Quantification of Groundwater Discharge in a Subalpine Stream Using Radon-222

During the dry months of the water year in Mediterranean climates, groundwater influx is essential to perennial streams for sustaining ecosystem health and regulating water temperature. Predicted earlier peak flow due to climate change may result in decreased baseflow and the transformation of perennial streams to intermittent streams. In this study, naturally occurring radon-222 (Rn-222) was used as a tracer of groundwater influx to Martis Creek, a subalpine stream near Lake Tahoe, CA. Groundwater Rn-222 is estimated based on measurements of Rn-222 activity in nearby deep wells and springs. To determine the degassing constant (needed for quantification of water and gas flux), an extrinsic tracer, xenon (Xe), was introduced to the stream and monitored at eight downstream locations. The degassing constant for Rn-222 is based on the degassing constant for Xe, and was determined to be 1.9-9.0 m/day. Applying a simple model in which stream Rn-222 activity is a balance between the main Rn-222 source (groundwater) and sink (volatilization), the influx in reaches of the upstream portion of Martis Creek was calculated to be <1 to 15 m(3)/day/m, which cumulatively constitutes a significant portion of the stream discharge. Experiments constraining Rn-222 emanation from hyporheic zone sediments suggest that this should be considered a maximum rate of influx. Groundwater influx is typically difficult to identify and quantify, and the method employed here is useful for identifying locations for focused stream flow measurements, for formulating a water budget, and for quantifying streamwater-groundwater interaction.