radon as a tracer of groundwater -- surface water

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

ii

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.

iii

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.

iv

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.

v

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

averyj2

Stamp

vi

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

vii

Xenon Tracer ...............................................................................................................62

Influx ............................................................................................................................66

CONCLUSION ...............................................................................................................71

REFERENCES ................................................................................................................73

viii

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

ix

17. Xenon Tracer Introduction Point ......................................................................39

18. Noble Gas Membrane Inlet Mass Spectrometer ..............................................39

19. Radon Activity in Wells ......................................................................................44

20. December 2011 Radon Activity Map ................................................................45

21. June 2012 Radon Activity Map .........................................................................46

22. October 2012 Radon Activity Map ....................................................................47

23. July 2012 Stream Survey .....................................................................................54

24. August 2012 Stream Survey ................................................................................54

25. July 2012 Radon Activity Map ..........................................................................56

26. August 2012 Radon Activity Map .....................................................................57

27. August 2012 Radon Activity Close-up ..............................................................58

28. March/April 2013 Radon Activity Map .............................................................59

29. Xenon Cross Sections ..........................................................................................64

30. Xenon Transect Along Stations .........................................................................64

31. Radon and Xenon With Distance Downstream.................................................65

32. Groundwater Influx .............................................................................................67

33. Groundwater Influx With Distance Downstream .............................................70

x

LIST OF TABLES

1. Variables Used ........................................................................................................5

2. Martis Creek Discharge ......................................................................................13

3. Similar Studies .....................................................................................................23

4. Sediment Sizes ......................................................................................................36

5. IDs and Rn Activity: Groundwater ....................................................................43

6. Rn Activity: Surface Water.................................................................................50

7. Surface Water Map IDs.......................................................................................55

8. Rn Emanation.......................................................................................................62

9. Degassing Constant ..............................................................................................65

10. Groundwater Influx .............................................................................................68

1

INTRODUCTION

Replenishment of groundwater basins is of great importance in California, due to

a generally dry climate and dependence on groundwater. The National Oceanic and

Atmospheric Agency (NOAA) has pointed out that over the next century, as air

temperature increases, there may be a higher frequency of droughts and generally

reduced water supplies (Stone & Lopez, 2009). As climate change occurs, precipitation

will begin to occur more frequently as rain rather than snow (currently approximately

77% of precipitation in the Martis Valley region occurs as snow on an average annual

basis), which could have drastic impact on stream flow and on groundwater recharge.

Snowpack in the Sierra Nevada allows for slow melting, and gradual groundwater

recharge in basins; however, as more precipitation occurs as rain, there will be limited

opportunity for groundwater recharge as there will only be some infiltration and water

that does not infiltrate will run off as overland flow. Thus, there may also be an increase

in flooding in Sierra Nevada groundwater basins (Brown & Caldwell, 2013).

Groundwater is essential to the area as it provides baseflow to Martis Creek during the

dry summer months, which keeps the stream ecosystem and the fish populations healthy.

If climate change does result in less groundwater recharge, natural discharge that ends

earlier in the summer or fall and groundwater pumping that continues into the summer

and fall may place stress on the baseflow of the stream. Groundwater discharge to the

stream also provides lower temperatures in the late summer and fall, which is essential to

the fish population in the stream. Pumping and other anthropogenic activities may cause

2

the amount of influx to unintentionally decrease, lowering the baseflow and affecting the

ecosystem of the stream.

Figure 1: 238

Uranium Decay Sequence. Radon-222 is a product of the 238

Uranium

(238

U) decay chain. 238

U is the most abundant isotope in natural uranium and is found in

most rock types, but is found in high concentrations in igneous rocks, especially granite,

which is abundant in the Sierra Nevada. Due to the presence of igneous rocks in Martis

Valley, groundwater may have a high Rn activity level.

Radon-222 (222

Rn) is radioactive with a half-life of 3.82 days and is a gaseous

daughter product in the Uranium-238 (238

U) decay chain (Figure 1). Radon is found in

groundwater due to flow through fractures and pore spaces of rock and sediment

containing uranium. In this study, naturally occurring Rn is used to trace reaches of

groundwater discharge to Martis Creek, since Rn activity is much higher in groundwater

than in surface water. Cook, Lamontagne, Berhane, and Clark (2006), Cook, Love, and

3

Dighton (1999), and Lamontagne and Cook (2006, 2007) wrote several seminal papers

concerning Rn tracers in streams, the studies of which took place in southeastern

Australia. Relatively high Rn activity levels in stream water indicated reaches of

groundwater influx, and introduced tracers were used to determine the degassing constant

in order to quantify groundwater influx to the river. Cox, Bibby, and Esser (2009)

performed a similar study nearby in the Sierra Nevada, at Squaw Creek. Determining

reaches of higher Rn activity allowed estimation of groundwater influx to the stream.

However, in that study, the degassing constant had to be estimated based on prior studies

in similar streams, such as the Cook et al. and Lamontagne and Cook studies. The

introduction of a xenon (Xe) tracer in this study allows direct quantification of the

degassing parameter. The Xe tracer is used to determine the degassing constant for Rn,

which allows for quantification of groundwater influx to the stream. In this study,

groundwater influx is determined by two completely independent methods:

geochemically (using Rn as a tracer) and physically (using measured stream discharge).

Naturally occurring Rn is used to determine influx by finding reaches of Martis

Creek with relatively high activity levels (higher than the background levels of about 20

pCi/L). Since surface water has lower Rn activity levels than groundwater, due to the

volatility of Rn, these relatively high Rn activities signal reaches of groundwater influx.

The equation

[

]

is used to calculate groundwater influx (I) based on measurements and estimates of

stream variables. While Rn activity in the surface water was measured, Rn activity in

4

groundwater is assumed to have similar activity to the wells and springs nearby. The

contribution of hyporheic exchange to Rn activity in the stream was accounted for by

sampling sediments from the stream bed and analyzing them for Rn emanation.

Parameters that were measured, calculated, and estimated are shown in Table 1.

This study provides valuable information for the region, as groundwater influx is

a factor in creating an accurate water budget, and until now, quantification of

groundwater influx has not been studied along Martis Creek. Lower order streams like

Martis Creek are usually not gauged and gaining and losing reaches are not known.

Since groundwater terms in basin water budgets have the greatest uncertainty, due to the

difficulty of determining the amount of groundwater discharge, it is important to make

this information as quantitative and accurate as possible.

5

Table 1: Variables Used. A table showing the variables which are measured, estimated,

and calculated in the equation used to determine groundwater influx to Martis Creek.

BACKGROUND

Geologic Setting

Martis Valley is located in Placer and Nevada counties, in northern California

near the Nevada border (Figure 2). It extends northward from Lake Tahoe, and is in the

transition zone between the Sierra Nevada and Basin and Range geomorphic provinces.

The Martis Valley Basin covers 148 km2 (35,600 acres), and is at an elevation between

1,737 and 1,798 m above mean sea level (California Groundwater Bulletin 118, 2006).

Symbo

l Description Method Value

k

gas transfer

velocity Measured m/day

w river width Measured m

d mean river depth Measured m

c

Rn activity in

surface water Measured pCi/L

cᵢ

Rn activity of

groundwater

inflow

Estimated from well

water measurements

- 500 pCi/L

v stream velocity Measured m/s

Q stream discharge Measured m³/day

dc/dx

change in Rn

activity per

distance

calculated from

measurements pCi/L m¯¹

I

groundwater

influx to stream

calculated from

measurements m³/m/day

6

Figure 2: Map of Nevada and Placer Counties. Map of Nevada and Placer counties,

with the locations of Martis Valley, Highway 267, and Lahontan Golf Club marked

(Okada, 2012).

The geology of the region is complex (Figures 3 and 4). Basin lithologies

comprise andesitic to basaltic flows, granite, tuffs, and breccias, and often there are

volcanics interbedded with more recent glacial and fluvial deposits. Though nearly all

rock types contain some uranium, granite (and igneous rocks in general) contains the

most, at 2.6 ppm for granite in the Sierra Nevada (Larsen & Gottfried, 1961), which adds

Rn to groundwater, since Rn is a product of the 238

U decay chain. At approximately 12

Ma, volcanic activity in the basin produced andesitic flows, tuffs, and breccias, while at

8-6 Ma, andesitic volcanism occurred around Martis Peak and Mount Lincoln, and at 5-3

Ma, andesitic to basaltic flows occurred to the north and west of Lake Tahoe. During the

Miocene and Pliocene, there were at least six instances of volcanism. At approximately 3

7

Ma, the Sierra Nevada range began to uplift, and the Tahoe and Truckee basins began to

form. When uplift and extension began in the region 3 Ma, deformation reshaped the

volcanics in the area, though there is disagreement about whether the basalt predates or

postdates the onset of extension. The northern portion of Martis Creek and the eastern

side of Martis Creek Dam run along the Dry Lake volcanic flows, which had between

four and six distinct flow events. While the volcanics in the Truckee basin often underlie

or are interbedded with the Prosser Creek alluvium, near the edge of the basin, the

volcanics lie directly on top of the basement, which is of Mesozoic age (Bedrosian,

Burton, Powers, Minsley, Phillips & Hunter, 2012).

Martis Valley is located in the Tahoe-Truckee fault zone. There are two known

faults near the section of Martis Creek where the tracer study occurred – Polaris Fault and

Martis Creek Fault. The Polaris Fault is nearly 35 km long, has dextral sense of slip

(evidenced by offsets in volcanic flows) and, from offset of Pleistocene geomorphic

features, has had an estimated 0.4 mm/year offset during the late Quaternary. Along the

East Martis Creek fan, magnetic field profiles and LiDAR (light detection and ranging)

imagery show evidence for two fault splays that cross the fan. The Martis Creek Fault is

a dip-slip fault in the vicinity of the study, approximately 2 km north of Martis Creek

Dam, but is inferred to extend beneath the dam and along the reservoir. Based on offset

of volcanic flows, there is an estimate of 30 m of movement on the fault since 1.3 Ma,

though there is no evidence that any movement has occurred in the Quaternary

(Bedrosian et al., 2012). In the Tahoe-Truckee fault zone, seismicity is diffuse, and the

last moderate magnitude earthquake in the study area was a M 6.0 earthquake that

8

occurred in 1966 (Schweickert, Lahren, Smith, Howle & Ichinose, 2004). Faults and

fractures in the rock provide secondary porosity, which allows for fracture flow and may

bring deep seated fluids to the surface, including as influx to streams.

