RADON AS A TRACER OF GROUNDWATER -- SURFACE WATER INTERACTION IN MARTIS VALLEY A University Thesis Presented to the Faculty of California State University, East Bay In Partial Fulfillment of the Requirements for the Degree Master of Science in Geology By Elizabeth Ann DeRubeis December, 2013 ABSTRACT During the dry months of the water year, groundwater influx is essential to perennial streams for maintaining flow and regulating water temperature. Decreased groundwater influx can cause perennial streams to become intermittent, deleteriously affecting the ecosystem and animal populations that live in the stream. Martis Creek is a perennial, sub-alpine stream located in the Martis Valley Groundwater Basin, approximately 16.1 km north of Lake Tahoe in the Sierra Nevada Mountains. On average, snow accounts for approximately 77% of the annual precipitation in the Truckee region, and groundwater recharge is currently supplied mainly by the slow melt of snowpack in the mountains, which infiltrates into late spring. As climate change progresses, more precipitation will occur as rain and less as snow in the range of Martis Valley elevations, with earlier peak runoff, which is likely to affect late season baseflow in sub-alpine streams like Martis Creek. In this study, naturally occurring radon (Rn) was used as a tracer of groundwater influx to Martis Creek, as groundwater has much higher Rn activity levels than surface water. Quantification of groundwater influx from Rn activity in the stream depends on knowledge of the concentration of Rn in the influent groundwater, and, since Rn is volatile, its rate of loss from the stream. In this study, groundwater Rn is estimated based on measurements of Rn activity in nearby, deep wells and nearby springs. To determine the degassing constant, an extrinsic tracer, xenon (Xe), which has properties similar to Rn, was introduced to the stream and measured at eight downstream locations. ii Based on a survey of Rn activity along Martis Creek, groundwater influx was determined to be occurring at upstream reaches in the Lahontan Golf Club property. Radon activity levels were approximately 100 pCi/L in the upstream reaches, while downstream activity levels were near the detection limit of approximately 15 pCi/L. By comparison, wells and springs in Martis Valley had Rn activity levels of between 300 and 1300 pCi/L. Sediment from the streambed was also analyzed for Rn emanation, and this was found to contribute a negligible amount of Rn activity to the stream. From the introduced Xe tracer results, the degassing constant for Xe was determined to be 3-5 m/day, and from this, the degassing constant for Rn in the stream was calculated at 2.25- 3.75 m/day. Applying a simple model in which stream Rn activity is a balance between the main Rn source (groundwater) and sink (volatilization), influx in reaches of the upstream portion of Martis Creek were calculated to be 1-3 m3/m/day. Groundwater influx is typically difficult to quantify, and the estimate determined here for Martis Creek is useful in formulating a more accurate water budget for the basin. iii ACKNOWLEDGMENTS I would like to acknowledge Dr. Jean Moran, Richard Bibby, Dr. Mitchell Craig, Dr. Bradley Esser, and Dr. Ate Visser for their mentoring and input on this thesis. I would also like to thank Stephanie Diaz, Andrew Benson, and Timothy Becker of UC Santa Barbara for their contributions during the mid-August 2012 stream survey, and Aaron Martin for his help during the March and April 2013 stream surveys. I would also like to acknowledge Lahontan Golf Club, Northstar CSD, and Truckee Donner PUD for their cooperation throughout this study. Finally, I would like to thank Jon Avery, Mary Ann Parins, Barbara Allcox, and Michael Libby for their support and patience. iv RADON AS A TRACER OF GROUNDWATER-- SURFACE WATER INTERACTION IN MARTIS VALLEY By Elizabeth Ann DeRubeis Approved: Date: Dr. Jean Moran Dr. Mitchell Craig Dr. Bradley Esser v TABLE OF CONTENTS ABSTRACT ....................................................................................................................... ii ACKNOWLEDGMENTS ............................................................................................... iv LIST OF FIGURES ....................................................................................................... viii LIST OF TABLES .............................................................................................................x INTRODUCTION..............................................................................................................1 BACKGROUND ................................................................................................................5 Geologic Setting ..............................................................................................................5 Hydrogeology ..................................................................................................................9 Stream Morphology ......................................................................................................15 Climate ..........................................................................................................................16 Land Cover and Canopy ............................................................................................17 ANALYSIS ......................................................................................................................21 Xenon: Introduced Gas Tracer to Constrain Degassing ..........................................24 Sediment Samples ........................................................................................................27 FIELD AND LABORATORY PROCEDURES ..........................................................32 Radon in Groundwater ................................................................................................32 Radon in Surface Water ...............................................................................................33 Radon From Sediment Samples ..................................................................................35 Xenon Tracer .................................................................................................................37 Stream Flow ...................................................................................................................40 RADON RESULTS IN WATER SAMPLES ................................................................40 Groundwater Data ......................................................................................................40 Surface Water Data .....................................................................................................48 Hyporheic Zone Data ..................................................................................................60 vi Xenon Tracer ...............................................................................................................62 Influx ............................................................................................................................66 CONCLUSION ...............................................................................................................71 REFERENCES ................................................................................................................73 vii LIST OF FIGURES 1. 238Uranium Decay Sequence .................................................................................2 2. Map of Nevada and Placer Counties ...................................................................6 3. Geologic Map .........................................................................................................8 4. Martis Valley Stratigraphy ...................................................................................9 5. Average Discharge for Martis Creek .................................................................13 6. Gaining and Losing Streams ...............................................................................14 7. Martis Creek Characteristics .............................................................................16 8. Land Cover in Martis Valley ..............................................................................18 9. Canopy in Martis Valley .....................................................................................19 10. Impervious Surfaces in Martis Valley................................................................20 11. Xenon Percentage vs. Distance Downstream ....................................................25 12. Hyporheic Zone ....................................................................................................28 13. Radon Inputs and Outputs ..................................................................................31 14. Well Sampling .....................................................................................................32 15. Surface Water Analysis ......................................................................................34 16. Sediment Analysis ...............................................................................................37 viii 17. Xenon Tracer Introduction Point ......................................................................39 18. Noble Gas Membrane Inlet Mass Spectrometer ..............................................39 19. Radon Activity in Wells ......................................................................................44 20. December 2011 Radon