon stream activities by varying groundwater discharge along the length of the streams. To quantify the gas emanation rate, we continuosly introduced a xenon tracer at a single station via submerged gas permeable silicon tubing and measured its concentration at 8 downstream locations. In Martis Creek,we used the smoothly decreasing Xe tracer profile to determine the gas transfer velocity for this 1-km reach. Radon also decreases, but at a rate less than predicted from gass loss. Radon mass balance requires groundwater influx of 3 m³/m/d upstream to 2 m³/m/d downstream along this reach. These estimates were in good agreement with observed increases in discharge measured using a hand-held flow meter.