Figure 3: Geologic Map. A geologic map of the study region (Brown & Caldwell,

2013).

9

Figure 4: Martis Valley Stratigraphy. Example of the stratigraphy of the Martis Valley

region. This only provides an example, however, as the region has a complex geology

that does not remain consistent through all areas of the Martis Valley Groundwater Basin

(Brown & Caldwell, 2013).

Hydrogeology

The Prosser Creek alluvium contains the oldest sediments mapped in the Truckee

basin, at 0.73-0.5 Ma, and consists of an upper fluvial layer, a lacustrine blue silt layer,

and a lower lacustrine layer. The blue silt layer acts as a confining layer to a deeper

10

aquifer, and the dam sits on this silt layer, with the Prosser Creek alluvium approximately

60 m thick near the dam. The Prosser Creek alluvium may have been deposited due to

basaltic flows blocking the drainage of the Truckee River, though there is some argument

by Latham (1985) that it was deposited from tectonic activity. Since deposition,

however, deformation of the alluvium has been minor. The Prosser Creek alluvium

differs from the glacial outwash in that it has higher clay content, greater consolidation,

and lower hydraulic conductivity (Bedrosian et al., 2012).

Glacial outwash, with an average depth of 20 to 30 m and varying thickness,

covers most of the valley (Bedrosian et al., 2012). There have been multiple glaciations

in the area, with the Wisconsin consisting of the Tahoe and Tioga glaciations, and the

Pre-Wisconsin consisting of the Hobart and Donner Lake glaciations (Schweickert et al.,

2004). The Donner Lake glaciation was the most widespread. The glacial outwash of the

Donner Lake glaciation was later overlain by the sediments of the Tahoe glaciation,

which filled some of the surrounding valleys between 118 and 56 ka. While both the

Donner Lake outwash and the Prosser Creek alluvium vary in material, both vertically

and laterally, the glacial outwash sediments have a higher hydraulic conductivity and less

clay than the Prosser Creek alluvium (Bedrosian et al., 2012). The hydrostratigraphic

units that are likely connected to Martis Creek in the study reach are the Prosser Creek

alluvium, glacial outwash, and volcanics (andesite). Figure 4 is an example of this

complex stratigraphy. The basement in the Truckee area does not contain or transmit

much groundwater (California Groundwater Bulletin 118, 2006).

11

A dam built by the Army Corps of Engineers is located on the south side of

Highway 267, just upstream from where Martis Creek joins the Truckee River. This dam

was built across a section of Martis Creek in 1972 to provide flood control and a water

supply, and it creates Martis Creek Lake. The dam is made out of rolled earth fill, and

seepage has been found to occur through the dam as well as through the glacial outwash

on the west side of the dam. It was originally thought that Martis Creek Dam sat on 100

m of sediments, but it has now been found that more than 2.5 km of sediments are

interbedded with the volcanic flows. The dam is built for water storage of approximately

2.5 x 107 m

3, though it has never held this much water. Generally, Martis Creek Lake is

kept at 25% storage capacity (Bedrosian et al., 2012) because of concerns about the

structural integrity of the dam.

In the lower elevations of the Martis Valley groundwater basin, the average

precipitation is 58.4 cm, while in the higher elevations it is 101.6 cm. Streams that run

through Martis Valley that are tributaries to the Truckee River include Donner Creek,

Prosser Creek, and Martis Creek, and surface water is primarily stored in Donner Lake,

Martis Creek Lake, and Prosser Creek Reservoir. In 2001, Nimbus Engineers found that

the total groundwater storage capacity in the Martis Valley Basin is approximately 1.2 x

1010

m3, with approximately 6.0 x 10

8 m

3 of groundwater in storage, and the average

annual recharge to the groundwater is approximately 3.6 x 107 m

3 per year. The annual

recharge includes approximately 2.9 x 107 m

3 of natural recharge and approximately 6.7

x 106 m

3 of artificial recharge (California Groundwater Bulletin 118, 2006). The

artificial recharge comes from the Tahoe-Truckee Sanitation Agency (Nimbus Engineers,

12

2001). About 8.7 x 106 m

3 of water is extracted each year for urban use, and Nimbus

Engineers estimate that there is 3.05 x 107 m

3 of groundwater available each year. The

basin has an unconfined storativity of 0.05 (California Groundwater Bulletin 118, 2006).

As for the groundwater characteristics, P-wave seismic refraction models have

shown the water level to be a relatively flat lying velocity contour of 1525 m/s, which is

consistent with saturated, un-compacted sediments. Along with the seismic refraction

models, water level has been investigated through several boreholes drilled in 2007. The

average water level is at an elevation of 1753 m and the water table follows the

topography. Even during the summer, when there is less precipitation, an average

groundwater level of 1753 m is maintained. The water level also remains at or above the

blue silt layer, which suggests that the blue silt is acting as an aquitard. The hydraulic

conductivity of the glacial outwash has been estimated to be 0.35 cm/s, while the

hydraulic conductivity of the Prosser Creek alluvium has not been estimated. Drawdown

tests performed between the glacial outwash and the Prosser Creek alluvium, however,

have shown a much slower rebound within the alluvium, which indicates that the

hydraulic conductivity is lower in the alluvium than in the outwash. Transient

electromagnetic (TEM) models also indicate low resistivities at depths of 120-180 m

through the area of the dam. The top of this low resistivity zone is more than 100 m

below the groundwater level, though a deeper aquifer does exist below the blue silt layer.

This deep region of low resistivity is likely due to clay rich zones within or below the

Prosser Creek alluvium, which may be clay from weathered volcanics. This is consistent

with driller’s logs from nearby water wells, which report intervals of highly weathered

13

volcanic rock (Bedrosian et al., 2012). The water quality in Martis Valley Basin is

considered good as of 2003, though as the Tahoe Truckee Sanitation Agency water

reclamation plant expands, it may affect water quality (California Groundwater Bulletin

118, 2006).

Table 2: Martis Creek Discharge. This table shows the average stream discharge each

month for Martis Creek (water year: 2009) in both ft3/s and m

3/s. The months of highest

discharge are February through May, while the summer and fall months consist mainly of

baseflow (Brown & Caldwell, 2013).

Watershed size: 37.2 mi2

Mean annual ft3/s: 27

Mean annual m3/s: 0.76

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

ft3/s 11 14 20 29 34 47 57 52 26 14 10 11

m3/s 0.31 0.40 0.57 0.82 0.96 1.33 1.61 1.47 0.74 0.40 0.28 0.31

Figure 5: Average Discharge for Martis Creek. A graphical representation of the

average discharge (m3/s) for Martis Creek during each month of the water year.

0

0.5

1

1.5

2

1 2 3 4 5 6 7 8 9 10 11 12

Ave

rage

Dis

char

ge (

m3 /

s)

Water Year Month (October to September)

Martis Creek Average Discharge

14

Figure 6: Gaining and Losing Streams. From top to bottom: cartoon of gaining, losing

– connected, and losing – disconnected streams (CSIRO and SKM, 2012). Martis Creek

is an example of a gaining stream over certain reaches.

Springs nearby support groundwater discharge to East Martis Creek (ID 11 on

Figure 21). An example of stream discharge for the year 2009 is shown in Table 2. For

Martis Creek, average stream discharge across the year is approximately 0.76 m3/s, with

the highest discharge occurring in the spring, and the lowest occurring in the summer. A

water balance performed by Interflow Hydrology in 2003 showed that streamflow losses

in October 2002 across Martis Valley were approximately 0.018 m3/s (0.65 ft

3/s), while

losses at Martis Creek Lake were approximately 0.044 m3/s (1.55 ft

3/s), which implies

that streams are losing to the groundwater basin over much of the valley (Brown &

Caldwell, 2013). My research shows that there are reaches of Martis Creek where it is a

gaining stream (Figure 6).

15

Stream Morphology

Martis Creek is located in a 96.3 km2 (37.2 mi

2) watershed (Brown & Caldwell,

2013). It has meanders and vegetation growing along its banks, and springs in the

vicinity of the creek support a wetland environment. Both riffles and deep pools are

present (Figure 7), which affect degassing rates for gasses dissolved in the water, and the

deep pools may be reaches of groundwater influx. There are also incisions and bank

failures seen along reaches of the creek, often where meanders and semi-deep pools are

located (Shaw, Hastings, Drake, Hogan & Lindstrom, 2012). No visible tributaries are

located over the reach of the study, so any increases in discharge can be attributed to

groundwater influx.

Due to roads and areas of former logging and cattle grazing, the stream has been

diverted and straightened in some areas. There are at least four diversions associated

with cattle grazing in the early to mid-twentieth century found in the reach between

Lahontan Golf Club and Highway 267. There are also modified channels, such as at

Highway 267, where there is a double box culvert under the highway. While roads do

cause modification of the stream, when paved roads replaced the former dirt roads there

was less sediment transport from the roads since paved roads have relatively less erosion

associated with them (Shaw et al., 2012).

16

Figure 7: Martis Creek Characteristics. Clockwise, from bottom left: 1. vegetation

that grows along the banks of the stream, 2. deep pools found in some of the bends of the

stream, and 3. riffles as water flows over natural flow obstructions in stream.

Climate

In the Martis Valley region, the summers are warm and dry (though summer

thunderstorms do occur), and the winters are cold and wet. Precipitation occurs most at

the higher elevations in the western part of Martis Valley, and mean annual precipitation

for the region ranges from about 76.2 cm (30 in) below 1981 m (6,500 ft) to about 114.3

cm (45 in) above 1981 m. Precipitation in the winter usually occurs as snow, though

mid-winter rains may occur. The trend for the area is that periods of below average

17

precipitation last longer than periods of above average precipitation (Brown & Caldwell,

2013). The relatively fast melting associated with lower amounts of precipitation

occurring as snow leads to a sharp peak in the hydrograph and then a long recession,

which must then be supplied by groundwater. The age of groundwater discharging into

Martis Creek is not known.

Land Cover and Canopy

In the region of the study, land cover consists of developed areas of high,

medium, and low intensity, developed open area, shrubs/scrub and herbaceous area, and

evergreen forest (Figure 8). Canopy is generally medium to none (Figure 9), and where

there is no canopy, this is either developed or shrubs/scrub and herbaceous area. While

there aren’t many impervious surfaces in the area of the study, some do exist (Figure 10).

