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Water 2018, 10(2), 100; https://doi.org/10.3390/w10020100

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

1
Department of Earth and Environmental Sciences, California State University East Bay, Hayward, CA 94542, USA
2
Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA
Current affiliation: Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506, USA
*
Author to whom correspondence should be addressed.
Received: 4 November 2017 / Revised: 21 January 2018 / Accepted: 22 January 2018 / Published: 25 January 2018
(This article belongs to the Special Issue Isotopes in Hydrology and Hydrogeology)
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Abstract

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 (222Rn) was used as a tracer of groundwater influx to Martis Creek, a subalpine stream near Lake Tahoe, CA. Groundwater 222Rn is estimated based on measurements of 222Rn 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 222Rn 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 222Rn activity is a balance between the main 222Rn source (groundwater) and sink (volatilization), the influx in reaches of the upstream portion of Martis Creek was calculated to be <1 to 15 m3/day/m, which cumulatively constitutes a significant portion of the stream discharge. Experiments constraining 222Rn 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. View Full-Text
Keywords: tracer; radioactive isotopes; groundwater influx tracer; radioactive isotopes; groundwater influx
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Avery, E.; Bibby, R.; Visser, A.; Esser, B.; Moran, J. Quantification of Groundwater Discharge in a Subalpine Stream Using Radon-222. Water 2018, 10, 100.

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