The ones immediately in the study area consist of the Tahoe-Truckee airport, roads such

as Highway 267, Lahontan Golf Club’s roads and homes, and the more developed areas

of Truckee, where there are schools, shops, restaurants, and homes. Northstar is the

closest ski resort, though others are nearby, and these have large areas where trees have

been removed from steep slopes. There are at least two golf courses in the immediate

area of the study (Lahontan and Martis Camp) (California Groundwater Bulletin 118,

2006), though nine golf courses use groundwater from the basin. Four golf courses use

water from Truckee-Donner Public Utilities Department (TDPUD), one uses water from

Northstar Community Services District (NCSD), and four are supplied from private

sources. Out of these, two use potable water, and the rest are assumed non-potable

(Brown & Caldwell, 2013). Currently, groundwater production from Martis Valley

18

Groundwater Basin is approximately1.15 x 107 m

3/year, while in the future a demand of

2.6 x 107 m

3/year is expected as the area becomes more populated (Brown & Caldwell,

2013).

Figure 8: Land Cover in Martis Valley. Land cover in the Martis Valley region. The

sample sites within the study area are indicated by the black markers. Most of the land

cover of the study area consists of evergreen forest, herbaceous/shrubs/scrub, and

developed land (National Land Cover Database, 2006).

19

Figure 9: Canopy in Martis Valley. A map showing canopy cover in Martis Valley.

The sample sites within the study area are indicated by the white markers. Canopy

ranges from medium intensity to none, with the darker green areas indicating heavier

canopy cover, and the black areas indicating no canopy cover (National Land Cover

Database, 2006).

20

Figure 10: Impervious Surfaces in Martis Valley. A map indicating regions of

impervious surfaces in Martis Valley. Most of the study region, with sample sites

indicated by the black markers, consists of pervious material, while the road surfaces and

developed areas consist of impervious material (National Land Cover Database, 2006).

21

ANALYSIS

The equation describing the change in flux of a dissolved gas with distance

downstream as a balance between the flux into the stream from groundwater and

hyporheic zone sediments and the flux out of the stream due to evaporation losses,

degassing (volatilization), decay, and losses to the hyporheic zone is reported in

Lamontagne and Cook (2006):

[

]

Where, at time t,

Q= stream discharge (m3/d),

I= influx (m3/d),

ci= initial radon activity (pCi/L) of groundwater discharge to stream,

c= radon activity (pCi/L) at location x,

w= mean stream width (m),

E= evaporation rate (m/d)

k= degassing constant (m/d),

d= mean stream depth (m),

λ= radioactive decay constant (d-1

) for 222

Rn,

γ= production rate for 222

Rn (pCi/d) within the hyporheic zone,

ѳ= porosity of sediment in the hyporheic zone, and

th= mean residence time of water (d) within hyporheic zone.

The conceptual model for radon emanation into porewater is shown in Figure 12.

If production in the hyporheic zone is effectively zero (discussed further below, in

Results section), the concentration of radon activity in the hyporheic zone porewater will

be equal to that in the streamwater, and the equation may be simplified by eliminating the

last three terms (

[

] ). The equation may be further simplified if

evaporation is neglected, which, in the case of Martis Creek, it may be, since the creek

has minimal relative evaporation over the short study reach. Common values for w, E,

22

and c for streams the size of Martis Creek are 117 to 658 cm, 10-3

to 10-2

m/d, and below

detection limit to 100 pCi/L, respectively. In that case, the term may be eliminated,

which leaves the equation for Martis Creek as

(Cox et al. 2009). Rearranging terms to solve for I gives

[

] .

In the study performed by Cox et al. (2009) at Squaw Creek, which was modeled

on a study performed on the Cockburn River by Cook et al. in 2006, k was assumed to

range from 5 to 10 m/day, based on previous estimates by Cook et al. (2006) for a

significantly larger river that were 0.5 to 25 m/day. The value of k in Squaw Creek was

unknown, however, so an estimate of 10 m/day was used, since k for a relatively small

stream is 14 m/day, while a large, low gradient river may have k of 2-4 m/day (Cook et

al., 2006 and Wanninkhof, Mulholland & Elwood, 1990). Small streams have higher

degassing constants because water will move relatively quickly when compared to a

large, low gradient river, where the water moves at a relatively slower velocity. For a

deeper river, there may also be fewer riffles to increase the degassing rate, since fallen

branches and trees may be able to sink to the river bed, rather than creating a riffle at the

river surface. From the assumptions and measurements made at Squaw Creek, in spring

of 2008, stream discharge was calculated to be 1.27 m3/s (Cox et al. 2009). Given that

degassing constants are site-specific, a more rigorous approach is to calculate degassing

from measured downstream losses of an introduced gas tracer, such as Xe.

23

Table 3: Similar Studies. A summary of the discharge (m3/s) and degassing constant

(m/day) found in similar studies.

Author Paper Title Location Discharge

(m3/sec)

k

(m/day)

for Rn

Cook et al.

Quantifying

Groundwater

Discharge to

Cockburn River,

Southeastern

Australia, Using

Dissolved Gas

Tracers 222

Rn and

SF6, 2006

Cockburn

River (New

South Wales,

Australia)

0.085 1.6

Lamontagne

et al.

Estimation of

Hyporheic Water

Residence Time In

Situ Using 222

Rn

Disequilibrium,

2007

Swamp Oak

Creek (New

South Wales,

Australia)

0.085 N/A

Cox et al.

Estimating

Groundwater Inflow

to Squaw Creek

Using Radon and

Radon Emanation

Experiments with

Squaw Creek

Sediments, 2009

Squaw Creek

(Northern

California,

United

States)

1.27 10

Clark et al.

Gas Exchange Rates

in the Tidal Hudson

River Using a Dual

Tracer Technique,

1994

Hudson River

(New York

State, United

States)

90 N/A

Zane

Reaeration in

Sagehen Creek near

Truckee, CA, 2010

Sagehen

Creek

(Northern

California,

United States

0.34 N/A

24

Xenon: Introduced Gas Tracer to Constrain Degassing

In this study, an introduced Xe gas tracer was used to determine the degassing

constant for Rn. Xe concentration (cm3/g) was measured at eight points downstream from

an injection point. Xe loss from degassing was represented as the slope of a semilog plot

of log xenon concentration against distance downstream (Figure 11), or by using the

equation

[(

) (

)]

where

K= slope (day-1

),

cx= Xe concentration (cm3/g) at distance x,

c0= Xe concentration (cm3/g) at first downstream station,

x= distance (m) from injection point, and

U= stream velocity (m/s).

The slope found in Figure 10 and K calculated from equation 4 give the same

value, as one is a graphical approach while the other is an algebraic method of finding K.

Once K has been calculated, the degassing constant (k) for the Xe can be calculated by

multiplying K (day-1

) by mean stream depth, which was measured as 0.16 m. Xenon

behaves in a similar way to Rn, due to similar chemical behavior and atomic weight

(since they are near each other on the periodic table). The degassing constant for Rn was

found using the equation

25

Where

kRn= degassing constant for Rn (m/day),

kXe= degassing constant for Xe (m/day),

DwRn= Diffusion coefficient for Rn (cm2/s) in water,

DwXe= Diffusion coefficient for Xe (cm2/s) in water, and

n= 0.5 to 1

with the diffusion coefficients of Rn and Xe on the order of 10-5

and 10-2

, respectively.

Therefore, the degassing constant (k) obtained for Xe was multiplied by 0.75 to find the

degassing constant for Rn based on equation 5 above, the fact that Rn (atomic weight

222) is heavier than Xe (atomic weight 131), and will leave the water at a slower rate

than Xe because of this. The ratio of kRn/kXe will range between 0.75 and 0.87, depending

on how it is calculated. Rn thus has approximately 0.75-0.87 the volatility of Xe.

Figure 11: Xenon Percentage vs. Distance Downstream. A graph plotting Xe

concentration on a logarithmic scale against distance from Xe tracer injection location.

y = 107.7e-0.002x R² = 0.9935

1.00

10.00

100.00

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00

Xe

Co

nce

ntr

atio

n (

cm³/

g)

Distance from injection (m)

26

M. Zane (2010) performed a similar study approximately 18 km north of Truckee,

in Sagehen Basin, concerning reaeration and gas exchange coefficients of dissolved

oxygen in a shallow alpine stream. SF6 was used as a tracer in Sagehen Creek, and the

average reaeration for the stream was found to be 35 day-1

. Even though reaeration rates

for different parts of the stream are similar, the gas exchange velocities differ because

they depend on depth, which changes along the river. The gas exchange rates for oxygen

in Sagehen Creek are 7.3 m/day in the upper sections of the stream, 9.8 m/day in the

lower sections of the stream, and 7.9 m/day over the entire stream. Zane also noted that

there were some sections of the stream where there was a large drop in SF6 over a short

distance, and this was attributed to a waterfall created by a USGS stream gauge.

Using Rn, groundwater discharge into the stream was also quantified. An average

discharge from 1954 to 2009 was found to be 0.34 m³/s (12.02 ft3/s), while peak

discharge in May was an average of 0.82 m³/s (29 ft3/s). The baseflow value was

considered to be the minimum discharge to the creek, which occurred in September, and

had an average of 0.04 m³/s (1.4 ft3/s) (Zane, 2010).

Clark, Wanninkhof, Schlosser, and Simpson (1994) found the gas exchange rates

for 3He and SF6 in the Hudson River by using introduced tracers of these two gases. The

tracers were injected fewer than 10 m below the water surface over a period of 20

minutes. Samples were then collected at stations 1 to 2 km from each other, one meter

below the water surface and one meter above the sediments, over a period of 16 days.

The samples were all analyzed within 12 hours of collection. Radon was not used as a

27

tracer since large Rn fluxes from sediments in estuaries make it difficult to determine gas

exchange rates.

The gas exchange rate is important to know in order to determine fluxes of gases

dissolved in the water, and how volatile contaminants may act in water. The mass flux of

a gas can be calculated from the first order gas transfer rate by

where h is the mean water depth, Cm is the mean concentration of the gas in the water, Ceq

is the concentration of the gas in equilibrium with the atmosphere, and K is the gas

transfer rate. The gas transfer velocity can be found from the equation

If the stream or river is vertically well mixed, the surface and mean gas concentrations

are equal and the gas transfer velocity simply equals the mean depth multiplied by the gas

transfer rate. The k values that were found in the Hudson River study ranged from 1.5 to

9.0 cm/h for SF6 (Clark et al 1994).

Sediment Samples

Where water samples are taken from the stream, sediment samples may also be

collected and analyzed for Rn activity. Of interest is hyporheic exchange, which occurs

when water flows through sediment below the stream bed, and then re-enters the stream.

When stream water flows through the sediment below the streambed, it may gain Rn that

emanates from the sediments and this hyporheic flux may obscure the Rn signal from

influent groundwater. This occurs when Ra, with a half-life of 1600 years, sorbed on the

28

surfaces of the sediments decays, and Rn diffuses from the sediments by alpha recoil

(Figure 12). In the hyporheic zone, Rn activity in porewater should theoretically reach

steady state with the Ra in the sediments. However, not all of the Rn produced by Ra

decay in the sediment actually reaches connected porewater, so Rn emanation rates are

often lower than Ra decay rates. If stream water dilutes the porewater, Rn activity in the

hyporheic zone will also be lower than expected. In this study, samples of sediment from

the streambed were taken from two locations that showed an increase in Rn activity with

distance (MC08 and MC05), and these sediment samples were analyzed using the LSC,

as described in the “Field and Laboratory Procedures” section.

Figure 12: Hyporheic Zone. Illustration of sediments and porewater in the hyporheic

zone. From point B, the 222

Rn can get into the pore water as point B is within recoil

range, and the path goes to the porewater (after Sakoda, Ishimori & Yamaoka, 2011).

29

Cox et al. (2009) performed a similar study, modeled after Cook et al. (2006), and

collected sediment samples from the hyporheic zone by digging a few inches into the

stream bed, and after the sediments were dried, 40 g of sediment was placed in a 60 mL

container with 25 mL of mineral oil, and the remainder was filled with deionized water.

A blank sample was also created from 40 g of commercial quartz sand, and three

standards were created using 0.5 mL of a 226

Ra standard. After being allowed to sit in the

refrigerator for approximately two weeks to reach secular equilibrium, the samples were

placed on the liquid scintillation counter. The Rn activities in the samples range from

70.3 to 254.1 pCi/kg, while the Rn activities in the groundwater samples are between 500

and 1000 pCi/L, and the equilibrium value was assumed to be 700 pCi/L, based on

groundwater values from wells near the creek. Porosity was estimated to be 0.4 and rock

density was estimated to be 2.9 g/cm3. From these approximations, an emanation rate of

160.8 pCi/kg can be calculated from the equation

E (700)

(1) (8)

where θ = porosity and ρ = density (Cox et al 2009). This emanation rate does not refer

to time because once secular equilibrium has been reached, the radon production rate

equals the radon loss. If 500 pCi/L is used as the equilibrium value for groundwater

entering Martis Creek, based on the wells near the creek and the springs near Highway

267, porosity is assumed to be 0.4, and rock density is assumed to be 2.9 g/cm3

(which is

higher than the average rock density of 2.7 g/cm3, since igneous rock tends to be a bit

more dense), then the expected emanation rate for the Martis Creek region is 114.9

pCi/kg.

30

The radon emanation rate, E, in pCi/kg, is related to the radon production rate, γ,

in pCi/L d-1

, by the equation

where ρ = density of the solid, λ= the decay constant, and ε = porosity (Lamontagne &

Cook, 2006).

Cook et al. (2006) estimated groundwater discharge to the Cockburn River in

Australia by using Rn activities in the river and an introduced tracer of SF6. Since water

velocity in the hyporheic zone is approximately two orders of magnitude lower than the

river velocity, it was assumed that there was no horizontal flow within the hyporheic

zone, since water residence times in the hyporheic zone are generally hours to days. To

process the sediment sample from the hyporheic zone, 40 g of sediment was placed in 60

mL containers, with 20 mL of mineral oil and 20 mL of deionized water. After the

samples were allowed to sit for six weeks, the radon activity in the mineral oil was

counted on the liquid scintillation counter. The Rn emanation from the sediments ranged

from 62.2 to 235.1 pCi/kg, similar to that found by Cox et al. (2009). Porosity was also

assumed to be 0.4 and rock density was assumed to be 2.7 g/cm3. Production rates were

calculated to be between 67.6 and 140.5 pCi/L/day, so radon activities in the hyporheic

zone would be between 270.1 to 811.0 pCi/L, while radon activities in the groundwater

was between 107.0 and 15,675.7 pCi/L (Cook et al., 2006). This range for groundwater

is not typical, however, and likely resulted from the study region overlying a granite

batholith.

31

The authors found that groundwater inflow over the entire reach of the river is

approximately 30,900 m3/day. In certain parts of the river, if radon contribution from the

hyporheic zone is ignored, groundwater inflow is overestimated by 30%, and over the

entire length of the river it is overestimated by 70%, which illustrates the importance of

considering Rn contribution by the hyporheic zone (Cook et al., 2006). The Cockburn

River, however, has a much larger area than Martis Creek, with higher Rn activities, and

higher stream discharge, so 70% is not a likely number to overestimate Rn contribution in

the study area. This is simply an example of the importance of testing for hyporheic zone

contribution. Since the study area of the Cockburn River overlies a granite batholith, and

some water samples yielded very high Rn activity rates, 70% overestimation is more

likely for the Cockburn River than for Martis Creek.

Figure 13: Radon Inputs and Outputs. Illustration of the different radon inputs and

outputs in a stream (after Agency for Toxic Substances and Disease Registry 2010).

32

FIELD AND LABORATORY PROCEDURES

Radon in Groundwater

Figure 14: Well Sampling. A photograph illustrating sampling from a monitoring well.

Once the well is purged, as indicated by stable water quality parameters (pH,

temperature, specific conductance), sample containers are filled according to protocol.

To measure the Rn activity in water samples, the water must first be collected in

the field with minimal exposure to the atmosphere. For groundwater samples, 250 mL

glass bottles are used, and in order to keep the water from contact with the atmosphere, a

pump is used to fill bottles directly when sampling from monitoring wells (Figure 13),

33

and capped while the water is still running so that the Rn is not lost from the sample. The

containers are capped with no headspace, so that the water has minimal contact with the

atmosphere. These samples are then measured on a RAD7 Radon Detector within a day

or two of being collected (while waiting to be measured, the samples are kept on ice).

Each sample is run for 35 minutes – this includes a 5 minute “bubbling” period in which

air is run through a closed loop on the machine to bubble the gas out of the sample, two 5

minute “grabbing” periods where the air is run from the sample, through drierite, to the

counter, and four 5 minute counting periods. The RAD7 then prints the Rn activities

found for each counting cycle, the average Rn activity for the sample, and the standard

deviation.

Radon in Surface Water

For surface water samples, 20 mL glass vials are filled with 10 mL of scintillation

cocktail (mineral oil). Glass vials with Teflon lined caps are used for volatile substances,

such as Rn, in order to reduce the amount that may escape from the vial. The cocktail is

used because Rn is more soluble in organic solvents than in water, so it will transfer from

the water to the cocktail. This sampling procedure aides in the analysis of Rn because

certain water soluble radionuclides such as radium (Ra) interfere with Rn counting. Ten

mL of mineral oil along with 10 mL of water are used in order to keep all the samples

with the same geometry, which is important during the counting procedure, in order to

interpret the Rn activity the same way for all samples, standards, and blanks. A hooked

syringe is used to collect 10 mL of water approximately 10 cm beneath the stream

surface, and the stream water is injected beneath the cocktail, so that the water does not

34

have contact with the atmosphere during transfer from the syringe to the vial (Figure 15).

The samples are then kept on ice in a cooler for transfer back to the laboratory. Once the

samples have been collected, they are allowed to equilibrate for at least four hours before

being counted. This allows the Ra and Rn to equilibrate in the sample and the

scintillation cocktail.

Figure 15: Surface Water Analysis. An illustration depicting the sequence of events

from surface water collection to measurement of the sample. From left to right: 1. A

curved syringe is used to collect 10 mL of water, 2. The water is collected from the

stream, 3. The water is injected beneath the scintillation cocktail (mineral oil), and 4.

Radon activity in the sample is measured on the Quantulus Liquid Scintillation Counter

After the surface water samples have been allowed to equilibrate, they are

analyzed on a Quantulus liquid scintillation counter (LSC) for 60 to 90 minutes. The Rn

samples are counted for a relatively short time due to the short half-life of Rn, which is

3.82 days. Samples with very low Rn activities are run twice to compare activity levels

between the two runs.

35

Once Rn decays from the samples have been counted on the LSC (in counts per

minute, or CPM), an Excel spreadsheet is used to translate CPM to Rn activity levels. To

do this, blanks made with deionized water are used to determine the background, which

was found to be approximately 0.1 counts per minute (CPM), and this background is

subtracted from the CPM of each sample. Along with blanks, two laboratory control

samples (500 μL of laboratory standard Ra liquid with water and 10 mL of mineral oil)

are counted, to give activity levels that can then be compared to samples counted after

sitting for several days (equilibrating Ra and Rn in the samples). The pulse shape

analyzer (PSA) is used to discriminate events not associated with radon decay. The PSA

setting that gave the best signal/noise ratio based on results from blanks was 75 keV. In

EASYView, part of the Quantulus software package by Perkin Elmer, the energy

spectrum is then viewed in a 650-875 keV window. This gives a CPM which can then

have the background CPM subtracted from it. Once the correct counts per minute have

been calculated, this is converted into Rn activities with the equation

where e is CPM/DPM (DPM being Decays Per Minute) and V is volume of sample.

Radon from Sediment Samples

To account for the possibility of hyphorheic zone contributions, sediment samples

are taken at several sample locations by digging approximately four inches below the

streambed with a trowel. Once samples are taken, they are dried for two days at 100˚C in

an oven. They are then sieved (see Table 4) to be separated into different sediment sizes:

36

gravel (>2 mm), coarse-medium sand (2mm-300 μm), fine-very fine sand (300-63 μm),

and silt (<63 μm).

Table 4: Sediment Sizes. This table shows sieve sizes used to separate sediment sizes

for samples gathered at Martis Creek. All of the sizes except gravel are processed and

measured for Rn emanation on the Quantulus LSC.

US Standard

Sieve

Nominal

opening Grain size caught Processed

No. 10 2.00 mm gravel no

No. 50 300 μm

coarse/medium

sand yes

No. 230 63 μm fine/very fine sand yes

tray n/a silt/clay yes

Once the samples are separated into size categories, 3.00 g of each category for

each sample is placed in a 20 mL glass vial, and the vial is filled to the 10 mL point with

water. Ten mL of liquid scintillation cocktail is then added, and the samples are left for

approximately two weeks so that the Rn can emanate from the sediments into the water,

and then into the mineral oil. Once the Rn has been transferred into the mineral oil, the

samples are placed on the LSC for 60 minutes each, and are counted in two rounds. They

are then allowed to sit for an additional five days, and then are re-run twice, for 60

minutes each. Laboratory control samples are made using a laboratory standard soil-

based uranium-thorium (0.5g and 1.0g), deionized water filled to the 10mL point, and

10mL of mineral oil, while blanks are made using commercial sand (considered to be Rn

free), water filled to the 10 mL point, and 10 mL of mineral oil. Events (CPM) in the

37

650-875 keV window are blank-subtracted and the corrected count rates are used to

determine radon activity.

Figure 16: Sediment Analysis. An illustration depicting the sequence of preparing and

measuring sediment samples. From left to right: 1. Sediments are collected, and once

they are dried for two days at 100˚C, are placed in a stack of sieves. 2. The sieves are

then shaken for at least 10 minutes to separate the sediment sizes. 3. Three grams of a

selected size is then placed in a 20 mL glass vial, deionized water is added to the 10 mL

point, and 10 mL of scintillation cocktail is added. 4. After the samples sit for two

weeks, Rn activity is measured on the Quantulus LSC.

Xenon Tracer

The Xe tracer is introduced through a one meter length of gas permeable tubing

(weighed down by a chain), and a regulator connected to a lecture bottle containing Xe

gas allowed the slow release of Xe into the tubing (Figure 17). The efficiency of

dissolution is near 100%, and the concentration of natural Xe in the water near the

location (measured at MC-01, a few meters upstream from the tracer introduction point,

is near zero. Three times a day (morning, afternoon, and evening) for three days, the

38

survey team took samples from the left and right banks and the center of the stream at

eight locations downstream, and one upstream, of the Xe introduction location. The

samples were taken by submerging 40 mL volatile organic analysis (VOA) vials and

capping them underwater to ensure that there was no headspace. These samples were

kept on ice/in the refrigerator at the lab after collection until they were analyzed at

Lawrence Livermore National Laboratory. The samples were analyzed by noble gas

membrane inlet mass spectrometry (NG-MIMS), which measures dissolved gasses by

pumping the water from the sample through a semi-permeable membrane inside the mass

spectrometer vacuum, then uses a residual gas analyzer to detect the Xe in the extracted

gas (Figure 18). As the sample is analyzed, in addition to each sample having no

headspace, gas exchange with the atmosphere is minimized by taking water from the

bottom of the uncapped vial. Each sample takes five minutes to run, and only takes 2.5

mL of the 40 mL sample to run the measurement. The Xe concentration is calculated by

the equation

where Cs is the sample concentration, Cstd is the standard concentration, and Mi,std, and

Mi,zero refer to the interpolated standard and the zero pressure measurements. The Xe

isotope concentrations are then converted to total Xe gas concentrations using known

atmospheric gas isotope ratios (Visser, Singleton, Hillegonds, Velsko, Moran & Esser,

2011).

39

Figure 17: Xenon Tracer Introduction Point. A picture of the chain that marks the

introduction point for the Xe tracer.

Figure 18: Noble Gas Membrane Inlet Mass Spectrometer. A picture of the noble gas

membrane inlet mass spectrometer (NG-MIMS), with the portions of the machine

labeled. A sample in a 40 mL volatile organic analysis (VOA) vial is shown in the part

labeled “sample” (Visser et al., 2011).

40

Stream Flow

Stream velocity was measured at five locations along the study reach using a

Global Water flow probe. First, a wooden stake was placed into the ground on either

side of the stream, and the width was measured based on foot markings on the attached

string. Then the water depth was measured using a tape measurer or ruler marking on the

flow probe and flow velocity was measured using the probe at each marking. The flow

was measured by lifting and lowering the probe slowly, so that water could flow through

the attached propeller. The probe then calculates an average stream velocity across the

water column. Discharge is calculated by taking the width and depths measured at the

stream, and creating a cross section. By multiplying the section areas by the flow

velocities and summing the resulting discharge for each section, total discharge (Q) is

obtained.

RADON RESULTS IN WATER SAMPLES

Groundwater Data

In order to quantify groundwater influx to Martis Creek, the Rn activity of

groundwater (ci) must be estimated. To estimate this variable, groundwater samples

were analyzed from both monitoring and production well sources across Martis Valley,

and the ones nearest to Martis Creek (N and O, Figures 20 and 21) were used to estimate

ci. Two springs (Map IDs Y and X, Figure 22) were also sampled in October 2012. All

of the wells, with the exception of one (M), had Rn activity levels of 362 pCi/L to 1300

pCi/L, which is consistent with common groundwater Rn activity levels. Location M was

41

a large diameter, open borehole that could not be properly purged and had a very low Rn

activity level of 9.19 pCi/L, due to re-equilibration with the atmosphere.

Some wells were sampled in both December 2011 and June 2012 (corresponding

to IDs C, D, E, G, H, J, K, L, N, and O on Figures 20 and 21). Whether the Rn activity is

similar or different between the winter and summer depends on the well. For instance,

Rn activity for well location E didn’t change much between winter and summer, staying

around 1300 pCi/L. Well locations O and N, however, had higher Rn activity levels in

June, going from about 900 pCi/L (for both) to about 1200 and 1100 pCi/L, respectively

(Figure 19). This is likely due to less recharge occurring during the relatively dry

summer months. When recharge occurs from precipitation, this water is lower in Rn

activity than groundwater that has had a relatively long residence time. So, during the

winter/spring the wells more affected by recent recharge may have a lower Rn activity

level than in the summer. The average Rn activity measured in the groundwater in

December 2011 was 788 pCi/L, with a high of 1270 pCi/L, a low of 362 pCi/L, and an

average error of 59.5 pCi/L. The average Rn activity measured in the groundwater in

June 2012 was 815 pCi/L, with a high of 1300 pCi/L, a low of 442 pCi/L, and an average

error of 27.6 pCi/L.

Most of the wells had Rn activities of less than 1000 pCi/L, and the ones closest

to Martis Creek (N and O) were both about 850 pCi/L in winter, and between 1100 and

1200 pCi/L in summer. The spring Y was closest to the study area out of all the wells

and springs, and this had a Rn activity of 322 pCi/L. The average for the springs was 459

pCi/L, and a value of 500 pCi/L was used as an estimate of the activity in groundwater

42

influx to the stream (ci). Since springs are more likely to tap a shallow aquifer system,

with lower Rn activity, which is more likely to contribute to groundwater inflow to the

stream, the average Rn activity for the springs was used to estimate groundwater

discharge to Martis Creek. Results for the Rn activities in the wells and springs can be

seen in Table 5, graphically in Figure 19, and on maps in Figures 20, 21, and 22.

43

Table 5: IDs and Rn Activity: Groundwater. Table showing Rn activity levels for

wells in the Martis Valley region.

Map

ID Collection Date Act (pCi/L)

Error 95%

CI+

O 12/19/11 8.58E+02 1.02E+02

K 12/19/11 4.19E+02 3.03E+01

N 12/19/11 8.68E+02 3.82E+01

E 12/19/11 1.27E+03 5.40E+01

G 12/19/11 7.72E+02 6.53E+01

D 12/19/11 6.44E+02 1.54E+01

C 12/19/11 3.62E+02 2.27E+01

H 12/19/11 1.18E+03 2.58E+01

J 12/20/11 5.00E+02 5.39E+01

L 12/20/11 1.01E+03 1.87E+02

A 06/19/12 6.87E+02 2.76E+01

B 06/19/12 4.97E+02 2.36E+01

C 06/19/12 4.63E+02 2.29E+01

D 06/19/12 7.69E+02 2.95E+01

E 06/19/12 1.28E+03 3.79E+01

F 06/19/12 7.86E+02 2.98E+01

G 06/19/12 9.52E+02 3.29E+01

H 06/19/12 1.30E+03 3.85E+01

I 06/20/12 6.06E+02 2.49E+01

J 06/20/12 4.42E+02 2.13E+01

K 06/20/12 5.43E+02 2.36E+01

L 06/20/12 7.22E+02 2.72E+01

M 06/20/12 9.19E+00 4.23E+00

N 06/20/12 1.13E+03 3.39E+01

O 06/20/12 1.23E+03 3.56E+01

Z 10/29/12 7.20E+02 2.75E+01

Z 10/29/12 7.39E+02 2.79E+01

Y 10/29/12 3.22E+02 1.87E+01

X 10/29/12 5.64E+02 2.52E+01

X 10/29/12 4.91E+02 2.36E+01

44

Figure 19: Radon Activity in Wells. Graph showing Rn activity levels plotted against

well IDs. Some wells were measured more than once, and while some like well E did not

change through the year, some wells such as N and O had higher Rn activity levels in

June than in December.

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1.40E+03

1.60E+03

O K N E G D C H J L A B C D E F G H I J K L M N O Z Z Y X X

Rad

on

Act

ivit

y (p

Ci/

L)

Well Location

December 2011

June 2012 October 2012

45

Figure 20: December 2011 Radon Activity Map. A map showing Rn activity levels for

wells sampled in December 2011.

46

Figure 21: June 2012 Radon Activity Map. A map showing Rn activity levels for

wells sampled in June 2012.

47

Figure 22: October 2012 Radon Activity Map. A map showing Rn activity levels for

springs (locations X, Y, and Z) and surface water (location 11) sampled in October 2012.

48

Surface Water Data

During the first stream survey in July 2012 (Table 6 and Figures 23 and 25),

samples were taken from locations far downstream (IDs 2, 5, 6, 7, 8, and 9), near Martis

Creek Lake and Highway 267. It was expected that the relatively low-lying meadow

portion of the valley would likely be a region of discharge of groundwater from the

nearby mountains, but the samples all showed Rn activities of 15.0 pCi/L or less,

indicating that these locations are not in the vicinity of groundwater influx. In addition,

samples were taken from the Truckee River (ID 1) and the inlet to Martis Creek Lake (ID

10). Both of these locations showed similarly low Rn activities of about 10.0 pCi/L. The

only sample from the initial survey that showed higher (91.2 pCi/L) Rn activity was a

sample taken from West Martis Creek (ID 3), in a reach of slowly moving water and

pools.

The next stream survey occurred on August 2, 2012 (Table 6). This time, samples

were taken farther upstream along Martis Creek (IDs 2 and 6). As a general trend, Rn

activities were higher the farther upstream samples were taken, some over 60 pCi/L. The

stream reach selected for intensive study (in mid-August 2012 on Lahontan Golf Club

property), was chosen based on the results of the preliminary surveys. During the mid-

August stream survey (Tables 6 and Figures 24 and 26-27), many samples were over 60

pCi/L in the reaches of the stream on Lahontan Golf Club property (IDs MC-17 through

MC14), and some farther upstream, MC-16 and MC-17, were over 100 pCi/L indicating

that groundwater influx is likely over these reaches. Several reaches of Martis Creek also

jumped up in Rn activity (MC08 and MC05), then went back down quickly away from

49

those points. Since this indicates potential groundwater influx, sediment samples were

taken at these locations to test for hyporheic zone influence (discussed in the “hyporheic

zone” section).

The samples from the March/April 2013 surveys (Table 6 and Figure 28) were

taken upstream from MC00 (longitude: -120.14, latitude: 39.30), including a few even

farther upstream than the August 2012 survey, though MC08 and MC05 were re-sampled.

The highest Rn activities were found in the reaches included in the August 2012 survey,

however. Compared to the mid-August 2012 stream survey, the March/April 2013

stream surveys show lower Rn activity levels, but still have higher levels than would be

expected without groundwater contribution. The lower Rn activity levels observed in

March and April are expected, as snowmelt and runoff increase discharge with little

additional Rn activity, so the Rn activity from groundwater influx becomes diluted.

50

Table 6: Rn Activity: Surface Water. A table showing surface water locations and Rn

activities.

Sample ID

Collection

Date

Act

(pCi/L)

Error 95%

CI Figure

West Martis Creek @ gage 12/19/11 4.05E+00 1.90E+00 N/A

Truckee River @ Don. Cr 12/20/11 1.26E+01 1.23E+01 N/A

Martis Creek @ #3 Bridge 12/20/11 5.75E+01 3.63E+01 N/A

Martis Creek at Hwy 267 12/20/11 1.15E+01 3.81E+00 N/A

Donner Creek (between

Truckee R and West R Rd) 06/21/12 2.17E+00 2.94E+00 N/A

Truckee R (40m dwnstrm

of Donner Cr. confl.) 06/21/12 2.43E+00 3.00E+00 N/A

Martis Creek (upstream

wooden bridge @267) 06/21/12 2.20E+00 2.98E+00 N/A

N Fork American R. @

Iowa Hill 06/21/12 1.23E+00 2.82E+00 N/A

Mid Martis Cr. @ bridge 07/09/12 9.19E+01 8.62E+00 22

Martis Cr. Dwnstrm survey 07/09/12 8.92E+00 3.41E+00 22

Martis Cr. Dwnstrm survey 07/09/12 6.35E+00 3.13E+00 N/A














Martis Cr. Upstream

survey 07/09/12 1.49E+01 4.50E+00 22


Martis Creek at Hwy 267 07/09/12 7.33E+00 3.71E+00

M. Martis Cr. Near Confl. 07/09/12 3.72E+00 3.26E+00 N/A

Martis Cr. Upstrm

confluence 07/09/12 3.50E+00 3.98E+00 22

Martis Lk In dwnstrm

surv, 07/09/12 4.79E+00 4.19E+00 N/A


surv. 07/09/12 6.44E+00 4.46E+00 N/A


surv. 07/09/12 4.25E+00 4.19E+00 N/A

Martis Lake Inlet 07/09/12 8.22E+00 4.78E+00 22

Martis Lake Inlet upstrm 07/09/12 1.70E+00 3.89E+00 N/A

51

Table 6 continued

Sample ID

Collection

Date Act (pCi/L)

Error 95%

CI Figure

Donner Creek 07/09/12 4.36E+00 4.30E+00 22

Truckee R. @ Donner

Cr. 07/09/12 2.73E+00 4.09E+00 22

Truckee R. @ Donner

Cr. 07/09/12 2.08E+00 4.03E+00 22

Jake's Bridge 08/02/12 6.04E-01 2.10E+00 N/A

upstream survey 1 08/02/12 8.14E-01 2.16E+00 N/A

Pappe's Bridge 08/02/12 3.44E+01 5.53E+00 N/A

upstream survey 2 08/02/12 4.14E+01 6.04E+00 N/A










MC05 08/02/12 2.47E+01 5.15E+00 N/A

Martis Deep Pool 08/02/12 1.09E+01 3.86E+00 N/A

Martis Surv. Dwnstrm 08/02/12 7.29E+00 3.44E+00 N/A

Martis Cr at Hwy 267 08/02/12 6.58E+00 3.35E+00 N/A



Middle Martis Creek 08/02/12 6.62E+01 8.13E+00 N/A

MC-17 08/15/12 1.15E+02 1.16E+01 23, 24

MC-16 08/15/12 1.05E+02 1.12E+01 23, 24

MC-15 08/15/12 8.82E+01 1.03E+01 23, 24

MC-14 08/15/12 8.23E+01 9.99E+00 23, 24

MC-13 08/15/12 7.40E+01 9.51E+00 23, 24

MC-12 08/15/12 7.99E+01 9.85E+00 23, 24

MC-11 08/15/12 8.11E+01 9.92E+00 23, 24

MC-10 08/15/12 6.55E+01 9.01E+00 23, 24

MC-09 08/15/12 7.43E+01 9.53E+00 23, 24

MC-08 08/14/12 4.49E+01 7.63E+00 23, 24

MC-08 08/16/12 5.54E+01 8.36E+00 23, 24

52

Table 6 continued

Sample ID

Collection

Date Act (pCi/L)

Error 95%

CI Figure

MC-07 08/14/12 3.07E+01 6.52E+00 23, 24

MC-06 08/14/12 4.09E+01 7.34E+00 23, 24

MC-05 08/14/12 3.69E+01 7.03E+00 23, 24

MC-05 08/16/12 6.98E+01 9.27E+00 23, 24

MC-04 08/14/12 3.50E+01 6.88E+00 23, 24

MC-03 08/14/12 3.66E+01 7.00E+00 23, 24

MC-03 08/16/12 6.43E+01 8.93E+00 23, 24

MC-01 08/15/12 5.60E+01 8.40E+00 23, 24

MC-01 08/16/12 6.01E+01 8.67E+00 23, 24

MC-01 08/16/12 6.25E+01 8.82E+00 23, 24

MC00 08/15/12 5.42E+01 8.28E+00 23, 24

MC01 08/15/12 3.83E+01 7.14E+00 23, 24

MC02 08/15/12 4.45E+01 7.61E+00 23, 24

MC03 08/15/12 4.37E+01 7.54E+00 23, 24

MC04 08/15/12 4.30E+01 7.49E+00 23, 24

MC05 08/15/12 4.37E+01 7.54E+00 23, 24

MC06 08/15/12 3.92E+01 7.20E+00 23, 24

MC07 08/15/12 5.04E+01 8.02E+00 23, 24

MC08 08/15/12 3.71E+01 7.04E+00 23, 24

MC09 08/15/12 2.79E+01 5.17E+00 23, 24

MC10 08/15/12 2.70E+01 5.11E+00 23, 24

MC11 08/15/12 2.89E+01 6.37E+00 23, 24

MC12 08/15/12 2.60E+01 6.11E+00 23, 24

MC13 08/15/12 4.24E+01 7.45E+00 23, 24

MC14 08/15/12 3.40E+01 6.79E+00 23, 24

MC15 08/15/12 3.36E+01 6.76E+00 23, 24

MC16 08/15/12 3.10E+01 6.55E+00 23, 24

MC17 08/15/12 3.48E+01 6.86E+00 23, 24

MC18 08/15/12 3.57E+01 6.93E+00 23, 24

MC19 08/15/12 4.09E+01 7.34E+00 23, 24

MC20 08/15/12 2.62E+01 6.12E+00 23, 24

MC21 08/15/12 2.77E+01 6.26E+00 23, 24

MC22 08/15/12 2.77E+01 6.26E+00 23, 24

MC23 08/15/12 2.91E+01 6.38E+00 23, 24

MC24 08/15/12 2.77E+01 6.26E+00 23, 24

53

Table 6 continued

Sample ID

Collection

Date Act (pCi/L) Error 95% CI Figure

MC25 08/15/12 2.95E+01 6.41E+00 23, 24

MC26 08/15/12 1.73E+01 5.26E+00 23, 24

MC27 08/15/12 2.03E+01 5.56E+00 23, 24

MC28 08/15/12 2.11E+01 5.65E+00 23, 24

MC29 08/15/12 1.77E+01 5.30E+00 23, 24

MC30 08/15/12 1.47E+01 4.98E+00 23, 24

MC31 08/15/12 1.85E+01 5.39E+00 23, 24

MC32 08/15/12 1.14E+01 4.60E+00 23, 24

MC33 08/15/12 1.32E+01 4.80E+00 23, 24

MC34 08/15/12 1.70E+01 5.22E+00 23, 24

East Martis Cr. 10/29/12 6.79E+00 3.75E+00 19

MC-02 3/29/13 1.15E+01 8.75E+00 25

MC-04 3/29/13 1.90E+01 9.87E+00 25

MC-09 3/29/13 3.11E+01 1.15E+01 25

MC-10 3/29/13 2.98E+01 1.14E+01 25

MC-13 3/29/13 2.40E+01 1.07E+01 25

MC-17 3/29/13 3.03E+01 1.16E+01 25

MC01 4/29/13 4.96E+01 9.74E+00 25

Surv. 1 (upstrm of

MC34) 4/29/13 4.48E+00 4.67E+00 N/A

MC02 4/29/13 6.07E+01 1.07E+01 25

Surv. 2 (upstrm of

MC34) 4/29/13 2.38E+00 4.35E+00 N/A

Surv. 3 (upstrm of

MC34) 4/29/13 9.83E+00 5.61E+00 N/A

MC34 4/29/13 8.57E+01 1.25E+01 25

MC-04 4/29/13 6.53E+01 1.12E+01 N/A

Golf Pass Br.

upstrm of August

survey 4/29/13 2.46E+00 4.50E+00 N/A

Large Golf Br.

upstrm of August

survey 4/29/13 2.48E+00 4.54E+00 N/A

MC08 4/29/13 1.74E+01 6.77E+00 25

MC05 4/29/13 1.93E+01 7.03E+00 25

MC03 4/29/13 7.95E+01 1.25E+01 25

54

Figure 23: July 2012 Stream Survey. This graph shows Rn activity against locations in

the July 2012 survey. In this survey, which took place downstream of the mid-August

2012 survey, Rn activities are all very low and ranged between approximately 5 and 10

pCi/L. The labels 7 and 8 refer to the Map ID locations (Table 7) of the beginning of the

Martis Creek downstream survey and the beginning of a two sample upstream survey,

respectively.

Figure 24: August 2012 Stream Survey. This graph shows results from the mid-August

2012 stream survey, with Rn activities plotted against station locations on Martis Creek.

The general trend was that Rn activities were highest upstream and lowest downstream.

0.00E+00

5.00E+00

1.00E+01

1.50E+01

2.00E+01

2.50E+01

July 2012 Downstream Survey 7 8

0.00E+00

5.00E+01

1.00E+02

1.50E+02

MC

-17

MC

-15

MC

-13

MC

-11

MC

-09

MC

-08

MC

-06

MC

-05

MC

-03

MC

-01

MC

-01

MC

01

MC

03

MC

05

MC

07

MC

09

MC

11

MC

13

MC

15

MC

17

MC

19

MC

21

MC

23

MC

25

MC

27

MC

29

MC

31

MC

33

August 2012 Stream Survey

55

Table 7: Surface Water Map IDs. This table shows locations on Martis Creek, along

with corresponding map IDs and Rn activity levels.

Sample ID

Map

ID

Collection

Date Act (pCi/L)

Error

95% CI

West Martis Creek @ USGS

gage 3 12/19/11 4.05E+00 1.90E+00

Truckee River @ Donner

Creek 1 12/20/11 1.26E+01 1.23E+01

Martis Creek @ #3 Bridge 4 12/20/11 5.75E+01 3.63E+01

Martis Creek at Hwy 267 2 12/20/11 1.15E+01 3.81E+00

Donner Creek (between

Truckee River and West River

Rd) 5 06/21/12 2.17E+00 2.94E+00

Truckee River (40m

downstream of Donner Cr.

confl.) 1 06/21/12 2.43E+00 3.00E+00

Mid Martis Cr. @ bridge 6 07/09/12 9.19E+01 8.62E+00

Martis Cr. Dwnstrm survey 7 07/09/12 8.92E+00 3.41E+00

Martis Cr. Upstream survey 8 07/09/12 1.49E+01 4.50E+00


Martis Cr. Upstrm confluence 9 07/09/12 3.50E+00 3.98E+00

Martis Lake Inlet 10 07/09/12 8.22E+00 4.78E+00

Donner Creek 5 07/09/12 4.36E+00 4.30E+00

Truckee R. @ Donner Cr. 1 07/09/12 2.73E+00 4.09E+00

Truckee R. @ Donner Cr. 1 07/09/12 2.08E+00 4.03E+00




Middle Martis Creek 6 08/02/12 6.62E+01 8.13E+00

East Martis Creek 11 10/29/12 6.79E+00 3.75E+00

56

Figure 25: July 2012 Radon Activity Map. Map showing Rn activity

levels for the June 2012 sampling trip (all surface water).

57

Figure 26: August 2012 Radon Activity Map. Map showing Rn activity

levels for the August 2012 sampling trip (all surface water locations).

58

Figure 27: August 2012 Radon Activity Close-up. Map showing Rn

activity levels for the mid-August 2012 stream survey.

59

Figure 28: March/April 2013 Radon Activity Map. Map showing Rn activity levels

for the March/April 2013 stream surveys.

60

Hyporheic Zone Results

Sediment samples were taken from locations MC05 and MC08 (Table 8), which

are locations where Rn activities in water samples were higher than expected, based on

comparison of the Xe and Rn curves (Figure 31). This was to test for hyporheic zone

influence, which could cause over estimation of groundwater influx if it is not accounted

for. For each of the samples, the sediment was divided into four size categories as

described in the laboratory procedures section.

Each of these categories shows little Rn contribution to the stream, with MC05

contributing 112 to 192 pCi/kg, and MC08 contributing 177 to 264 pCi/kg. The finer

grained sediments have a higher Rn contribution, due to the greater surface area to

volume. These contributions are consistent with what Cox et al. (2009) found in Squaw

Creek, and what Cook et al. (2006) found from sediments from the Cockburn River, as

described in the “Analysis” section.

Based on the decay rate of Rn, these activities should be within 10% of steady-

state values where radon emanation is balanced by radon decay. The calculated

emanation rate for the Martis Creek region, based on equation 8, is 114.9 pCi/kg, when

using 500 pCi/L as the Rn activity in the groundwater, a porosity of 0.4, and a sediment

density of 2.9 g/cm3. This is consistent with emanation rates for MC05 (Table 8), though

the emanation rate for MC08 is a bit higher than the calculated rate. This may be due to

the estimation of 500 pCi/L – if 700 pCi/L is used, then the calculated emanation rate

changes to 160 pCi/kg. This is still a bit low when compared to the finer sediments found

61

at MC08, but the measured values are still within the steady-state values calculated using

groundwater radon activities and sediment properties observed in the Martis Creek basin.

While these results are similar to those listed in the Cook et al. (2006) and Cox et

al. (2009) studies, hyporheic zone thickness, permeability, and residence time of water in

the hyporheic zone must be known to determine hyporheic zone contribution to Rn

activity in Martis Creek. If water has a longer residence time within the hyporheic zone,

Rn in the water will decay before the water leaves the hyporheic zone, whereas if the

water has a shorter residence time, more Rn will be added to the stream water (the

amount depends on Rn emanation from the sediments). Therefore, the hyporheic zone

may either be a sink (decreases Rn activity in the stream water) or a source (increases Rn

activity in the stream water). Since streambed piezometers were not used, the location

and thickness of the hyporheic zone is not known, thus hyporheic zone contribution to Rn

activity in the stream is not quantified. It is likely that the hyporheic zone is a source of

Rn activity, however, so the influx found in the results is probably a maximum rather

than a minimum.

62

Table 8: Rn Emanation. Table showing Ra decays per minute/kg and Rn activities in

pCi/kg for stations MC05 and MC08. Each station has three sediment sizes – sieve 50,

for coarse/medium sand, sieve 230, for fine/very fine sand, and the tray for silt/clay.

Xenon Tracer

The location of the introduction point of the Xe tracer is marked by a chain in the

stream (Figure 18), and this spot is given the name MC00 in Table 6. To determine the

degassing constant for Rn, Xe was used in this study as an alternative tracer to SF6, which

has been applied in previous studies that use an introduced, dissolved gas tracer. The

advantages to using an introduced tracer are that the exact time and place of introduction

are known, so the decay constant can actually be quantified. This is in contrast to using

exclusively a natural tracer, since we do not know the activity in the groundwater influx,

nor the exact time or place of introduction to the surface water. The advantages to using

Xe in particular as an introduced tracer are that, as a noble gas, it is non-reactive, there

are no health risks to Xe being introduced in the water, there are low background

concentrations in the environment, Xe is widely available, and it is not a greenhouse gas

ID

226Ra

DPM/kg

95%

CI+

222Rn

pCi/kg

95%

CI+

MC05 50 247.78 9.27 111.61 4.17

MC05

230 244.44 9.24 110.11 4.16

MC05

SILT 425.56 10.54 191.69 4.75

MC08 50 392.22 10.31 176.68 4.65

MC08

230 477.78 10.89 215.22 4.90

MC08

SILT 585.56 11.57 263.76 5.21

63

(which is an advantage over SF6) (Visser et. al., 2011). A Xe tracer was introduced near

one of the two large golf bridges at Lahontan Golf Club, in a reach of Martis Creek where

relatively high Rn activity had been measured. After the tracer was introduced, it mixed

into the flowing water relatively quickly and thoroughly (Figure 29). As the first graph

illustrates, when samples were taken across the width of the stream at MC01, all showed

approximately the same concentration. Samples taken at MC08 showed a similar state,

though there was less Xe dissolved in the water, as much of it had outgassed to the

atmosphere by the time it reached that location in the stream. The Xe transect along the

eight stations downstream from the introduction point showed a smooth decrease in Xe

concentration (Figure 30), which is expected as Xe degasses from the stream. The one

station upstream from the injection point also showed that very little Xe was present in

the stream to begin with, which is expected at background, environmental levels.

The degassing rates calculated based on the Xe results are shown in Table 9. The

rates found vary between about 2.5 and 15 m/day, with the variance likely due to the

nature of the creek – there are some deep pools, some riffles, some shallower areas, and

some areas with vegetation growing into the creek. While the average stream velocity

was measured at 0.36 m/s during the mid-August 2012 stream survey, the water has

different discharge rates depending on the characteristics of the particular reach of the

stream. For instance, in the deep pools, the width and depth of the creek is relatively

greater, and water will take longer to move through these areas. Then, there are some

reaches where the depth is shallow, the width is relatively short, and the water moves

64

quickly over a riffle nearby. This morphology will increase the rate that a gas leaves the

water.

Figure 29: Xenon Cross Sections. Graphs showing percentage of Xe remaining at cross

sections of station 1 and station 8 shortly after tracer introduction. Both cross sections

show that the tracer was well mixed in the stream.

Figure 30: Xenon Transect Along Stations. Graph showing the decrease in Xe

concentration as it reaches checkpoints 1 through 8, and the naturally low environmental

level of Xe at station -1.

0

10

20

30

40

50

60

70

80

90

100

MC-1C MC+1C MC+2C MC+3C MC+4C MC+5C MC+6C MC+7C MC+8C

Pe

rce

nt

Xe

re

mai

nin

g

Xenon transect along stations (8/17)

65

Figure 31: Radon and Xenon With Distance Downstream. Graph showing percentage

of Xe remaining and Rn activity level plotted against distance downstream from Xe

tracer introduction point. Both show a fairly smooth decrease with distance downstream,

though the Rn activity jumps up at stations 4 and 8, which indicates groundwater influx.

Table 9: Degassing Constant. This table shows checkpoints 1 through 8 with distances

from the Xe tracer introduction point, velocity (v), degassing constant (k), Xe

concentration, and 222

Rn activity.

checkpoint

(Xe)

Xe

(cm3/g)

Rn

(pCi/L) v (m/s)

K

(m/d)

k

(m/day)

distance

(m) from

MC00

SF+1 93.6 83.79 2.51E-01 61

SF+2 86.47 69.89 1.53E-01 1.35 -2.5 128

SF+3 73.4 71.22 4.00E-01 2.23 -7.38 251

SF+4 64.33 82.2 4.00E-01 1.72 -7.07 354

SF+5 52.59 66.26 6.46E-01 2.41 -15.18 473

SF+6 42.85 44.58 4.61E-01 2.28 -8.7 624

SF+7 29.15 38.18 4.61E-01 7.24 -14.24 797

SF+8 23.24 40.19 2.77E-01 2.96 -5.14 965

66

Influx

As noted in the “Analysis” section above, equation 3,

[

]

may be used to calculate groundwater influx (I) to Martis Creek. Stream discharge (Q),

gas transfer velocity (k), mean stream width (w), and stream Rn activity (c) were all

measured, while groundwater Rn activity (ci) was estimated (Table 1). Radon activities

collected between MC01 and MC08 during the mid-August 2012 stream survey were fit

to a simple polynomial, with different fits used for reaches where radon was rising and

falling (see Figure 32). The model was then fit to the polynomial curve by iteration with

different values of groundwater influx (I) until a best fit was achieved. The model

matches observed data fairly well, with the possible exception of radon activity at 600m,

which is lower than predicted by the model. Groundwater influx was calculated from the

best-fit model over each of the modeled 50 m intervals.

The influx over the modeled 50 m intervals ranges between 0.1 and 3.0 m3/m/day.

The greatest influx is found between 250 and 350 m downstream (2.75, 3.00, and 2.00

m3/m/day) and between 950 and 1000 m (1.85 and 2.00 m

3/m/day) downstream. These

results show that certain reaches of the stream contribute more to streamflow than others

(Figure 30), especially between 200 to 400 m downstream from the Xe introduction

point. The results also show that 900-1000 m downstream from the Xe introduction

point, groundwater influx is occurring. With the similar Rn values upstream of the area

of the stream that the Xe tracer was introduced, it is possible that similar influx occurs in

some reaches upstream as well.

67

Figure 32: Groundwater Influx. This graph shows measured Rn activities (red

squares), with the matched model (blue line). From this polynomial fit, it is possible to

find the groundwater influx over each 50 m interval, as shown in Table 10.

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Cn (

pC

i/L)

distance (m)

Model

Martis

68

Table 10: . Groundwater Influx. Groundwater influx (I) in m3/m/day found

downstream from the Xe tracer introduction point, based on matching the curve to the

measured Rn activities. Also shown in Table 9 are the I step interval (m3/day), stream

discharge (m3/day), the Rn activity (C) in pCi/L, and the change in Rn activity (dC) over

a distance (x).

Distance (m) I (m^3/m/d) I step int Qm (m^3/day) Cn-1 (pCi/L) dc/dx Cn (pCi/L)

0 0.00 4,010 90.00

50 0.30 7.50 4,018 90.00 -1.37E-01 83.13

100 0.10 10.00 4,028 83.13 -1.44E-01 75.91

150 0.00 2.50 4,030 75.91 -1.41E-01 68.84

200 0.00 0.00 4,030 68.84 -1.28E-01 62.44

250 2.75 68.75 4,099 62.44 1.79E-01 71.40

300 3.00 143.75 4,243 71.40 1.77E-01 80.25

350 2.00 125.00 4,368 80.25 5.44E-02 82.97

400 0.00 50.00 4,418 82.97 -1.41E-01 75.92

450 0.00 0.00 4,418 75.92 -1.29E-01 69.48

500 0.00 0.00 4,418 69.48 -1.18E-01 63.58

550 0.00 0.00 4,418 63.58 -1.08E-01 58.18

600 0.00 0.00 4,418 58.18 -9.88E-02 53.24

650 0.00 0.00 4,418 53.24 -9.04E-02 48.72

700 0.00 0.00 4,418 48.72 -8.27E-02 44.59

750 0.15 3.75 4,421 44.59 -6.02E-02 41.58

800 0.00 3.75 4,425 41.58 -7.05E-02 38.06

850 0.00 0.00 4,425 38.06 -6.45E-02 34.83

900 0.00 0.00 4,425 34.83 -5.90E-02 31.88

950 1.85 46.25 4,471 31.88 1.40E-01 38.89

1000 2.00 96.25 4,568 38.89 1.38E-01 45.79

Model Parameters model reference value

Co 90 100.00 Initial Rn concentration (pCi/L)

Qo 4010 0.10 Initial Discharge (m^3/d)

dx 50 50.00 Step Size (m)

w 3.00 5.00 stream width (m)

k 2.50 5.70 Gas transfer velocity (m/d)

Ci 500 500.00 Groundwater Rn activity (pCi/L)

Io 1.00 1.00 Initial Groundwater Input (m3/m/d)

69

The average stream discharge (Q) measured during the mid-August 2012 stream

survey was 4,910 m3/day, with the lowest being 4,150 m

3/day and the highest being 5,444

m3/day. At MC08 the discharge was 4,150 m

3/day, at MC23 it was 5,135 m

3/day, and at

MC34 it was 5,444 m3/day. This is slightly higher than the discharge calculated in the

model for the same reaches (Table 10), which has an average discharge of 4,309 m3/day,

with the lowest being 4,010 m3/day and the highest being 4,568 m

3/day. The measured

discharge is lower than the typical 24,468 m3/day average for the month of August (Table

2). The measurements and the model do share a similar pattern in discharge in that it

increases with distance downstream. Although the measured discharge is about 12%

higher than the modeled discharge, the measurements and calculations do overlap.

Twelve percent is an acceptable difference, as it is near the typically accepted values for

error. Furthermore, only three stations in the modeling reaches had flow measurements

taken (as listed above), so it is possible that with more data points, the averages may have

been closer. Because only three stations had flow measurements taken, the modeled

discharge is likely more accurate, since it uses data from more than three stations.

In this study, the groundwater influx of 0.10 to 0.30 m3/m/day in three intervals,

2.00 to 3.00 m3/m/day in five intervals, and zero influx in the thirteen other intervals

modeled are reasonable values. Over the total 1 km length of the modeled reach,

groundwater influx is 12.15 m3/day. The flow values were not adjusted, they are

calculated based on the curve, and independent measurements were taken at Martis Creek

for comparison. In comparison to the modeled influx, Cox et al. (2009) found a

groundwater influx of 1.61 to 5.10 m3/m/day for Squaw Creek. This is a bit higher than

70

the influx found at Martis Creek, but the values are similar. For the 32.8 km length of the

Cockburn River studied by Cook et al. (2006), there was 18,500 m3/day groundwater

(0.56 m3/m/day). This is a larger stream than Martis Creek, with a longer length studied,

so total discharge is larger as well. Wanninkhof et al. (1990) studied groundwater influx

on a 300 m reach of a small stream, and measured a maximum groundwater influx of

14.00 m3/m/day (Cox et al., 2009). The influx modeled in this study are well within this

value.

Figure 33: Groundwater Influx With Distance Downstream. Modeled groundwater

influx graphed against distance downstream from the Xe introduction point.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0 200 400 600 800 1000 1200

Infl

ux

(m^3

/m/d

)

Distance (m)

71

CONCLUSION

Baseflow keeps perennial streams from becoming intermittent streams during the

dry months of the water year and regulates water temperature by providing cool

groundwater to the stream. By quantifying groundwater influx to Martis Creek, the water

budget can be improved in the region by integrating more complete groundwater data.

Using naturally occurring Rn as a water tracer, it was possible to identify reaches

of groundwater influx where relatively high Rn activities were measured in the creek.

Both groundwater and surface water samples were taken to estimate Rn activities in the

the groundwater entering the stream and to measure Rn activities in the surface water.

Sediment samples were also taken to measure Rn emanation to determine if the

hyporheic zone contributed to Rn activities in the surface water. A Xe tracer was also

introduced to provide a way to quantify the gas transfer velocity for Rn, since Xe and Rn

share similar properties and interact with water similarly.

By using an introduced Xe tracer, it was possible to quantify, rather than estimate,

the degassing constant, which in turn allowed quantification of groundwater influx. The

Xe tracer is also a relatively new water tracer, and more environmentally friendly than

the typically used SF6. It is also seen that the hyporheic zone of Martis Creek, like that of

Squaw Creek, provides a relatively negligible amount of Rn activity to the stream,

allowing for simplification of the equation used to calculate groundwater influx.

It has been determined that groundwater influx does play a part in streamflow in

Martis Creek, and more research may be done farther upstream to determine other

reaches of influx. In general, groundwater influx adds 2 to 3 m3/m/day to the stream in

72

several reaches, and 0.1 to 0.3 m3/m/day in a couple others, with a groundwater influx of

12.15 m3/m/day over the 1 km reach that was modeled, with an average streamflow is

64,800 m3/day. Though the area of the Xe tracer study has shown the highest Rn activity

levels (based on the March/April 2013 stream surveys), the entirety of Martis Creek was

not surveyed, and the Rn activities found upstream of the Xe tracer study certainly were

not low. Thus, there may be reaches of groundwater influx upstream, and immediately

downstream of the end of the Xe tracer study. Once more research has been done with

regions farther upstream, further planning may be done concerning water demands for

groundwater.

As climate change occurs, more precipitation will occur as rain and less as snow,

while currently snow accounts for 77% of precipitation. Snowpack is essential for the

slow release of water for groundwater recharge, which then allows for enough baseflow

in Martis Creek to keep the stream from becoming intermittent. If more precipitation

begins to occur as rain rather than snow, there may be less recharge of groundwater and

more overland flow. With less groundwater available, pumping and other anthropogenic

activities may place stress on the baseflow of the stream. An accurate, balanced water

budget is essential to keep Martis Creek from becoming an intermittent stream.

73

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